163 lines
6.7 KiB
Plaintext
163 lines
6.7 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 "torture"). 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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nfakewriters This is the number of RCU fake writer threads to run. Fake
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writer threads repeatedly use the synchronous "wait for
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current readers" function of the interface selected by
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torture_type, with a delay between calls to allow for various
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different numbers of writers running in parallel.
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nfakewriters defaults to 4, which provides enough parallelism
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to trigger special cases caused by multiple writers, such as
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the synchronize_srcu() early return optimization.
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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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shuffle_interval
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The number of seconds to keep the test threads affinitied
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to a particular subset of the CPUs, defaults to 5 seconds.
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Used in conjunction with test_no_idle_hz.
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test_no_idle_hz Whether or not to test the ability of RCU to operate in
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a kernel that disables the scheduling-clock interrupt to
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idle CPUs. Boolean parameter, "1" to test, "0" otherwise.
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Defaults to omitting this test.
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torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API,
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"rcu_sync" for rcu_read_lock() with synchronous reclamation,
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"rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for
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rcu_read_lock_bh() with synchronous reclamation, "srcu" for
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the "srcu_read_lock()" API, and "sched" for the use of
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preempt_disable() together with synchronize_sched().
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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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rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
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rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
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rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0
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rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0
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rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
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rcu-torture: --- End of test
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The command "dmesg | grep torture:" 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 "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 that
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it is sometimes 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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Different implementations of RCU can provide implementation-specific
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additional information. For example, SRCU provides the following:
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srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0
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srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0
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srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0
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srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0
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srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
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The first four lines are similar to those for RCU. The last line shows
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the per-CPU counter state. The numbers in parentheses are the values
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of the "old" and "current" counters for the corresponding CPU. The
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"idx" value maps the "old" and "current" values to the underlying array,
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and is useful for debugging.
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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 torture:
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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. The "rmmod" command forces a "SUCCESS" or
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"FAILURE" indication to be printk()ed.
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