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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.

1	RCU Torture Test Operation
6	The CONFIG_RCU_TORTURE_TEST config option is available for all RCU
7	implementations.  It creates an rcutorture kernel module that can
8	be loaded to run a torture test.  The test periodically outputs
9	status messages via printk(), which can be examined via the dmesg
10	command (perhaps grepping for "torture").  The test is started
11	when the module is loaded, and stops when the module is unloaded.
16	This module has the following parameters:
18	fqs_duration	Duration (in microseconds) of artificially induced bursts
19			of force_quiescent_state() invocations.  In RCU
20			implementations having force_quiescent_state(), these
21			bursts help force races between forcing a given grace
22			period and that grace period ending on its own.
24	fqs_holdoff	Holdoff time (in microseconds) between consecutive calls
25			to force_quiescent_state() within a burst.
27	fqs_stutter	Wait time (in seconds) between consecutive bursts
28			of calls to force_quiescent_state().
30	gp_normal	Make the fake writers use normal synchronous grace-period
31			primitives.
33	gp_exp		Make the fake writers use expedited synchronous grace-period
34			primitives.  If both gp_normal and gp_exp are set, or
35			if neither gp_normal nor gp_exp are set, then randomly
36			choose the primitive so that about 50% are normal and
37			50% expedited.  By default, neither are set, which
38			gives best overall test coverage.
40	irqreader	Says to invoke RCU readers from irq level.  This is currently
41			done via timers.  Defaults to "1" for variants of RCU that
42			permit this.  (Or, more accurately, variants of RCU that do
43			-not- permit this know to ignore this variable.)
45	n_barrier_cbs	If this is nonzero, RCU barrier testing will be conducted,
46			in which case n_barrier_cbs specifies the number of
47			RCU callbacks (and corresponding kthreads) to use for
48			this testing.  The value cannot be negative.  If you
49			specify this to be non-zero when torture_type indicates a
50			synchronous RCU implementation (one for which a member of
51			the synchronize_rcu() rather than the call_rcu() family is
52			used -- see the documentation for torture_type below), an
53			error will be reported and no testing will be carried out.
55	nfakewriters	This is the number of RCU fake writer threads to run.  Fake
56			writer threads repeatedly use the synchronous "wait for
57			current readers" function of the interface selected by
58			torture_type, with a delay between calls to allow for various
59			different numbers of writers running in parallel.
60			nfakewriters defaults to 4, which provides enough parallelism
61			to trigger special cases caused by multiple writers, such as
62			the synchronize_srcu() early return optimization.
64	nreaders	This is the number of RCU reading threads supported.
65			The default is twice the number of CPUs.  Why twice?
66			To properly exercise RCU implementations with preemptible
67			read-side critical sections.
69	onoff_interval
70			The number of seconds between each attempt to execute a
71			randomly selected CPU-hotplug operation.  Defaults to
72			zero, which disables CPU hotplugging.  In HOTPLUG_CPU=n
73			kernels, rcutorture will silently refuse to do any
74			CPU-hotplug operations regardless of what value is
75			specified for onoff_interval.
77	onoff_holdoff	The number of seconds to wait until starting CPU-hotplug
78			operations.  This would normally only be used when
79			rcutorture was built into the kernel and started
80			automatically at boot time, in which case it is useful
81			in order to avoid confusing boot-time code with CPUs
82			coming and going.
84	shuffle_interval
85			The number of seconds to keep the test threads affinitied
86			to a particular subset of the CPUs, defaults to 3 seconds.
87			Used in conjunction with test_no_idle_hz.
89	shutdown_secs	The number of seconds to run the test before terminating
90			the test and powering off the system.  The default is
91			zero, which disables test termination and system shutdown.
92			This capability is useful for automated testing.
94	stall_cpu	The number of seconds that a CPU should be stalled while
95			within both an rcu_read_lock() and a preempt_disable().
