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Based on kernel version 4.8. Page generated on 2016-10-06 23:18 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.
15	It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will
16	result in the tests being loaded into the base kernel.  In this case,
17	the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify
18	whether the RCU torture tests are to be started immediately during
19	boot or whether the /proc/sys/kernel/rcutorture_runnable file is used
20	to enable them.  This /proc file can be used to repeatedly pause and
21	restart the tests, regardless of the initial state specified by the
24	You will normally -not- want to start the RCU torture tests during boot
25	(and thus the default is CONFIG_RCU_TORTURE_TEST_RUNNABLE=n), but doing
26	this can sometimes be useful in finding boot-time bugs.
31	This module has the following parameters:
33	fqs_duration	Duration (in microseconds) of artificially induced bursts
34			of force_quiescent_state() invocations.  In RCU
35			implementations having force_quiescent_state(), these
36			bursts help force races between forcing a given grace
37			period and that grace period ending on its own.
39	fqs_holdoff	Holdoff time (in microseconds) between consecutive calls
40			to force_quiescent_state() within a burst.
42	fqs_stutter	Wait time (in seconds) between consecutive bursts
43			of calls to force_quiescent_state().
45	gp_normal	Make the fake writers use normal synchronous grace-period
46			primitives.
48	gp_exp		Make the fake writers use expedited synchronous grace-period
49			primitives.  If both gp_normal and gp_exp are set, or
50			if neither gp_normal nor gp_exp are set, then randomly
51			choose the primitive so that about 50% are normal and
52			50% expedited.  By default, neither are set, which
53			gives best overall test coverage.
55	irqreader	Says to invoke RCU readers from irq level.  This is currently
56			done via timers.  Defaults to "1" for variants of RCU that
57			permit this.  (Or, more accurately, variants of RCU that do
58			-not- permit this know to ignore this variable.)
60	n_barrier_cbs	If this is nonzero, RCU barrier testing will be conducted,
61			in which case n_barrier_cbs specifies the number of
62			RCU callbacks (and corresponding kthreads) to use for
63			this testing.  The value cannot be negative.  If you
64			specify this to be non-zero when torture_type indicates a
65			synchronous RCU implementation (one for which a member of
66			the synchronize_rcu() rather than the call_rcu() family is
67			used -- see the documentation for torture_type below), an
68			error will be reported and no testing will be carried out.
70	nfakewriters	This is the number of RCU fake writer threads to run.  Fake
71			writer threads repeatedly use the synchronous "wait for
72			current readers" function of the interface selected by
73			torture_type, with a delay between calls to allow for various
74			different numbers of writers running in parallel.
75			nfakewriters defaults to 4, which provides enough parallelism
76			to trigger special cases caused by multiple writers, such as
77			the synchronize_srcu() early return optimization.
79	nreaders	This is the number of RCU reading threads supported.
80			The default is twice the number of CPUs.  Why twice?
81			To properly exercise RCU implementations with preemptible
82			read-side critical sections.
84	onoff_interval
85			The number of seconds between each attempt to execute a
86			randomly selected CPU-hotplug operation.  Defaults to
87			zero, which disables CPU hotplugging.  In HOTPLUG_CPU=n
88			kernels, rcutorture will silently refuse to do any
89			CPU-hotplug operations regardless of what value is
90			specified for onoff_interval.
92	onoff_holdoff	The number of seconds to wait until starting CPU-hotplug
93			operations.  This would normally only be used when
94			rcutorture was built into the kernel and started
95			automatically at boot time, in which case it is useful
96			in order to avoid confusing boot-time code with CPUs
97			coming and going.
99	shuffle_interval
100			The number of seconds to keep the test threads affinitied
101			to a particular subset of the CPUs, defaults to 3 seconds.
102			Used in conjunction with test_no_idle_hz.
104	shutdown_secs	The number of seconds to run the test before terminating
105			the test and powering off the system.  The default is
106			zero, which disables test termination and system shutdown.
107			This capability is useful for automated testing.
109	stall_cpu	The number of seconds that a CPU should be stalled while
110			within both an rcu_read_lock() and a preempt_disable().
111			This stall happens only once per rcutorture run.
112			If you need multiple stalls, use modprobe and rmmod to
113			repeatedly run rcutorture.  The default for stall_cpu
114			is zero, which prevents rcutorture from stalling a CPU.
116			Note that attempts to rmmod rcutorture while the stall
117			is ongoing will hang, so be careful what value you
118			choose for this module parameter!  In addition, too-large
119			values for stall_cpu might well induce failures and
120			warnings in other parts of the kernel.  You have been
121			warned!
123	stall_cpu_holdoff
124			The number of seconds to wait after rcutorture starts
125			before stalling a CPU.  Defaults to 10 seconds.
