2116 lines
87 KiB
ReStructuredText
2116 lines
87 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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======================
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Histogram Design Notes
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======================
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:Author: Tom Zanussi <zanussi@kernel.org>
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This document attempts to provide a description of how the ftrace
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histograms work and how the individual pieces map to the data
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structures used to implement them in trace_events_hist.c and
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tracing_map.c.
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Note: All the ftrace histogram command examples assume the working
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directory is the ftrace /tracing directory. For example::
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# cd /sys/kernel/debug/tracing
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Also, the histogram output displayed for those commands will be
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generally be truncated - only enough to make the point is displayed.
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'hist_debug' trace event files
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==============================
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If the kernel is compiled with CONFIG_HIST_TRIGGERS_DEBUG set, an
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event file named 'hist_debug' will appear in each event's
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subdirectory. This file can be read at any time and will display some
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of the hist trigger internals described in this document. Specific
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examples and output will be described in test cases below.
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Basic histograms
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================
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First, basic histograms. Below is pretty much the simplest thing you
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can do with histograms - create one with a single key on a single
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event and cat the output::
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# echo 'hist:keys=pid' >> events/sched/sched_waking/trigger
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# cat events/sched/sched_waking/hist
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{ pid: 18249 } hitcount: 1
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{ pid: 13399 } hitcount: 1
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{ pid: 17973 } hitcount: 1
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{ pid: 12572 } hitcount: 1
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...
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{ pid: 10 } hitcount: 921
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{ pid: 18255 } hitcount: 1444
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{ pid: 25526 } hitcount: 2055
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{ pid: 5257 } hitcount: 2055
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{ pid: 27367 } hitcount: 2055
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{ pid: 1728 } hitcount: 2161
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Totals:
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Hits: 21305
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Entries: 183
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Dropped: 0
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What this does is create a histogram on the sched_waking event using
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pid as a key and with a single value, hitcount, which even if not
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explicitly specified, exists for every histogram regardless.
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The hitcount value is a per-bucket value that's automatically
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incremented on every hit for the given key, which in this case is the
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pid.
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So in this histogram, there's a separate bucket for each pid, and each
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bucket contains a value for that bucket, counting the number of times
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sched_waking was called for that pid.
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Each histogram is represented by a hist_data struct.
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To keep track of each key and value field in the histogram, hist_data
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keeps an array of these fields named fields[]. The fields[] array is
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an array containing struct hist_field representations of each
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histogram val and key in the histogram (variables are also included
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here, but are discussed later). So for the above histogram we have one
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key and one value; in this case the one value is the hitcount value,
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which all histograms have, regardless of whether they define that
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value or not, which the above histogram does not.
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Each struct hist_field contains a pointer to the ftrace_event_field
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from the event's trace_event_file along with various bits related to
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that such as the size, offset, type, and a hist_field_fn_t function,
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which is used to grab the field's data from the ftrace event buffer
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(in most cases - some hist_fields such as hitcount don't directly map
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to an event field in the trace buffer - in these cases the function
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implementation gets its value from somewhere else). The flags field
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indicates which type of field it is - key, value, variable, variable
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reference, etc., with value being the default.
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The other important hist_data data structure in addition to the
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fields[] array is the tracing_map instance created for the histogram,
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which is held in the .map member. The tracing_map implements the
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lock-free hash table used to implement histograms (see
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kernel/trace/tracing_map.h for much more discussion about the
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low-level data structures implementing the tracing_map). For the
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purposes of this discussion, the tracing_map contains a number of
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buckets, each bucket corresponding to a particular tracing_map_elt
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object hashed by a given histogram key.
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Below is a diagram the first part of which describes the hist_data and
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associated key and value fields for the histogram described above. As
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you can see, there are two fields in the fields array, one val field
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for the hitcount and one key field for the pid key.
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Below that is a diagram of a run-time snapshot of what the tracing_map
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might look like for a given run. It attempts to show the
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relationships between the hist_data fields and the tracing_map
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elements for a couple hypothetical keys and values.::
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+------------------+
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| hist_data |
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+------------------+ +----------------+
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| .fields[] |---->| val = hitcount |----------------------------+
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+----------------+ +----------------+ |
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| .map | | .size | |
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+----------------+ +--------------+ |
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| .offset | |
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+--------------+ |
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| .fn() | |
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+--------------+ |
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. |
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. |
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. |
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+----------------+ <--- n_vals |
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| key = pid |----------------------------|--+
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+----------------+ | |
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| .size | | |
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+--------------+ | |
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| .offset | | |
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+--------------+ | |
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| .fn() | | |
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+----------------+ <--- n_fields | |
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| unused | | |
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+----------------+ | |
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| | | |
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+--------------+ | |
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| | | |
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+--------------+ | |
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| | | |
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+--------------+ | |
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n_keys = n_fields - n_vals | |
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The hist_data n_vals and n_fields delineate the extent of the fields[] | |
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array and separate keys from values for the rest of the code. | |
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Below is a run-time representation of the tracing_map part of the | |
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histogram, with pointers from various parts of the fields[] array | |
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to corresponding parts of the tracing_map. | |
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The tracing_map consists of an array of tracing_map_entrys and a set | |
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of preallocated tracing_map_elts (abbreviated below as map_entry and | |
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map_elt). The total number of map_entrys in the hist_data.map array = | |
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map->max_elts (actually map->map_size but only max_elts of those are | |
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used. This is a property required by the map_insert() algorithm). | |
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If a map_entry is unused, meaning no key has yet hashed into it, its | |
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.key value is 0 and its .val pointer is NULL. Once a map_entry has | |
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been claimed, the .key value contains the key's hash value and the | |
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.val member points to a map_elt containing the full key and an entry | |
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for each key or value in the map_elt.fields[] array. There is an | |
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entry in the map_elt.fields[] array corresponding to each hist_field | |
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in the histogram, and this is where the continually aggregated sums | |
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corresponding to each histogram value are kept. | |
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The diagram attempts to show the relationship between the | |
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hist_data.fields[] and the map_elt.fields[] with the links drawn | |
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between diagrams::
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+-----------+ | |
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| hist_data | | |
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+-----------+ | |
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| .fields | | |
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+---------+ +-----------+ | |
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| .map |---->| map_entry | | |
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+---------+ +-----------+ | |
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| .key |---> 0 | |
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+---------+ | |
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| .val |---> NULL | |
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+-----------+ | |
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| map_entry | | |
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+-----------+ | |
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| .key |---> pid = 999 | |
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+---------+ +-----------+ | |
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| .val |--->| map_elt | | |
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+---------+ +-----------+ | |
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. | .key |---> full key * | |
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. +---------+ +---------------+ | |
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. | .fields |--->| .sum (val) |<-+ |
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+-----------+ +---------+ | 2345 | | |
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| map_entry | +---------------+ | |
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+-----------+ | .offset (key) |<----+
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| .key |---> 0 | 0 | | |
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+---------+ +---------------+ | |
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| .val |---> NULL . | |
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+-----------+ . | |
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| map_entry | . | |
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+-----------+ +---------------+ | |
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| .key | | .sum (val) or | | |
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+---------+ +---------+ | .offset (key) | | |
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| .val |--->| map_elt | +---------------+ | |
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+-----------+ +---------+ | .sum (val) or | | |
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| map_entry | | .offset (key) | | |
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+-----------+ +---------------+ | |
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| .key |---> pid = 4444 | |
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+---------+ +-----------+ | |
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| .val | | map_elt | | |
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+---------+ +-----------+ | |
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| .key |---> full key * | |
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+---------+ +---------------+ | |
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| .fields |--->| .sum (val) |<-+ |
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+---------+ | 65523 | |
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+---------------+ |
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| .offset (key) |<----+
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| 0 |
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+---------------+
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.