96			This stall happens only once per rcutorture run.
97			If you need multiple stalls, use modprobe and rmmod to
98			repeatedly run rcutorture.  The default for stall_cpu
99			is zero, which prevents rcutorture from stalling a CPU.
101			Note that attempts to rmmod rcutorture while the stall
102			is ongoing will hang, so be careful what value you
103			choose for this module parameter!  In addition, too-large
104			values for stall_cpu might well induce failures and
105			warnings in other parts of the kernel.  You have been
106			warned!
108	stall_cpu_holdoff
109			The number of seconds to wait after rcutorture starts
110			before stalling a CPU.  Defaults to 10 seconds.
112	stat_interval	The number of seconds between output of torture
113			statistics (via printk()).  Regardless of the interval,
114			statistics are printed when the module is unloaded.
115			Setting the interval to zero causes the statistics to
116			be printed -only- when the module is unloaded, and this
117			is the default.
119	stutter		The length of time to run the test before pausing for this
120			same period of time.  Defaults to "stutter=5", so as
121			to run and pause for (roughly) five-second intervals.
122			Specifying "stutter=0" causes the test to run continuously
123			without pausing, which is the old default behavior.
125	test_boost	Whether or not to test the ability of RCU to do priority
126			boosting.  Defaults to "test_boost=1", which performs
127			RCU priority-inversion testing only if the selected
128			RCU implementation supports priority boosting.  Specifying
129			"test_boost=0" never performs RCU priority-inversion
130			testing.  Specifying "test_boost=2" performs RCU
131			priority-inversion testing even if the selected RCU
132			implementation does not support RCU priority boosting,
133			which can be used to test rcutorture's ability to
134			carry out RCU priority-inversion testing.
136	test_boost_interval
137			The number of seconds in an RCU priority-inversion test
138			cycle.	Defaults to "test_boost_interval=7".  It is
139			usually wise for this value to be relatively prime to
140			the value selected for "stutter".
142	test_boost_duration
143			The number of seconds to do RCU priority-inversion testing
144			within any given "test_boost_interval".  Defaults to
145			"test_boost_duration=4".
147	test_no_idle_hz	Whether or not to test the ability of RCU to operate in
148			a kernel that disables the scheduling-clock interrupt to
149			idle CPUs.  Boolean parameter, "1" to test, "0" otherwise.
150			Defaults to omitting this test.
152	torture_type	The type of RCU to test, with string values as follows:
154			"rcu":  rcu_read_lock(), rcu_read_unlock() and call_rcu(),
155				along with expedited, synchronous, and polling
156				variants.
158			"rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
159				call_rcu_bh(), along with expedited and synchronous
160				variants.
162			"rcu_busted": This tests an intentionally incorrect version
163				of RCU in order to help test rcutorture itself.
165			"srcu": srcu_read_lock(), srcu_read_unlock() and
166				call_srcu(), along with expedited and
167				synchronous variants.
169			"sched": preempt_disable(), preempt_enable(), and
170				call_rcu_sched(), along with expedited,
171				synchronous, and polling variants.
173			"tasks": voluntary context switch and call_rcu_tasks(),
174				along with expedited and synchronous variants.
176			Defaults to "rcu".
178	verbose		Enable debug printk()s.  Default is disabled.
183	The statistics output is as follows:
185		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
186		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
187		rcu-torture: Reader Pipe:  727860534 34213 0 0 0 0 0 0 0 0 0
188		rcu-torture: Reader Batch:  727877838 17003 0 0 0 0 0 0 0 0 0
189		rcu-torture: Free-Block Circulation:  155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
190		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
192	The command "dmesg | grep torture:" will extract this information on
193	most systems.  On more esoteric configurations, it may be necessary to
194	use other commands to access the output of the printk()s used by
195	the RCU torture test.  The printk()s use KERN_ALERT, so they should
196	be evident.  ;-)
198	The first and last lines show the rcutorture module parameters, and the
199	last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
200	automatic determination as to whether RCU operated correctly.
