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+RCU Torture Test Operation
+The CONFIG_RCU_TORTURE_TEST config option is available for all RCU
+implementations. It creates an rcutorture kernel module that can
+be loaded to run a torture test. The test periodically outputs
+status messages via printk(), which can be examined via the dmesg
+command (perhaps grepping for "torture"). The test is started
+when the module is loaded, and stops when the module is unloaded.
+It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will
+result in the tests being loaded into the base kernel. In this case,
+the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify
+whether the RCU torture tests are to be started immediately during
+boot or whether the /proc/sys/kernel/rcutorture_runnable file is used
+to enable them. This /proc file can be used to repeatedly pause and
+restart the tests, regardless of the initial state specified by the
+You will normally -not- want to start the RCU torture tests during boot
+(and thus the default is CONFIG_RCU_TORTURE_TEST_RUNNABLE=n), but doing
+this can sometimes be useful in finding boot-time bugs.
+This module has the following parameters:
+fqs_duration Duration (in microseconds) of artificially induced bursts
+ of force_quiescent_state() invocations. In RCU
+ implementations having force_quiescent_state(), these
+ bursts help force races between forcing a given grace
+ period and that grace period ending on its own.
+fqs_holdoff Holdoff time (in microseconds) between consecutive calls
+ to force_quiescent_state() within a burst.
+fqs_stutter Wait time (in seconds) between consecutive bursts
+ of calls to force_quiescent_state().
+irqreader Says to invoke RCU readers from irq level. This is currently
+ done via timers. Defaults to "1" for variants of RCU that
+ permit this. (Or, more accurately, variants of RCU that do
+ -not- permit this know to ignore this variable.)
+n_barrier_cbs If this is nonzero, RCU barrier testing will be conducted,
+ in which case n_barrier_cbs specifies the number of
+ RCU callbacks (and corresponding kthreads) to use for
+ this testing. The value cannot be negative. If you
+ specify this to be non-zero when torture_type indicates a
+ synchronous RCU implementation (one for which a member of
+ the synchronize_rcu() rather than the call_rcu() family is
+ used -- see the documentation for torture_type below), an
+ error will be reported and no testing will be carried out.
+nfakewriters This is the number of RCU fake writer threads to run. Fake
+ writer threads repeatedly use the synchronous "wait for
+ current readers" function of the interface selected by
+ torture_type, with a delay between calls to allow for various
+ different numbers of writers running in parallel.
+ nfakewriters defaults to 4, which provides enough parallelism
+ to trigger special cases caused by multiple writers, such as
+ the synchronize_srcu() early return optimization.
+nreaders This is the number of RCU reading threads supported.
+ The default is twice the number of CPUs. Why twice?
+ To properly exercise RCU implementations with preemptible
+ read-side critical sections.
+ The number of seconds between each attempt to execute a
+ randomly selected CPU-hotplug operation. Defaults to
+ zero, which disables CPU hotplugging. In HOTPLUG_CPU=n
+ kernels, rcutorture will silently refuse to do any
+ CPU-hotplug operations regardless of what value is
+ specified for onoff_interval.
+onoff_holdoff The number of seconds to wait until starting CPU-hotplug
+ operations. This would normally only be used when
+ rcutorture was built into the kernel and started
+ automatically at boot time, in which case it is useful
+ in order to avoid confusing boot-time code with CPUs
+ coming and going.
+ The number of seconds to keep the test threads affinitied
+ to a particular subset of the CPUs, defaults to 3 seconds.
+ Used in conjunction with test_no_idle_hz.
+shutdown_secs The number of seconds to run the test before terminating
+ the test and powering off the system. The default is
+ zero, which disables test termination and system shutdown.
+ This capability is useful for automated testing.
+stall_cpu The number of seconds that a CPU should be stalled while
+ within both an rcu_read_lock() and a preempt_disable().
+ This stall happens only once per rcutorture run.