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.
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.
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+---------------+
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| .sum (val) or |
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| .offset (key) |
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+---------------+
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| .sum (val) or |
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| .offset (key) |
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+---------------+
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Abbreviations used in the diagrams::
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hist_data = struct hist_trigger_data
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hist_data.fields = struct hist_field
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fn = hist_field_fn_t
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map_entry = struct tracing_map_entry
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map_elt = struct tracing_map_elt
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map_elt.fields = struct tracing_map_field
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Whenever a new event occurs and it has a hist trigger associated with
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it, event_hist_trigger() is called. event_hist_trigger() first deals
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with the key: for each subkey in the key (in the above example, there
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is just one subkey corresponding to pid), the hist_field that
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represents that subkey is retrieved from hist_data.fields[] and the
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hist_field_fn_t fn() associated with that field, along with the
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field's size and offset, is used to grab that subkey's data from the
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current trace record.
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Once the complete key has been retrieved, it's used to look that key
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up in the tracing_map. If there's no tracing_map_elt associated with
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that key, an empty one is claimed and inserted in the map for the new
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key. In either case, the tracing_map_elt associated with that key is
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returned.
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Once a tracing_map_elt available, hist_trigger_elt_update() is called.
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As the name implies, this updates the element, which basically means
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updating the element's fields. There's a tracing_map_field associated
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with each key and value in the histogram, and each of these correspond
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to the key and value hist_fields created when the histogram was
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created. hist_trigger_elt_update() goes through each value hist_field
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and, as for the keys, uses the hist_field's fn() and size and offset
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to grab the field's value from the current trace record. Once it has
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that value, it simply adds that value to that field's
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continually-updated tracing_map_field.sum member. Some hist_field
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fn()s, such as for the hitcount, don't actually grab anything from the
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trace record (the hitcount fn() just increments the counter sum by 1),
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but the idea is the same.
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Once all the values have been updated, hist_trigger_elt_update() is
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done and returns. Note that there are also tracing_map_fields for
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each subkey in the key, but hist_trigger_elt_update() doesn't look at
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them or update anything - those exist only for sorting, which can
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happen later.
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Basic histogram test
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--------------------
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This is a good example to try. It produces 3 value fields and 2 key
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fields in the output::
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# echo 'hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
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To see the debug data, cat the kmem/kmalloc's 'hist_debug' file. It
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will show the trigger info of the histogram it corresponds to, along
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with the address of the hist_data associated with the histogram, which
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will become useful in later examples. It then displays the number of
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total hist_fields associated with the histogram along with a count of
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how many of those correspond to keys and how many correspond to values.
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It then goes on to display details for each field, including the
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field's flags and the position of each field in the hist_data's
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fields[] array, which is useful information for verifying that things
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internally appear correct or not, and which again will become even
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more useful in further examples::
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# cat events/kmem/kmalloc/hist_debug
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# event histogram
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#
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# trigger info: hist:keys=common_pid,call_site.sym:vals=hitcount,bytes_req,bytes_alloc:sort=hitcount:size=2048 [active]
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#
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hist_data: 000000005e48c9a5
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n_vals: 3
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n_keys: 2
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n_fields: 5
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val fields:
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hist_data->fields[0]:
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flags:
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VAL: HIST_FIELD_FL_HITCOUNT
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type: u64
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size: 8
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is_signed: 0
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hist_data->fields[1]:
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flags:
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VAL: normal u64 value
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ftrace_event_field name: bytes_req
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type: size_t
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size: 8
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is_signed: 0
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hist_data->fields[2]:
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flags:
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VAL: normal u64 value
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ftrace_event_field name: bytes_alloc
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type: size_t
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size: 8
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is_signed: 0
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key fields:
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hist_data->fields[3]:
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flags:
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HIST_FIELD_FL_KEY
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ftrace_event_field name: common_pid
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type: int
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size: 8
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is_signed: 1
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hist_data->fields[4]:
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flags:
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HIST_FIELD_FL_KEY
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ftrace_event_field name: call_site
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type: unsigned long
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size: 8
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is_signed: 0
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The commands below can be used to clean things up for the next test::
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# echo '!hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
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Variables
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=========
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Variables allow data from one hist trigger to be saved by one hist
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trigger and retrieved by another hist trigger. For example, a trigger
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on the sched_waking event can capture a timestamp for a particular
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pid, and later a sched_switch event that switches to that pid event
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can grab the timestamp and use it to calculate a time delta between
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the two events::
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# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
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events/sched/sched_waking/trigger
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# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >>
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events/sched/sched_switch/trigger
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In terms of the histogram data structures, variables are implemented
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as another type of hist_field and for a given hist trigger are added
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to the hist_data.fields[] array just after all the val fields. To
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distinguish them from the existing key and val fields, they're given a
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new flag type, HIST_FIELD_FL_VAR (abbreviated FL_VAR) and they also
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make use of a new .var.idx field member in struct hist_field, which
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maps them to an index in a new map_elt.vars[] array added to the
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map_elt specifically designed to store and retrieve variable values.
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The diagram below shows those new elements and adds a new variable
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entry, ts0, corresponding to the ts0 variable in the sched_waking
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trigger above.
