269 lines
11 KiB
Plaintext
269 lines
11 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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CONFIG_RCU_TORTURE_TEST_RUNNABLE
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It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will
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result in the tests being loaded into the base kernel. In this case,
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the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify
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whether the RCU torture tests are to be started immediately during
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boot or whether the /proc/sys/kernel/rcutorture_runnable file is used
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to enable them. This /proc file can be used to repeatedly pause and
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restart the tests, regardless of the initial state specified by the
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CONFIG_RCU_TORTURE_TEST_RUNNABLE config option.
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You will normally -not- want to start the RCU torture tests during boot
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(and thus the default is CONFIG_RCU_TORTURE_TEST_RUNNABLE=n), but doing
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this can sometimes be useful in finding boot-time bugs.
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MODULE PARAMETERS
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This module has the following parameters:
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fqs_duration Duration (in microseconds) of artificially induced bursts
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of force_quiescent_state() invocations. In RCU
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implementations having force_quiescent_state(), these
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bursts help force races between forcing a given grace
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period and that grace period ending on its own.
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fqs_holdoff Holdoff time (in microseconds) between consecutive calls
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to force_quiescent_state() within a burst.
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fqs_stutter Wait time (in seconds) between consecutive bursts
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of calls to force_quiescent_state().
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irqreader Says to invoke RCU readers from irq level. This is currently
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done via timers. Defaults to "1" for variants of RCU that
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permit this. (Or, more accurately, variants of RCU that do
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-not- permit this know to ignore this variable.)
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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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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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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 3 seconds.
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Used in conjunction with test_no_idle_hz.
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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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stutter The length of time to run the test before pausing for this
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same period of time. Defaults to "stutter=5", so as
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to run and pause for (roughly) five-second intervals.
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Specifying "stutter=0" causes the test to run continuously
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without pausing, which is the old default behavior.
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test_boost Whether or not to test the ability of RCU to do priority
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boosting. Defaults to "test_boost=1", which performs
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RCU priority-inversion testing only if the selected
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RCU implementation supports priority boosting. Specifying
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"test_boost=0" never performs RCU priority-inversion
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testing. Specifying "test_boost=2" performs RCU
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priority-inversion testing even if the selected RCU
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implementation does not support RCU priority boosting,
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which can be used to test rcutorture's ability to
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carry out RCU priority-inversion testing.
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test_boost_interval
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The number of seconds in an RCU priority-inversion test
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cycle. Defaults to "test_boost_interval=7". It is
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usually wise for this value to be relatively prime to
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the value selected for "stutter".
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test_boost_duration
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The number of seconds to do RCU priority-inversion testing
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within any given "test_boost_interval". Defaults to
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"test_boost_duration=4".
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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, with string values as follows:
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"rcu": rcu_read_lock(), rcu_read_unlock() and call_rcu().
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"rcu_sync": rcu_read_lock(), rcu_read_unlock(), and
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synchronize_rcu().
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"rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and
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synchronize_rcu_expedited().
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"rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
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call_rcu_bh().
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"rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(),
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and synchronize_rcu_bh().
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"rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(),
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and synchronize_rcu_bh_expedited().
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"srcu": srcu_read_lock(), srcu_read_unlock() and
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synchronize_srcu().
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"srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
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synchronize_srcu_expedited().
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"sched": preempt_disable(), preempt_enable(), and
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call_rcu_sched().
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"sched_sync": preempt_disable(), preempt_enable(), and
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synchronize_sched().
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"sched_expedited": preempt_disable(), preempt_enable(), and
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synchronize_sched_expedited().
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Defaults to "rcu".
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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 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4
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rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767
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rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0
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rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0
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rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
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rcu-torture:--- End of test: SUCCESS: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4
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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 first and last lines show the rcutorture module parameters, and the
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last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
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automatic determination as to whether RCU operated correctly.
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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 RCU 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 structures 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. It is not unusual for this
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to be non-zero, but it is bad for it to be a large fraction of
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the value indicated by "rta".
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o "rtf": Number of frees into the torture freelist.
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o "rtmbe": A non-zero value indicates that rcutorture believes that
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rcu_assign_pointer() and rcu_dereference() are not working
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correctly. This value should be zero.
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o "rtbke": rcutorture was unable to create the real-time kthreads
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used to force RCU priority inversion. This value should be zero.
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o "rtbre": Although rcutorture successfully created the kthreads
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used to force RCU priority inversion, it was unable to set them
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to the real-time priority level of 1. This value should be zero.
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o "rtbf": The number of times that RCU priority boosting failed
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to resolve RCU priority inversion.
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o "rtb": The number of times that rcutorture attempted to force
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an RCU priority inversion condition. If you are testing RCU
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priority boosting via the "test_boost" module parameter, this
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value should be non-zero.
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o "nt": The number of times rcutorture ran RCU read-side code from
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within a timer handler. This value should be non-zero only
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if you specified the "irqreader" module parameter.
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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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additional line:
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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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This line shows the per-CPU counter state. The numbers in parentheses are
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the values of the "old" and "current" counters for the corresponding CPU.
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The "idx" value maps the "old" and "current" values to the underlying
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array, 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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