+ If you need multiple stalls, use modprobe and rmmod to
+ repeatedly run rcutorture. The default for stall_cpu
+ is zero, which prevents rcutorture from stalling a CPU.
+ Note that attempts to rmmod rcutorture while the stall
+ is ongoing will hang, so be careful what value you
+ choose for this module parameter! In addition, too-large
+ values for stall_cpu might well induce failures and
+ warnings in other parts of the kernel. You have been
+ warned!
+ The number of seconds to wait after rcutorture starts
+ before stalling a CPU. Defaults to 10 seconds.
+stat_interval The number of seconds between output of torture
+ statistics (via printk()). Regardless of the interval,
+ statistics are printed when the module is unloaded.
+ Setting the interval to zero causes the statistics to
+ be printed -only- when the module is unloaded, and this
+ is the default.
+stutter The length of time to run the test before pausing for this
+ same period of time. Defaults to "stutter=5", so as
+ to run and pause for (roughly) five-second intervals.
+ Specifying "stutter=0" causes the test to run continuously
+ without pausing, which is the old default behavior.
+test_boost Whether or not to test the ability of RCU to do priority
+ boosting. Defaults to "test_boost=1", which performs
+ RCU priority-inversion testing only if the selected
+ RCU implementation supports priority boosting. Specifying
+ "test_boost=0" never performs RCU priority-inversion
+ testing. Specifying "test_boost=2" performs RCU
+ priority-inversion testing even if the selected RCU
+ implementation does not support RCU priority boosting,
+ which can be used to test rcutorture's ability to
+ carry out RCU priority-inversion testing.
+ The number of seconds in an RCU priority-inversion test
+ cycle. Defaults to "test_boost_interval=7". It is
+ usually wise for this value to be relatively prime to
+ the value selected for "stutter".
+ The number of seconds to do RCU priority-inversion testing
+ within any given "test_boost_interval". Defaults to
+ "test_boost_duration=4".
+test_no_idle_hz Whether or not to test the ability of RCU to operate in
+ a kernel that disables the scheduling-clock interrupt to
+ idle CPUs. Boolean parameter, "1" to test, "0" otherwise.
+ Defaults to omitting this test.
+torture_type The type of RCU to test, with string values as follows:
+ "rcu": rcu_read_lock(), rcu_read_unlock() and call_rcu().
+ "rcu_sync": rcu_read_lock(), rcu_read_unlock(), and
+ synchronize_rcu().
+ "rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and
+ synchronize_rcu_expedited().
+ "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
+ call_rcu_bh().
+ "rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(),
+ and synchronize_rcu_bh().
+ "rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(),
+ and synchronize_rcu_bh_expedited().
+ "srcu": srcu_read_lock(), srcu_read_unlock() and
+ call_srcu().
+ "srcu_sync": srcu_read_lock(), srcu_read_unlock() and
+ synchronize_srcu().
+ "srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
+ synchronize_srcu_expedited().
+ "srcu_raw": srcu_read_lock_raw(), srcu_read_unlock_raw(),
+ and call_srcu().
+ "srcu_raw_sync": srcu_read_lock_raw(), srcu_read_unlock_raw(),
+ and synchronize_srcu().
+ "sched": preempt_disable(), preempt_enable(), and
+ call_rcu_sched().
+ "sched_sync": preempt_disable(), preempt_enable(), and
+ synchronize_sched().
+ "sched_expedited": preempt_disable(), preempt_enable(), and
+ synchronize_sched_expedited().
+ Defaults to "rcu".
+verbose Enable debug printk()s. Default is disabled.
+The statistics output is as follows:
+ 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
+ rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767
+ rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0
+ rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0
+ rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
+ 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
+The command "dmesg | grep torture:" will extract this information on
+most systems. On more esoteric configurations, it may be necessary to
+use other commands to access the output of the printk()s used by
+the RCU torture test. The printk()s use KERN_ALERT, so they should
+be evident. ;-)
+The first and last lines show the rcutorture module parameters, and the
+last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
+automatic determination as to whether RCU operated correctly.