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sched_waking histogram
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----------------------::
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+------------------+
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| hist_data |<-------------------------------------------------------+
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+------------------+ +-------------------+ |
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| .fields[] |-->| val = hitcount | |
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+----------------+ +-------------------+ |
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| .map | | .size | |
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+----------------+ +-----------------+ |
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| .offset | |
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+-----------------+ |
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| .fn() | |
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+-----------------+ |
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| .flags | |
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+-----------------+ |
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| .var.idx | |
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+-------------------+ |
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| var = ts0 | |
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+-------------------+ |
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| .size | |
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+-----------------+ |
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| .offset | |
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+-----------------+ |
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| .fn() | |
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+-----------------+ |
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| .flags & FL_VAR | |
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+-----------------+ |
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| .var.idx |----------------------------+-+ |
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+-----------------+ | | |
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. | | |
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. | | |
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. | | |
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+-------------------+ <--- n_vals | | |
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| key = pid | | | |
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+-------------------+ | | |
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| .size | | | |
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+-----------------+ | | |
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| .offset | | | |
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+-----------------+ | | |
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| .fn() | | | |
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+-----------------+ | | |
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| .flags & FL_KEY | | | |
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+-----------------+ | | |
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| .var.idx | | | |
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+-------------------+ <--- n_fields | | |
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| unused | | | |
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+-------------------+ | | |
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| | | | |
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+-----------------+ | | |
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| | | | |
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+-----------------+ | | |
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| | | | |
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+-----------------+ | | |
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| | | | |
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+-----------------+ | | |
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+-----------------+ | | |
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n_keys = n_fields - n_vals | | |
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| | |
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This is very similar to the basic case. In the above diagram, we can | | |
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see a new .flags member has been added to the struct hist_field | | |
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struct, and a new entry added to hist_data.fields representing the ts0 | | |
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variable. For a normal val hist_field, .flags is just 0 (modulo | | |
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modifier flags), but if the value is defined as a variable, the .flags | | |
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contains a set FL_VAR bit. | | |
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As you can see, the ts0 entry's .var.idx member contains the index | | |
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into the tracing_map_elts' .vars[] array containing variable values. | | |
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This idx is used whenever the value of the variable is set or read. | | |
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The map_elt.vars idx assigned to the given variable is assigned and | | |
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saved in .var.idx by create_tracing_map_fields() after it calls | | |
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tracing_map_add_var(). | | |
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Below is a representation of the histogram at run-time, which | | |
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populates the map, along with correspondence to the above hist_data and | | |
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hist_field data structures. | | |
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The diagram attempts to show the relationship between the | | |
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hist_data.fields[] and the map_elt.fields[] and map_elt.vars[] with | | |
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the links drawn between diagrams. For each of the map_elts, you can | | |
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see that the .fields[] members point to the .sum or .offset of a key | | |
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or val and the .vars[] members point to the value of a variable. The | | |
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arrows between the two diagrams show the linkages between those | | |
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tracing_map members and the field definitions in the corresponding | | |
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hist_data fields[] members.::
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+-----------+ | | |
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| hist_data | | | |
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+-----------+ | | |
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| .fields | | | |
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+---------+ +-----------+ | | |
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| .map |---->| map_entry | | | |
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+---------+ +-----------+ | | |
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| .key |---> 0 | | |
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+---------+ | | |
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| .val |---> NULL | | |
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+-----------+ | | |
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| map_entry | | | |
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+-----------+ | | |
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| .key |---> pid = 999 | | |
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+---------+ +-----------+ | | |
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| .val |--->| map_elt | | | |
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+---------+ +-----------+ | | |
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. | .key |---> full key * | | |
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. +---------+ +---------------+ | | |
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. | .fields |--->| .sum (val) | | | |
|
|
. +---------+ | 2345 | | | |
|
|
. +--| .vars | +---------------+ | | |
|
|
. | +---------+ | .offset (key) | | | |
|
|
. | | 0 | | | |
|
|
. | +---------------+ | | |
|
|
. | . | | |
|
|
. | . | | |
|
|
. | . | | |
|
|
. | +---------------+ | | |
|
|
. | | .sum (val) or | | | |
|
|
. | | .offset (key) | | | |
|
|
. | +---------------+ | | |
|
|
. | | .sum (val) or | | | |
|
|
. | | .offset (key) | | | |
|
|
. | +---------------+ | | |
|
|
. | | | |
|
|
. +---------------->+---------------+ | | |
|
|
. | ts0 |<--+ | |
|
|
. | 113345679876 | | | |
|
|
. +---------------+ | | |
|
|
. | unused | | | |
|
|
. | | | | |
|
|
. +---------------+ | | |
|
|
. . | | |
|
|
. . | | |
|
|
. . | | |
|
|
. +---------------+ | | |
|
|
. | unused | | | |
|
|
. | | | | |
|
|
. +---------------+ | | |
|
|
. | unused | | | |
|
|
. | | | | |
|
|
. +---------------+ | | |
|
|
. | | |
|
|
+-----------+ | | |
|
|
| map_entry | | | |
|
|
+-----------+ | | |
|
|
| .key |---> pid = 4444 | | |
|
|
+---------+ +-----------+ | | |
|
|
| .val |--->| map_elt | | | |
|
|
+---------+ +-----------+ | | |
|
|
. | .key |---> full key * | | |
|
|
. +---------+ +---------------+ | | |
|
|
. | .fields |--->| .sum (val) | | | |
|
|
+---------+ | 2345 | | | |
|
|
+--| .vars | +---------------+ | | |
|
|
| +---------+ | .offset (key) | | | |
|
|
| | 0 | | | |
|
|
| +---------------+ | | |
|
|
| . | | |
|
|
| . | | |
|
|
| . | | |
|
|
| +---------------+ | | |
|
|
| | .sum (val) or | | | |
|
|
| | .offset (key) | | | |
|
|
| +---------------+ | | |
|
|
| | .sum (val) or | | | |
|
|
| | .offset (key) | | | |
|
|
| +---------------+ | | |
|
|
| | | |
|
|
| +---------------+ | | |
|
|
+---------------->| ts0 |<--+ | |
|
|
| 213499240729 | | |
|
|
+---------------+ | |
|
|
| unused | | |
|
|
| | | |
|
|
+---------------+ | |
|
|
. | |
|
|
. | |
|