202	The entries are as follows:
204	o	"rtc": The hexadecimal address of the structure currently visible
205		to readers.
207	o	"ver": The number of times since boot that the RCU writer task
208		has changed the structure visible to readers.
210	o	"tfle": If non-zero, indicates that the "torture freelist"
211		containing structures to be placed into the "rtc" area is empty.
212		This condition is important, since it can fool you into thinking
213		that RCU is working when it is not.  :-/
215	o	"rta": Number of structures allocated from the torture freelist.
217	o	"rtaf": Number of allocations from the torture freelist that have
218		failed due to the list being empty.  It is not unusual for this
219		to be non-zero, but it is bad for it to be a large fraction of
220		the value indicated by "rta".
222	o	"rtf": Number of frees into the torture freelist.
224	o	"rtmbe": A non-zero value indicates that rcutorture believes that
225		rcu_assign_pointer() and rcu_dereference() are not working
226		correctly.  This value should be zero.
228	o	"rtbe": A non-zero value indicates that one of the rcu_barrier()
229		family of functions is not working correctly.
231	o	"rtbke": rcutorture was unable to create the real-time kthreads
232		used to force RCU priority inversion.  This value should be zero.
234	o	"rtbre": Although rcutorture successfully created the kthreads
235		used to force RCU priority inversion, it was unable to set them
236		to the real-time priority level of 1.  This value should be zero.
238	o	"rtbf": The number of times that RCU priority boosting failed
239		to resolve RCU priority inversion.
241	o	"rtb": The number of times that rcutorture attempted to force
242		an RCU priority inversion condition.  If you are testing RCU
243		priority boosting via the "test_boost" module parameter, this
244		value should be non-zero.
246	o	"nt": The number of times rcutorture ran RCU read-side code from
247		within a timer handler.  This value should be non-zero only
248		if you specified the "irqreader" module parameter.
250	o	"Reader Pipe": Histogram of "ages" of structures seen by readers.
251		If any entries past the first two are non-zero, RCU is broken.
252		And rcutorture prints the error flag string "!!!" to make sure
253		you notice.  The age of a newly allocated structure is zero,
254		it becomes one when removed from reader visibility, and is
255		incremented once per grace period subsequently -- and is freed
256		after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
258		The output displayed above was taken from a correctly working
259		RCU.  If you want to see what it looks like when broken, break
260		it yourself.  ;-)
262	o	"Reader Batch": Another histogram of "ages" of structures seen
263		by readers, but in terms of counter flips (or batches) rather
264		than in terms of grace periods.  The legal number of non-zero
265		entries is again two.  The reason for this separate view is that
266		it is sometimes easier to get the third entry to show up in the
267		"Reader Batch" list than in the "Reader Pipe" list.
269	o	"Free-Block Circulation": Shows the number of torture structures
270		that have reached a given point in the pipeline.  The first element
271		should closely correspond to the number of structures allocated,
272		the second to the number that have been removed from reader view,
273		and all but the last remaining to the corresponding number of
274		passes through a grace period.  The last entry should be zero,
275		as it is only incremented if a torture structure's counter
276		somehow gets incremented farther than it should.
278	Different implementations of RCU can provide implementation-specific
279	additional information.  For example, SRCU provides the following
280	additional line:
282		srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
284	This line shows the per-CPU counter state.  The numbers in parentheses are
285	the values of the "old" and "current" counters for the corresponding CPU.
286	The "idx" value maps the "old" and "current" values to the underlying
287	array, and is useful for debugging.
292	The following script may be used to torture RCU:
294		#!/bin/sh
296		modprobe rcutorture
297		sleep 3600
298		rmmod rcutorture
299		dmesg | grep torture:
301	The output can be manually inspected for the error flag of "!!!".
302	One could of course create a more elaborate script that automatically
303	checked for such errors.  The "rmmod" command forces a "SUCCESS",
304	"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed.  The first
305	two are self-explanatory, while the last indicates that while there
306	were no RCU failures, CPU-hotplug problems were detected.
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