+The entries are as follows:
+o "rtc": The hexadecimal address of the structure currently visible
+ to readers.
+o "ver": The number of times since boot that the RCU writer task
+ has changed the structure visible to readers.
+o "tfle": If non-zero, indicates that the "torture freelist"
+ containing structures to be placed into the "rtc" area is empty.
+ This condition is important, since it can fool you into thinking
+ that RCU is working when it is not. :-/
+o "rta": Number of structures allocated from the torture freelist.
+o "rtaf": Number of allocations from the torture freelist that have
+ failed due to the list being empty. It is not unusual for this
+ to be non-zero, but it is bad for it to be a large fraction of
+ the value indicated by "rta".
+o "rtf": Number of frees into the torture freelist.
+o "rtmbe": A non-zero value indicates that rcutorture believes that
+ rcu_assign_pointer() and rcu_dereference() are not working
+ correctly. This value should be zero.
+o "rtbe": A non-zero value indicates that one of the rcu_barrier()
+ family of functions is not working correctly.
+o "rtbke": rcutorture was unable to create the real-time kthreads
+ used to force RCU priority inversion. This value should be zero.
+o "rtbre": Although rcutorture successfully created the kthreads
+ used to force RCU priority inversion, it was unable to set them
+ to the real-time priority level of 1. This value should be zero.
+o "rtbf": The number of times that RCU priority boosting failed
+ to resolve RCU priority inversion.
+o "rtb": The number of times that rcutorture attempted to force
+ an RCU priority inversion condition. If you are testing RCU
+ priority boosting via the "test_boost" module parameter, this
+ value should be non-zero.
+o "nt": The number of times rcutorture ran RCU read-side code from
+ within a timer handler. This value should be non-zero only
+ if you specified the "irqreader" module parameter.
+o "Reader Pipe": Histogram of "ages" of structures seen by readers.
+ If any entries past the first two are non-zero, RCU is broken.
+ And rcutorture prints the error flag string "!!!" to make sure
+ you notice. The age of a newly allocated structure is zero,
+ it becomes one when removed from reader visibility, and is
+ incremented once per grace period subsequently -- and is freed
+ after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
+ The output displayed above was taken from a correctly working
+ RCU. If you want to see what it looks like when broken, break
+ it yourself. ;-)
+o "Reader Batch": Another histogram of "ages" of structures seen
+ by readers, but in terms of counter flips (or batches) rather
+ than in terms of grace periods. The legal number of non-zero
+ entries is again two. The reason for this separate view is that
+ it is sometimes easier to get the third entry to show up in the
+ "Reader Batch" list than in the "Reader Pipe" list.
+o "Free-Block Circulation": Shows the number of torture structures
+ that have reached a given point in the pipeline. The first element
+ should closely correspond to the number of structures allocated,
+ the second to the number that have been removed from reader view,
+ and all but the last remaining to the corresponding number of
+ passes through a grace period. The last entry should be zero,
+ as it is only incremented if a torture structure's counter
+ somehow gets incremented farther than it should.
+Different implementations of RCU can provide implementation-specific
+additional information. For example, SRCU provides the following
+additional line:
+ srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
+This line shows the per-CPU counter state. The numbers in parentheses are
+the values of the "old" and "current" counters for the corresponding CPU.
+The "idx" value maps the "old" and "current" values to the underlying
+array, and is useful for debugging.
+The following script may be used to torture RCU:
+ #!/bin/sh
+ modprobe rcutorture
+ sleep 3600
+ rmmod rcutorture
+ dmesg | grep torture:
+The output can be manually inspected for the error flag of "!!!".
+One could of course create a more elaborate script that automatically
+checked for such errors. The "rmmod" command forces a "SUCCESS",
+"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first
+two are self-explanatory, while the last indicates that while there
+were no RCU failures, CPU-hotplug problems were detected.