|
. | |
|
|
+---------------+ | |
|
|
| unused | | |
|
|
| | | |
|
|
+---------------+ | |
|
|
| unused | | |
|
|
| | | |
|
|
+---------------+ | |
|
|
|
|
For each used map entry, there's a map_elt pointing to an array of | |
|
|
.vars containing the current value of the variables associated with | |
|
|
that histogram entry. So in the above, the timestamp associated with | |
|
|
pid 999 is 113345679876, and the timestamp variable in the same | |
|
|
.var.idx for pid 4444 is 213499240729. | |
|
|
|
|
sched_switch histogram | |
|
|
---------------------- | |
|
|
|
|
The sched_switch histogram paired with the above sched_waking | |
|
|
histogram is shown below. The most important aspect of the | |
|
|
sched_switch histogram is that it references a variable on the | |
|
|
sched_waking histogram above. | |
|
|
|
|
The histogram diagram is very similar to the others so far displayed, | |
|
|
but it adds variable references. You can see the normal hitcount and | |
|
|
key fields along with a new wakeup_lat variable implemented in the | |
|
|
same way as the sched_waking ts0 variable, but in addition there's an | |
|
|
entry with the new FL_VAR_REF (short for HIST_FIELD_FL_VAR_REF) flag. | |
|
|
|
|
Associated with the new var ref field are a couple of new hist_field | |
|
|
members, var.hist_data and var_ref_idx. For a variable reference, the | |
|
|
var.hist_data goes with the var.idx, which together uniquely identify | |
|
|
a particular variable on a particular histogram. The var_ref_idx is | |
|
|
just the index into the var_ref_vals[] array that caches the values of | |
|
|
each variable whenever a hist trigger is updated. Those resulting | |
|
|
values are then finally accessed by other code such as trace action | |
|
|
code that uses the var_ref_idx values to assign param values. | |
|
|
|
|
The diagram below describes the situation for the sched_switch | |
|
|
histogram referred to before::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> | |
|
|
events/sched/sched_switch/trigger | |
|
|
| |
|
|
+------------------+ | |
|
|
| hist_data | | |
|
|
+------------------+ +-----------------------+ | |
|
|
| .fields[] |-->| val = hitcount | | |
|
|
+----------------+ +-----------------------+ | |
|
|
| .map | | .size | | |
|
|
+----------------+ +---------------------+ | |
|
|
+--| .var_refs[] | | .offset | | |
|
|
| +----------------+ +---------------------+ | |
|
|
| | .fn() | | |
|
|
| var_ref_vals[] +---------------------+ | |
|
|
| +-------------+ | .flags | | |
|
|
| | $ts0 |<---+ +---------------------+ | |
|
|
| +-------------+ | | .var.idx | | |
|
|
| | | | +---------------------+ | |
|
|
| +-------------+ | | .var.hist_data | | |
|
|
| | | | +---------------------+ | |
|
|
| +-------------+ | | .var_ref_idx | | |
|
|
| | | | +-----------------------+ | |
|
|
| +-------------+ | | var = wakeup_lat | | |
|
|
| . | +-----------------------+ | |
|
|
| . | | .size | | |
|
|
| . | +---------------------+ | |
|
|
| +-------------+ | | .offset | | |
|
|
| | | | +---------------------+ | |
|
|
| +-------------+ | | .fn() | | |
|
|
| | | | +---------------------+ | |
|
|
| +-------------+ | | .flags & FL_VAR | | |
|
|
| | +---------------------+ | |
|
|
| | | .var.idx | | |
|
|
| | +---------------------+ | |
|
|
| | | .var.hist_data | | |
|
|
| | +---------------------+ | |
|
|
| | | .var_ref_idx | | |
|
|
| | +---------------------+ | |
|
|
| | . | |
|
|
| | . | |
|
|
| | . | |
|
|
| | +-----------------------+ <--- n_vals | |
|
|
| | | key = pid | | |
|
|
| | +-----------------------+ | |
|
|
| | | .size | | |
|
|
| | +---------------------+ | |
|
|
| | | .offset | | |
|
|
| | +---------------------+ | |
|
|
| | | .fn() | | |
|
|
| | +---------------------+ | |
|
|
| | | .flags | | |
|
|
| | +---------------------+ | |
|
|
| | | .var.idx | | |
|
|
| | +-----------------------+ <--- n_fields | |
|
|
| | | unused | | |
|
|
| | +-----------------------+ | |
|
|
| | | | | |
|
|
| | +---------------------+ | |
|
|
| | | | | |
|
|
| | +---------------------+ | |
|
|
| | | | | |
|
|
| | +---------------------+ | |
|
|
| | | | | |
|
|
| | +---------------------+ | |
|
|
| | | | | |
|
|
| | +---------------------+ | |
|
|
| | n_keys = n_fields - n_vals | |
|
|
| | | |
|
|
| | | |
|
|
| | +-----------------------+ | |
|
|
+---------------------->| var_ref = $ts0 | | |
|
|
| +-----------------------+ | |
|
|
| | .size | | |
|
|
| +---------------------+ | |
|
|
| | .offset | | |
|
|
| +---------------------+ | |
|
|
| | .fn() | | |
|
|
| +---------------------+ | |
|
|
| | .flags & FL_VAR_REF | | |
|
|
| +---------------------+ | |
|
|
| | .var.idx |--------------------------+ |
|
|
| +---------------------+ |
|
|
| | .var.hist_data |----------------------------+
|
|
| +---------------------+
|
|
+---| .var_ref_idx |
|
|
+---------------------+
|
|
|
|
Abbreviations used in the diagrams::
|
|
|
|
hist_data = struct hist_trigger_data
|
|
hist_data.fields = struct hist_field
|
|
fn = hist_field_fn_t
|
|
FL_KEY = HIST_FIELD_FL_KEY
|
|
FL_VAR = HIST_FIELD_FL_VAR
|
|
FL_VAR_REF = HIST_FIELD_FL_VAR_REF
|
|
|
|
When a hist trigger makes use of a variable, a new hist_field is
|
|
created with flag HIST_FIELD_FL_VAR_REF. For a VAR_REF field, the
|
|
var.idx and var.hist_data take the same values as the referenced
|
|
variable, as well as the referenced variable's size, type, and
|
|
is_signed values. The VAR_REF field's .name is set to the name of the
|
|
variable it references. If a variable reference was created using the
|
|
explicit system.event.$var_ref notation, the hist_field's system and
|
|
event_name variables are also set.
|
|
|
|
So, in order to handle an event for the sched_switch histogram,
|
|
because we have a reference to a variable on another histogram, we
|
|
need to resolve all variable references first. This is done via the
|
|
resolve_var_refs() calls made from event_hist_trigger(). What this
|
|
does is grabs the var_refs[] array from the hist_data representing the
|
|
sched_switch histogram. For each one of those, the referenced
|
|
variable's var.hist_data along with the current key is used to look up
|
|
the corresponding tracing_map_elt in that histogram. Once found, the
|
|
referenced variable's var.idx is used to look up the variable's value
|
|
using tracing_map_read_var(elt, var.idx), which yields the value of
|
|
the variable for that element, ts0 in the case above. Note that both
|
|
the hist_fields representing both the variable and the variable
|
|
reference have the same var.idx, so this is straightforward.
|
|
|
|
Variable and variable reference test
|
|
------------------------------------
|
|
|
|
This example creates a variable on the sched_waking event, ts0, and
|
|
uses it in the sched_switch trigger. The sched_switch trigger also
|
|
creates its own variable, wakeup_lat, but nothing yet uses it::
|
|
|
|
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
|
|
|
|
Looking at the sched_waking 'hist_debug' output, in addition to the
|
|
normal key and value hist_fields, in the val fields section we see a
|
|
field with the HIST_FIELD_FL_VAR flag, which indicates that that field
|
|
represents a variable. Note that in addition to the variable name,
|
|
contained in the var.name field, it includes the var.idx, which is the
|
|
index into the tracing_map_elt.vars[] array of the actual variable
|
|
location. Note also that the output shows that variables live in the
|
|
same part of the hist_data->fields[] array as normal values::
|
|
|
|
# cat events/sched/sched_waking/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 000000009536f554
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
Moving on to the sched_switch trigger hist_debug output, in addition
|
|
to the unused wakeup_lat variable, we see a new section displaying
|
|
variable references. Variable references are displayed in a separate
|
|
section because in addition to being logically separate from
|
|
variables and values, they actually live in a separate hist_data
|
|
array, var_refs[].
|
|
|
|
In this example, the sched_switch trigger has a reference to a
|
|
variable on the sched_waking trigger, $ts0. Looking at the details,
|
|
we can see that the var.hist_data value of the referenced variable
|
|
matches the previously displayed sched_waking trigger, and the var.idx
|
|
value matches the previously displayed var.idx value for that
|
|
variable. Also displayed is the var_ref_idx value for that variable
|
|
reference, which is where the value for that variable is cached for
|
|
use when the trigger is invoked::
|
|
|
|
# cat events/sched/sched_switch/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 00000000f4ee8006
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
variable reference fields:
|
|
|
|
hist_data->var_refs[0]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 000000009536f554
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
The commands below can be used to clean things up for the next test::
|
|
|
|
# echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
|
|
|
|
# echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
Actions and Handlers
|
|
====================
|
|
|
|
Adding onto the previous example, we will now do something with that
|
|
wakeup_lat variable, namely send it and another field as a synthetic
|
|
event.
|
|
|
|
The onmatch() action below basically says that whenever we have a
|
|
sched_switch event, if we have a matching sched_waking event, in this
|
|
case if we have a pid in the sched_waking histogram that matches the
|
|
next_pid field on this sched_switch event, we retrieve the
|
|
variables specified in the wakeup_latency() trace action, and use
|
|
them to generate a new wakeup_latency event into the trace stream.
|
|
|
|
Note that the way the trace handlers such as wakeup_latency() (which
|
|
could equivalently be written trace(wakeup_latency,$wakeup_lat,next_pid)
|
|
are implemented, the parameters specified to the trace handler must be
|
|
variables. In this case, $wakeup_lat is obviously a variable, but
|
|
next_pid isn't, since it's just naming a field in the sched_switch
|
|
trace event. Since this is something that almost every trace() and
|
|
save() action does, a special shortcut is implemented to allow field
|
|
names to be used directly in those cases. How it works is that under
|
|
the covers, a temporary variable is created for the named field, and
|
|
this variable is what is actually passed to the trace handler. In the
|
|
code and documentation, this type of variable is called a 'field
|
|
variable'.
