mirror of https://gitee.com/openkylin/linux.git
138 lines
5.3 KiB
Plaintext
138 lines
5.3 KiB
Plaintext
PMU Event Based Branches
|
|
========================
|
|
|
|
Event Based Branches (EBBs) are a feature which allows the hardware to
|
|
branch directly to a specified user space address when certain events occur.
|
|
|
|
The full specification is available in Power ISA v2.07:
|
|
|
|
https://www.power.org/documentation/power-isa-version-2-07/
|
|
|
|
One type of event for which EBBs can be configured is PMU exceptions. This
|
|
document describes the API for configuring the Power PMU to generate EBBs,
|
|
using the Linux perf_events API.
|
|
|
|
|
|
Terminology
|
|
-----------
|
|
|
|
Throughout this document we will refer to an "EBB event" or "EBB events". This
|
|
just refers to a struct perf_event which has set the "EBB" flag in its
|
|
attr.config. All events which can be configured on the hardware PMU are
|
|
possible "EBB events".
|
|
|
|
|
|
Background
|
|
----------
|
|
|
|
When a PMU EBB occurs it is delivered to the currently running process. As such
|
|
EBBs can only sensibly be used by programs for self-monitoring.
|
|
|
|
It is a feature of the perf_events API that events can be created on other
|
|
processes, subject to standard permission checks. This is also true of EBB
|
|
events, however unless the target process enables EBBs (via mtspr(BESCR)) no
|
|
EBBs will ever be delivered.
|
|
|
|
This makes it possible for a process to enable EBBs for itself, but not
|
|
actually configure any events. At a later time another process can come along
|
|
and attach an EBB event to the process, which will then cause EBBs to be
|
|
delivered to the first process. It's not clear if this is actually useful.
|
|
|
|
|
|
When the PMU is configured for EBBs, all PMU interrupts are delivered to the
|
|
user process. This means once an EBB event is scheduled on the PMU, no non-EBB
|
|
events can be configured. This means that EBB events can not be run
|
|
concurrently with regular 'perf' commands, or any other perf events.
|
|
|
|
It is however safe to run 'perf' commands on a process which is using EBBs. The
|
|
kernel will in general schedule the EBB event, and perf will be notified that
|
|
its events could not run.
|
|
|
|
The exclusion between EBB events and regular events is implemented using the
|
|
existing "pinned" and "exclusive" attributes of perf_events. This means EBB
|
|
events will be given priority over other events, unless they are also pinned.
|
|
If an EBB event and a regular event are both pinned, then whichever is enabled
|
|
first will be scheduled and the other will be put in error state. See the
|
|
section below titled "Enabling an EBB event" for more information.
|
|
|
|
|
|
Creating an EBB event
|
|
---------------------
|
|
|
|
To request that an event is counted using EBB, the event code should have bit
|
|
63 set.
|
|
|
|
EBB events must be created with a particular, and restrictive, set of
|
|
attributes - this is so that they interoperate correctly with the rest of the
|
|
perf_events subsystem.
|
|
|
|
An EBB event must be created with the "pinned" and "exclusive" attributes set.
|
|
Note that if you are creating a group of EBB events, only the leader can have
|
|
these attributes set.
|
|
|
|
An EBB event must NOT set any of the "inherit", "sample_period", "freq" or
|
|
"enable_on_exec" attributes.
|
|
|
|
An EBB event must be attached to a task. This is specified to perf_event_open()
|
|
by passing a pid value, typically 0 indicating the current task.
|
|
|
|
All events in a group must agree on whether they want EBB. That is all events
|
|
must request EBB, or none may request EBB.
|
|
|
|
EBB events must specify the PMC they are to be counted on. This ensures
|
|
userspace is able to reliably determine which PMC the event is scheduled on.
|
|
|
|
|
|
Enabling an EBB event
|
|
---------------------
|
|
|
|
Once an EBB event has been successfully opened, it must be enabled with the
|
|
perf_events API. This can be achieved either via the ioctl() interface, or the
|
|
prctl() interface.
|
|
|
|
However, due to the design of the perf_events API, enabling an event does not
|
|
guarantee that it has been scheduled on the PMU. To ensure that the EBB event
|
|
has been scheduled on the PMU, you must perform a read() on the event. If the
|
|
read() returns EOF, then the event has not been scheduled and EBBs are not
|
|
enabled.
|
|
|
|
This behaviour occurs because the EBB event is pinned and exclusive. When the
|
|
EBB event is enabled it will force all other non-pinned events off the PMU. In
|
|
this case the enable will be successful. However if there is already an event
|
|
pinned on the PMU then the enable will not be successful.
|
|
|
|
|
|
Reading an EBB event
|
|
--------------------
|
|
|
|
It is possible to read() from an EBB event. However the results are
|
|
meaningless. Because interrupts are being delivered to the user process the
|
|
kernel is not able to count the event, and so will return a junk value.
|
|
|
|
|
|
Closing an EBB event
|
|
--------------------
|
|
|
|
When an EBB event is finished with, you can close it using close() as for any
|
|
regular event. If this is the last EBB event the PMU will be deconfigured and
|
|
no further PMU EBBs will be delivered.
|
|
|
|
|
|
EBB Handler
|
|
-----------
|
|
|
|
The EBB handler is just regular userspace code, however it must be written in
|
|
the style of an interrupt handler. When the handler is entered all registers
|
|
are live (possibly) and so must be saved somehow before the handler can invoke
|
|
other code.
|
|
|
|
It's up to the program how to handle this. For C programs a relatively simple
|
|
option is to create an interrupt frame on the stack and save registers there.
|
|
|
|
Fork
|
|
----
|
|
|
|
EBB events are not inherited across fork. If the child process wishes to use
|
|
EBBs it should open a new event for itself. Similarly the EBB state in
|
|
BESCR/EBBHR/EBBRR is cleared across fork().
|