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123 lines
4.7 KiB
Plaintext
123 lines
4.7 KiB
Plaintext
RCU Torture Test Operation
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CONFIG_RCU_TORTURE_TEST
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The CONFIG_RCU_TORTURE_TEST config option is available for all RCU
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implementations. It creates an rcutorture kernel module that can
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be loaded to run a torture test. The test periodically outputs
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status messages via printk(), which can be examined via the dmesg
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command (perhaps grepping for "rcutorture"). The test is started
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when the module is loaded, and stops when the module is unloaded.
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However, actually setting this config option to "y" results in the system
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running the test immediately upon boot, and ending only when the system
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is taken down. Normally, one will instead want to build the system
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with CONFIG_RCU_TORTURE_TEST=m and to use modprobe and rmmod to control
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the test, perhaps using a script similar to the one shown at the end of
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this document. Note that you will need CONFIG_MODULE_UNLOAD in order
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to be able to end the test.
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MODULE PARAMETERS
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This module has the following parameters:
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nreaders This is the number of RCU reading threads supported.
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The default is twice the number of CPUs. Why twice?
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To properly exercise RCU implementations with preemptible
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read-side critical sections.
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stat_interval The number of seconds between output of torture
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statistics (via printk()). Regardless of the interval,
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statistics are printed when the module is unloaded.
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Setting the interval to zero causes the statistics to
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be printed -only- when the module is unloaded, and this
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is the default.
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verbose Enable debug printk()s. Default is disabled.
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OUTPUT
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The statistics output is as follows:
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rcutorture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
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rcutorture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
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rcutorture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0
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rcutorture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0
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rcutorture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
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rcutorture: --- End of test
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The command "dmesg | grep rcutorture:" will extract this information on
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most systems. On more esoteric configurations, it may be necessary to
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use other commands to access the output of the printk()s used by
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the RCU torture test. The printk()s use KERN_ALERT, so they should
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be evident. ;-)
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The entries are as follows:
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o "ggp": The number of counter flips (or batches) since boot.
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o "rtc": The hexadecimal address of the structure currently visible
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to readers.
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o "ver": The number of times since boot that the rcutw writer task
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has changed the structure visible to readers.
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o "tfle": If non-zero, indicates that the "torture freelist"
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containing structure to be placed into the "rtc" area is empty.
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This condition is important, since it can fool you into thinking
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that RCU is working when it is not. :-/
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o "rta": Number of structures allocated from the torture freelist.
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o "rtaf": Number of allocations from the torture freelist that have
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failed due to the list being empty.
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o "rtf": Number of frees into the torture freelist.
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o "Reader Pipe": Histogram of "ages" of structures seen by readers.
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If any entries past the first two are non-zero, RCU is broken.
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And rcutorture prints the error flag string "!!!" to make sure
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you notice. The age of a newly allocated structure is zero,
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it becomes one when removed from reader visibility, and is
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incremented once per grace period subsequently -- and is freed
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after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
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The output displayed above was taken from a correctly working
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RCU. If you want to see what it looks like when broken, break
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it yourself. ;-)
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o "Reader Batch": Another histogram of "ages" of structures seen
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by readers, but in terms of counter flips (or batches) rather
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than in terms of grace periods. The legal number of non-zero
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entries is again two. The reason for this separate view is
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that it is easier to get the third entry to show up in the
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"Reader Batch" list than in the "Reader Pipe" list.
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o "Free-Block Circulation": Shows the number of torture structures
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that have reached a given point in the pipeline. The first element
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should closely correspond to the number of structures allocated,
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the second to the number that have been removed from reader view,
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and all but the last remaining to the corresponding number of
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passes through a grace period. The last entry should be zero,
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as it is only incremented if a torture structure's counter
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somehow gets incremented farther than it should.
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USAGE
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The following script may be used to torture RCU:
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#!/bin/sh
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modprobe rcutorture
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sleep 100
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rmmod rcutorture
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dmesg | grep rcutorture:
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The output can be manually inspected for the error flag of "!!!".
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One could of course create a more elaborate script that automatically
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checked for such errors.
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