|
|
|
|
Fields on other trace event's histograms can be used as well. In that
|
|
case we have to generate a new histogram and an unfortunately named
|
|
'synthetic_field' (the use of synthetic here has nothing to do with
|
|
synthetic events) and use that special histogram field as a variable.
|
|
|
|
The diagram below illustrates the new elements described above in the
|
|
context of the sched_switch histogram using the onmatch() handler and
|
|
the trace() action.
|
|
|
|
First, we define the wakeup_latency synthetic event::
|
|
|
|
# echo 'wakeup_latency u64 lat; pid_t pid' >> synthetic_events
|
|
|
|
Next, the sched_waking hist trigger as before::
|
|
|
|
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
|
|
events/sched/sched_waking/trigger
|
|
|
|
Finally, we create a hist trigger on the sched_switch event that
|
|
generates a wakeup_latency() trace event. In this case we pass
|
|
next_pid into the wakeup_latency synthetic event invocation, which
|
|
means it will be automatically converted into a field variable::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
|
|
onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid)' >>
|
|
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
|
|
|
|
The diagram for the sched_switch event is similar to previous examples
|
|
but shows the additional field_vars[] array for hist_data and shows
|
|
the linkages between the field_vars and the variables and references
|
|
created to implement the field variables. The details are discussed
|
|
below::
|
|
|
|
+------------------+
|
|
| hist_data |
|
|
+------------------+ +-----------------------+
|
|
| .fields[] |-->| val = hitcount |
|
|
+----------------+ +-----------------------+
|
|
| .map | | .size |
|
|
+----------------+ +---------------------+
|
|
+---| .field_vars[] | | .offset |
|
|
| +----------------+ +---------------------+
|
|
|+--| .var_refs[] | | .offset |
|
|
|| +----------------+ +---------------------+
|
|
|| | .fn() |
|
|
|| var_ref_vals[] +---------------------+
|
|
|| +-------------+ | .flags |
|
|
|| | $ts0 |<---+ +---------------------+
|
|
|| +-------------+ | | .var.idx |
|
|
|| | $next_pid |<-+ | +---------------------+
|
|
|| +-------------+ | | | .var.hist_data |
|
|
||+>| $wakeup_lat | | | +---------------------+
|
|
||| +-------------+ | | | .var_ref_idx |
|
|
||| | | | | +-----------------------+
|
|
||| +-------------+ | | | var = wakeup_lat |
|
|
||| . | | +-----------------------+
|
|
||| . | | | .size |
|
|
||| . | | +---------------------+
|
|
||| +-------------+ | | | .offset |
|
|
||| | | | | +---------------------+
|
|
||| +-------------+ | | | .fn() |
|
|
||| | | | | +---------------------+
|
|
||| +-------------+ | | | .flags & FL_VAR |
|
|
||| | | +---------------------+
|
|
||| | | | .var.idx |
|
|
||| | | +---------------------+
|
|
||| | | | .var.hist_data |
|
|
||| | | +---------------------+
|
|
||| | | | .var_ref_idx |
|
|
||| | | +---------------------+
|
|
||| | | .
|
|
||| | | .
|
|
||| | | .
|
|
||| | | .
|
|
||| +--------------+ | | .
|
|
+-->| field_var | | | .
|
|
|| +--------------+ | | .
|
|
|| | var | | | .
|
|
|| +------------+ | | .
|
|
|| | val | | | .
|
|
|| +--------------+ | | .
|
|
|| | field_var | | | .
|
|
|| +--------------+ | | .
|
|
|| | var | | | .
|
|
|| +------------+ | | .
|
|
|| | val | | | .
|
|
|| +------------+ | | .
|
|
|| . | | .
|
|
|| . | | .
|
|
|| . | | +-----------------------+ <--- n_vals
|
|
|| +--------------+ | | | key = pid |
|
|
|| | field_var | | | +-----------------------+
|
|
|| +--------------+ | | | .size |
|
|
|| | var |--+| +---------------------+
|
|
|| +------------+ ||| | .offset |
|
|
|| | val |-+|| +---------------------+
|
|
|| +------------+ ||| | .fn() |
|
|
|| ||| +---------------------+
|
|
|| ||| | .flags |
|
|
|| ||| +---------------------+
|
|
|| ||| | .var.idx |
|
|
|| ||| +---------------------+ <--- n_fields
|
|
|| |||
|
|
|| ||| n_keys = n_fields - n_vals
|
|
|| ||| +-----------------------+
|
|
|| |+->| var = next_pid |
|
|
|| | | +-----------------------+
|
|
|| | | | .size |
|
|
|| | | +---------------------+
|
|
|| | | | .offset |
|
|
|| | | +---------------------+
|
|
|| | | | .flags & FL_VAR |
|
|
|| | | +---------------------+
|
|
|| | | | .var.idx |
|
|
|| | | +---------------------+
|
|
|| | | | .var.hist_data |
|
|
|| | | +-----------------------+
|
|
|| +-->| val for next_pid |
|
|
|| | | +-----------------------+
|
|
|| | | | .size |
|
|
|| | | +---------------------+
|
|
|| | | | .offset |
|
|
|| | | +---------------------+
|
|
|| | | | .fn() |
|
|
|| | | +---------------------+
|
|
|| | | | .flags |
|
|
|| | | +---------------------+
|
|
|| | | | |
|
|
|| | | +---------------------+
|
|
|| | |
|
|
|| | |
|
|
|| | | +-----------------------+
|
|
+|------------------|-|>| var_ref = $ts0 |
|
|
| | | +-----------------------+
|
|
| | | | .size |
|
|
| | | +---------------------+
|
|
| | | | .offset |
|
|
| | | +---------------------+
|
|
| | | | .fn() |
|
|
| | | +---------------------+
|
|
| | | | .flags & FL_VAR_REF |
|
|
| | | +---------------------+
|
|
| | +---| .var_ref_idx |
|
|
| | +-----------------------+
|
|
| | | var_ref = $next_pid |
|
|
| | +-----------------------+
|
|
| | | .size |
|
|
| | +---------------------+
|
|
| | | .offset |
|
|
| | +---------------------+
|
|
| | | .fn() |
|
|
| | +---------------------+
|
|
| | | .flags & FL_VAR_REF |
|
|
| | +---------------------+
|
|
| +-----| .var_ref_idx |
|
|
| +-----------------------+
|
|
| | var_ref = $wakeup_lat |
|
|
| +-----------------------+
|
|
| | .size |
|
|
| +---------------------+
|
|
| | .offset |
|
|
| +---------------------+
|
|
| | .fn() |
|
|
| +---------------------+
|
|
| | .flags & FL_VAR_REF |
|
|
| +---------------------+
|
|
+------------------------| .var_ref_idx |
|
|
+---------------------+
|
|
|
|
As you can see, for a field variable, two hist_fields are created: one
|
|
representing the variable, in this case next_pid, and one to actually
|
|
get the value of the field from the trace stream, like a normal val
|
|
field does. These are created separately from normal variable
|
|
creation and are saved in the hist_data->field_vars[] array. See
|
|
below for how these are used. In addition, a reference hist_field is
|
|
also created, which is needed to reference the field variables such as
|
|
$next_pid variable in the trace() action.
|
|
|
|
Note that $wakeup_lat is also a variable reference, referencing the
|
|
value of the expression common_timestamp-$ts0, and so also needs to
|
|
have a hist field entry representing that reference created.
|
|
|
|
When hist_trigger_elt_update() is called to get the normal key and
|
|
value fields, it also calls update_field_vars(), which goes through
|
|
each field_var created for the histogram, and available from
|
|
hist_data->field_vars and calls val->fn() to get the data from the
|
|
current trace record, and then uses the var's var.idx to set the
|
|
variable at the var.idx offset in the appropriate tracing_map_elt's
|
|
variable at elt->vars[var.idx].
|
|
|
|
Once all the variables have been updated, resolve_var_refs() can be
|
|
called from event_hist_trigger(), and not only can our $ts0 and
|
|
$next_pid references be resolved but the $wakeup_lat reference as
|
|
well. At this point, the trace() action can simply access the values
|
|
assembled in the var_ref_vals[] array and generate the trace event.
|
|
|
|
The same process occurs for the field variables associated with the
|
|
save() action.
|
|
|
|
Abbreviations used in the diagram::
|
|
|
|
hist_data = struct hist_trigger_data
|
|
hist_data.fields = struct hist_field
|
|
field_var = struct field_var
|
|
fn = hist_field_fn_t
|
|
FL_KEY = HIST_FIELD_FL_KEY
|
|
FL_VAR = HIST_FIELD_FL_VAR
|
|
FL_VAR_REF = HIST_FIELD_FL_VAR_REF
|
|
|
|
trace() action field variable test
|
|
----------------------------------
|
|
|
|
This example adds to the previous test example by finally making use
|
|
of the wakeup_lat variable, but in addition also creates a couple of
|
|
field variables that then are all passed to the wakeup_latency() trace
|
|
action via the onmatch() handler.
|
|
|
|
First, we create the wakeup_latency synthetic event::
|
|
|
|
# echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
|
|
|
|
Next, the sched_waking trigger from previous examples::
|
|
|
|
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
Finally, as in the previous test example, we calculate and assign the
|
|
wakeup latency using the $ts0 reference from the sched_waking trigger
|
|
to the wakeup_lat variable, and finally use it along with a couple
|
|
sched_switch event fields, next_pid and next_comm, to generate a
|
|
wakeup_latency trace event. The next_pid and next_comm event fields
|
|
are automatically converted into field variables for this purpose::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
|
|
|
|
The sched_waking hist_debug output shows the same data as in the
|
|
previous test example::
|
|
|
|
# cat events/sched/sched_waking/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 00000000d60ff61f
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
The sched_switch hist_debug output shows the same key and value fields
|
|
as in the previous test example - note that wakeup_lat is still in the
|
|
val fields section, but that the new field variables are not there -
|
|
although the field variables are variables, they're held separately in
|
|
the hist_data's field_vars[] array. Although the field variables and
|
|
the normal variables are located in separate places, you can see that
|
|
the actual variable locations for those variables in the
|
|
tracing_map_elt.vars[] do have increasing indices as expected:
|
|
wakeup_lat takes the var.idx = 0 slot, while the field variables for
|
|
next_pid and next_comm have values var.idx = 1, and var.idx = 2. Note
|
|
also that those are the same values displayed for the variable
|
|
references corresponding to those variables in the variable reference
|
|
fields section. Since there are two triggers and thus two hist_data
|
|
addresses, those addresses also need to be accounted for when doing
|
|
the matching - you can see that the first variable refers to the 0
|
|
var.idx on the previous hist trigger (see the hist_data address
|
|
associated with that trigger), while the second variable refers to the
|
|
0 var.idx on the sched_switch hist trigger, as do all the remaining
|
|
variable references.
|
|
|
|
Finally, the action tracking variables section just shows the system
|
|
and event name for the onmatch() handler::
|
|
|
|
# cat events/sched/sched_switch/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm) [active]
|
|
#
|
|
|
|
hist_data: 0000000008f551b7
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
variable reference fields:
|
|
|
|
hist_data->var_refs[0]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 00000000d60ff61f
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 0000000008f551b7
|
|
var_ref_idx (into hist_data->var_refs[]): 1
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[2]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: next_pid
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
var.hist_data: 0000000008f551b7
|
|
var_ref_idx (into hist_data->var_refs[]): 2
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[3]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: next_comm
|
|
var.idx (into tracing_map_elt.vars[]): 2
|
|
var.hist_data: 0000000008f551b7
|
|
var_ref_idx (into hist_data->var_refs[]): 3
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
field variables:
|
|
|
|
hist_data->field_vars[0]:
|
|
|
|
field_vars[0].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: next_pid
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
|
|
field_vars[0].val:
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->field_vars[1]:
|
|
|
|
field_vars[1].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: next_comm
|
|
var.idx (into tracing_map_elt.vars[]): 2
|
|
|
|
field_vars[1].val:
|
|
ftrace_event_field name: next_comm
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
action tracking variables (for onmax()/onchange()/onmatch()):
|
|
|
|
hist_data->actions[0].match_data.event_system: sched
|
|
hist_data->actions[0].match_data.event: sched_waking
|
|
|
|
The commands below can be used to clean things up for the next test::
|
|
|
|
# echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
|
|
|
|
# echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
# echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
|
|
|
|
action_data and the trace() action
|
|
----------------------------------
|
|
|
|
As mentioned above, when the trace() action generates a synthetic
|
|
event, all the parameters to the synthetic event either already are
|
|
variables or are converted into variables (via field variables), and
|
|
finally all those variable values are collected via references to them
|
|
into a var_ref_vals[] array.
|
|
|
|
The values in the var_ref_vals[] array, however, don't necessarily
|
|
follow the same ordering as the synthetic event params. To address
|
|
that, struct action_data contains another array, var_ref_idx[] that
|
|
maps the trace action params to the var_ref_vals[] values. Below is a
|
|
diagram illustrating that for the wakeup_latency() synthetic event::
|
|
|
|
+------------------+ wakeup_latency()
|
|
| action_data | event params var_ref_vals[]
|
|
+------------------+ +-----------------+ +-----------------+
|
|
| .var_ref_idx[] |--->| $wakeup_lat idx |---+ | |
|
|
+----------------+ +-----------------+ | +-----------------+
|
|
| .synth_event | | $next_pid idx |---|-+ | $wakeup_lat val |
|
|
+----------------+ +-----------------+ | | +-----------------+
|
|
. | +->| $next_pid val |
|
|
. | +-----------------+
|
|
. | .
|
|
+-----------------+ | .
|
|
| | | .
|
|
+-----------------+ | +-----------------+
|
|
+--->| $wakeup_lat val |
|
|
+-----------------+
|
|
|
|
Basically, how this ends up getting used in the synthetic event probe
|
|
function, trace_event_raw_event_synth(), is as follows::
|
|
|
|
for each field i in .synth_event
|
|
val_idx = .var_ref_idx[i]
|
|
val = var_ref_vals[val_idx]
|
|
|
|
action_data and the onXXX() handlers
|
|
------------------------------------
|
|
|
|
The hist trigger onXXX() actions other than onmatch(), such as onmax()
|
|
and onchange(), also make use of and internally create hidden
|
|
variables. This information is contained in the
|
|
action_data.track_data struct, and is also visible in the hist_debug
|
|
output as will be described in the example below.
|
|
|
|
Typically, the onmax() or onchange() handlers are used in conjunction
|
|
with the save() and snapshot() actions. For example::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
|
|
onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >>
|
|
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
|
|
|
|
or::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
|
|
onmax($wakeup_lat).snapshot()' >>
|
|
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
|
|
|
|
save() action field variable test
|
|
---------------------------------
|
|
|
|
For this example, instead of generating a synthetic event, the save()
|
|
action is used to save field values whenever an onmax() handler
|
|
detects that a new max latency has been hit. As in the previous
|
|
example, the values being saved are also field values, but in this
|
|
case, are kept in a separate hist_data array named save_vars[].
|
|
|
|
As in previous test examples, we set up the sched_waking trigger::
|
|
|
|
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
In this case, however, we set up the sched_switch trigger to save some
|
|
sched_switch field values whenever we hit a new maximum latency. For
|
|
both the onmax() handler and save() action, variables will be created,
|
|
which we can use the hist_debug files to examine::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
|
|
|
|
The sched_waking hist_debug output shows the same data as in the
|
|
previous test examples::
|
|
|
|
# cat events/sched/sched_waking/hist_debug
|
|
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 00000000e6290f48
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
The output of the sched_switch trigger shows the same val and key
|
|
values as before, but also shows a couple new sections.
|
|
|
|
First, the action tracking variables section now shows the
|
|
actions[].track_data information describing the special tracking
|
|
variables and references used to track, in this case, the running
|
|
maximum value. The actions[].track_data.var_ref member contains the
|
|
reference to the variable being tracked, in this case the $wakeup_lat
|
|
variable. In order to perform the onmax() handler function, there
|
|
also needs to be a variable that tracks the current maximum by getting
|
|
updated whenever a new maximum is hit. In this case, we can see that
|
|
an auto-generated variable named ' __max' has been created and is
|
|
visible in the actions[].track_data.track_var variable.
|
|
|
|
Finally, in the new 'save action variables' section, we can see that
|
|
the 4 params to the save() function have resulted in 4 field variables
|
|
being created for the purposes of saving the values of the named
|
|
fields when the max is hit. These variables are kept in a separate
|
|
save_vars[] array off of hist_data, so are displayed in a separate
|
|
section::
|
|
|
|
# cat events/sched/sched_switch/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) [active]
|
|
#
|
|
|
|
hist_data: 0000000057bcd28d
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
variable reference fields:
|
|
|
|
hist_data->var_refs[0]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 00000000e6290f48
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 0000000057bcd28d
|
|
var_ref_idx (into hist_data->var_refs[]): 1
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
action tracking variables (for onmax()/onchange()/onmatch()):
|
|
|
|
hist_data->actions[0].track_data.var_ref:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 0000000057bcd28d
|
|
var_ref_idx (into hist_data->var_refs[]): 1
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
hist_data->actions[0].track_data.track_var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: __max
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
save action variables (save() params):
|
|
|
|
hist_data->save_vars[0]:
|
|
|
|
save_vars[0].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: next_comm
|
|
var.idx (into tracing_map_elt.vars[]): 2
|
|
|
|
save_vars[0].val:
|
|
ftrace_event_field name: next_comm
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
hist_data->save_vars[1]:
|
|
|
|
save_vars[1].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: prev_pid
|
|
var.idx (into tracing_map_elt.vars[]): 3
|
|
|
|
save_vars[1].val:
|
|
ftrace_event_field name: prev_pid
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->save_vars[2]:
|
|
|
|
save_vars[2].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: prev_prio
|
|
var.idx (into tracing_map_elt.vars[]): 4
|
|
|
|
save_vars[2].val:
|
|
ftrace_event_field name: prev_prio
|
|
type: int
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->save_vars[3]:
|
|
|
|
save_vars[3].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: prev_comm
|
|
var.idx (into tracing_map_elt.vars[]): 5
|
|
|
|
save_vars[3].val:
|
|
ftrace_event_field name: prev_comm
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
The commands below can be used to clean things up for the next test::
|
|
|
|
# echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
|
|
|
|
# echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
A couple special cases
|
|
======================
|
|
|
|
While the above covers the basics of the histogram internals, there
|
|
are a couple of special cases that should be discussed, since they
|
|
tend to create even more confusion. Those are field variables on other
|
|
histograms, and aliases, both described below through example tests
|
|
using the hist_debug files.
|
|
|
|
Test of field variables on other histograms
|
|
-------------------------------------------
|
|
|
|
This example is similar to the previous examples, but in this case,
|
|
the sched_switch trigger references a hist trigger field on another
|
|
event, namely the sched_waking event. In order to accomplish this, a
|
|
field variable is created for the other event, but since an existing
|
|
histogram can't be used, as existing histograms are immutable, a new
|
|
histogram with a matching variable is created and used, and we'll see
|
|
that reflected in the hist_debug output shown below.
|
|
|
|
First, we create the wakeup_latency synthetic event. Note the
|
|
addition of the prio field::
|
|
|
|
# echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
|
|
|
|
As in previous test examples, we set up the sched_waking trigger::
|
|
|
|
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
Here we set up a hist trigger on sched_switch to send a wakeup_latency
|
|
event using an onmatch handler naming the sched_waking event. Note
|
|
that the third param being passed to the wakeup_latency() is prio,
|
|
which is a field name that needs to have a field variable created for
|
|
it. There isn't however any prio field on the sched_switch event so
|
|
it would seem that it wouldn't be possible to create a field variable
|
|
for it. The matching sched_waking event does have a prio field, so it
|
|
should be possible to make use of it for this purpose. The problem
|
|
with that is that it's not currently possible to define a new variable
|
|
on an existing histogram, so it's not possible to add a new prio field
|
|
variable to the existing sched_waking histogram. It is however
|
|
possible to create an additional new 'matching' sched_waking histogram
|
|
for the same event, meaning that it uses the same key and filters, and
|
|
define the new prio field variable on that.
|
|
|
|
Here's the sched_switch trigger::
|
|
|
|
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
|
|
|
|
And here's the output of the hist_debug information for the
|
|
sched_waking hist trigger. Note that there are two histograms
|
|
displayed in the output: the first is the normal sched_waking
|
|
histogram we've seen in the previous examples, and the second is the
|
|
special histogram we created to provide the prio field variable.
|
|
|
|
Looking at the second histogram below, we see a variable with the name
|
|
synthetic_prio. This is the field variable created for the prio field
|
|
on that sched_waking histogram::
|
|
|
|
# cat events/sched/sched_waking/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 00000000349570e4
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active]
|
|
#
|
|
|
|
hist_data: 000000006920cf38
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
ftrace_event_field name: prio
|
|
var.name: synthetic_prio
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: int
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
Looking at the sched_switch histogram below, we can see a reference to
|
|
the synthetic_prio variable on sched_waking, and looking at the
|
|
associated hist_data address we see that it is indeed associated with
|
|
the new histogram. Note also that the other references are to a
|
|
normal variable, wakeup_lat, and to a normal field variable, next_pid,
|
|
the details of which are in the field variables section::
|
|
|
|
# cat events/sched/sched_switch/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio) [active]
|
|
#
|
|
|
|
hist_data: 00000000a73b67df
|
|
|
|
n_vals: 2
|
|
n_keys: 1
|
|
n_fields: 3
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
variable reference fields:
|
|
|
|
hist_data->var_refs[0]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 00000000349570e4
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 00000000a73b67df
|
|
var_ref_idx (into hist_data->var_refs[]): 1
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[2]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: next_pid
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
var.hist_data: 00000000a73b67df
|
|
var_ref_idx (into hist_data->var_refs[]): 2
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[3]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: synthetic_prio
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 000000006920cf38
|
|
var_ref_idx (into hist_data->var_refs[]): 3
|
|
type: int
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
field variables:
|
|
|
|
hist_data->field_vars[0]:
|
|
|
|
field_vars[0].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: next_pid
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
|
|
field_vars[0].val:
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
action tracking variables (for onmax()/onchange()/onmatch()):
|
|
|
|
hist_data->actions[0].match_data.event_system: sched
|
|
hist_data->actions[0].match_data.event: sched_waking
|
|
|
|
The commands below can be used to clean things up for the next test::
|
|
|
|
# echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
|
|
|
|
# echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
# echo '!wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
|
|
|
|
Alias test
|
|
----------
|
|
|
|
This example is very similar to previous examples, but demonstrates
|
|
the alias flag.
|
|
|
|
First, we create the wakeup_latency synthetic event::
|
|
|
|
# echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
|
|
|
|
Next, we create a sched_waking trigger similar to previous examples,
|
|
but in this case we save the pid in the waking_pid variable::
|
|
|
|
# echo 'hist:keys=pid:waking_pid=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
For the sched_switch trigger, instead of using $waking_pid directly in
|
|
the wakeup_latency synthetic event invocation, we create an alias of
|
|
$waking_pid named $woken_pid, and use that in the synthetic event
|
|
invocation instead::
|
|
|
|
# echo 'hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
|
|
|
|
Looking at the sched_waking hist_debug output, in addition to the
|
|
normal fields, we can see the waking_pid variable::
|
|
|
|
# cat events/sched/sched_waking/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=pid:vals=hitcount:waking_pid=pid,ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
|
|
#
|
|
|
|
hist_data: 00000000a250528c
|
|
|
|
n_vals: 3
|
|
n_keys: 1
|
|
n_fields: 4
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
ftrace_event_field name: pid
|
|
var.name: waking_pid
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[3]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
The sched_switch hist_debug output shows that a variable named
|
|
woken_pid has been created but that it also has the
|
|
HIST_FIELD_FL_ALIAS flag set. It also has the HIST_FIELD_FL_VAR flag
|
|
set, which is why it appears in the val field section.
|
|
|
|
Despite that implementation detail, an alias variable is actually more
|
|
like a variable reference; in fact it can be thought of as a reference
|
|
to a reference. The implementation copies the var_ref->fn() from the
|
|
variable reference being referenced, in this case, the waking_pid
|
|
fn(), which is hist_field_var_ref() and makes that the fn() of the
|
|
alias. The hist_field_var_ref() fn() requires the var_ref_idx of the
|
|
variable reference it's using, so waking_pid's var_ref_idx is also
|
|
copied to the alias. The end result is that when the value of alias
|
|
is retrieved, in the end it just does the same thing the original
|
|
reference would have done and retrieves the same value from the
|
|
var_ref_vals[] array. You can verify this in the output by noting
|
|
that the var_ref_idx of the alias, in this case woken_pid, is the same
|
|
as the var_ref_idx of the reference, waking_pid, in the variable
|
|
reference fields section.
|
|
|
|
Additionally, once it gets that value, since it is also a variable, it
|
|
then saves that value into its var.idx. So the var.idx of the
|
|
woken_pid alias is 0, which it fills with the value from var_ref_idx 0
|
|
when its fn() is called to update itself. You'll also notice that
|
|
there's a woken_pid var_ref in the variable refs section. That is the
|
|
reference to the woken_pid alias variable, and you can see that it
|
|
retrieves the value from the same var.idx as the woken_pid alias, 0,
|
|
and then in turn saves that value in its own var_ref_idx slot, 3, and
|
|
the value at this position is finally what gets assigned to the
|
|
$woken_pid slot in the trace event invocation::
|
|
|
|
# cat events/sched/sched_switch/hist_debug
|
|
|
|
# event histogram
|
|
#
|
|
# trigger info: hist:keys=next_pid:vals=hitcount:woken_pid=$waking_pid,wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm) [active]
|
|
#
|
|
|
|
hist_data: 0000000055d65ed0
|
|
|
|
n_vals: 3
|
|
n_keys: 1
|
|
n_fields: 4
|
|
|
|
val fields:
|
|
|
|
hist_data->fields[0]:
|
|
flags:
|
|
VAL: HIST_FIELD_FL_HITCOUNT
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->fields[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
HIST_FIELD_FL_ALIAS
|
|
var.name: woken_pid
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->fields[2]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
key fields:
|
|
|
|
hist_data->fields[3]:
|
|
flags:
|
|
HIST_FIELD_FL_KEY
|
|
ftrace_event_field name: next_pid
|
|
type: pid_t
|
|
size: 8
|
|
is_signed: 1
|
|
|
|
variable reference fields:
|
|
|
|
hist_data->var_refs[0]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: waking_pid
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 00000000a250528c
|
|
var_ref_idx (into hist_data->var_refs[]): 0
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->var_refs[1]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: ts0
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
var.hist_data: 00000000a250528c
|
|
var_ref_idx (into hist_data->var_refs[]): 1
|
|
type: u64
|
|
size: 8
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[2]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: wakeup_lat
|
|
var.idx (into tracing_map_elt.vars[]): 1
|
|
var.hist_data: 0000000055d65ed0
|
|
var_ref_idx (into hist_data->var_refs[]): 2
|
|
type: u64
|
|
size: 0
|
|
is_signed: 0
|
|
|
|
hist_data->var_refs[3]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: woken_pid
|
|
var.idx (into tracing_map_elt.vars[]): 0
|
|
var.hist_data: 0000000055d65ed0
|
|
var_ref_idx (into hist_data->var_refs[]): 3
|
|
type: pid_t
|
|
size: 4
|
|
is_signed: 1
|
|
|
|
hist_data->var_refs[4]:
|
|
flags:
|
|
HIST_FIELD_FL_VAR_REF
|
|
name: next_comm
|
|
var.idx (into tracing_map_elt.vars[]): 2
|
|
var.hist_data: 0000000055d65ed0
|
|
var_ref_idx (into hist_data->var_refs[]): 4
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
field variables:
|
|
|
|
hist_data->field_vars[0]:
|
|
|
|
field_vars[0].var:
|
|
flags:
|
|
HIST_FIELD_FL_VAR
|
|
var.name: next_comm
|
|
var.idx (into tracing_map_elt.vars[]): 2
|
|
|
|
field_vars[0].val:
|
|
ftrace_event_field name: next_comm
|
|
type: char[16]
|
|
size: 256
|
|
is_signed: 0
|
|
|
|
action tracking variables (for onmax()/onchange()/onmatch()):
|
|
|
|
hist_data->actions[0].match_data.event_system: sched
|
|
hist_data->actions[0].match_data.event: sched_waking
|
|
|
|
The commands below can be used to clean things up for the next test::
|
|
|
|
# echo '!hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
|
|
|
|
# echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
|
|
|
|
# echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
|