127	stat_interval	The number of seconds between output of torture
128			statistics (via printk()).  Regardless of the interval,
129			statistics are printed when the module is unloaded.
130			Setting the interval to zero causes the statistics to
131			be printed -only- when the module is unloaded, and this
132			is the default.
134	stutter		The length of time to run the test before pausing for this
135			same period of time.  Defaults to "stutter=5", so as
136			to run and pause for (roughly) five-second intervals.
137			Specifying "stutter=0" causes the test to run continuously
138			without pausing, which is the old default behavior.
140	test_boost	Whether or not to test the ability of RCU to do priority
141			boosting.  Defaults to "test_boost=1", which performs
142			RCU priority-inversion testing only if the selected
143			RCU implementation supports priority boosting.  Specifying
144			"test_boost=0" never performs RCU priority-inversion
145			testing.  Specifying "test_boost=2" performs RCU
146			priority-inversion testing even if the selected RCU
147			implementation does not support RCU priority boosting,
148			which can be used to test rcutorture's ability to
149			carry out RCU priority-inversion testing.
151	test_boost_interval
152			The number of seconds in an RCU priority-inversion test
153			cycle.	Defaults to "test_boost_interval=7".  It is
154			usually wise for this value to be relatively prime to
155			the value selected for "stutter".
157	test_boost_duration
158			The number of seconds to do RCU priority-inversion testing
159			within any given "test_boost_interval".  Defaults to
160			"test_boost_duration=4".
162	test_no_idle_hz	Whether or not to test the ability of RCU to operate in
163			a kernel that disables the scheduling-clock interrupt to
164			idle CPUs.  Boolean parameter, "1" to test, "0" otherwise.
165			Defaults to omitting this test.
167	torture_type	The type of RCU to test, with string values as follows:
169			"rcu":  rcu_read_lock(), rcu_read_unlock() and call_rcu(),
170				along with expedited, synchronous, and polling
171				variants.
173			"rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
174				call_rcu_bh(), along with expedited and synchronous
175				variants.
177			"rcu_busted": This tests an intentionally incorrect version
178				of RCU in order to help test rcutorture itself.
180			"srcu": srcu_read_lock(), srcu_read_unlock() and
181				call_srcu(), along with expedited and
182				synchronous variants.
184			"sched": preempt_disable(), preempt_enable(), and
185				call_rcu_sched(), along with expedited,
186				synchronous, and polling variants.
188			"tasks": voluntary context switch and call_rcu_tasks(),
189				along with expedited and synchronous variants.
191			Defaults to "rcu".
193	verbose		Enable debug printk()s.  Default is disabled.
198	The statistics output is as follows:
200		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
201		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
202		rcu-torture: Reader Pipe:  727860534 34213 0 0 0 0 0 0 0 0 0
203		rcu-torture: Reader Batch:  727877838 17003 0 0 0 0 0 0 0 0 0
204		rcu-torture: Free-Block Circulation:  155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
205		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
207	The command "dmesg | grep torture:" will extract this information on
208	most systems.  On more esoteric configurations, it may be necessary to
209	use other commands to access the output of the printk()s used by
210	the RCU torture test.  The printk()s use KERN_ALERT, so they should
211	be evident.  ;-)
213	The first and last lines show the rcutorture module parameters, and the
214	last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
215	automatic determination as to whether RCU operated correctly.
217	The entries are as follows:
219	o	"rtc": The hexadecimal address of the structure currently visible
220		to readers.
222	o	"ver": The number of times since boot that the RCU writer task
223		has changed the structure visible to readers.
225	o	"tfle": If non-zero, indicates that the "torture freelist"
226		containing structures to be placed into the "rtc" area is empty.
227		This condition is important, since it can fool you into thinking
228		that RCU is working when it is not.  :-/
230	o	"rta": Number of structures allocated from the torture freelist.
232	o	"rtaf": Number of allocations from the torture freelist that have
233		failed due to the list being empty.  It is not unusual for this
234		to be non-zero, but it is bad for it to be a large fraction of
235		the value indicated by "rta".
237	o	"rtf": Number of frees into the torture freelist.
239	o	"rtmbe": A non-zero value indicates that rcutorture believes that
240		rcu_assign_pointer() and rcu_dereference() are not working
241		correctly.  This value should be zero.
243	o	"rtbe": A non-zero value indicates that one of the rcu_barrier()
244		family of functions is not working correctly.
246	o	"rtbke": rcutorture was unable to create the real-time kthreads
247		used to force RCU priority inversion.  This value should be zero.
249	o	"rtbre": Although rcutorture successfully created the kthreads
250		used to force RCU priority inversion, it was unable to set them
251		to the real-time priority level of 1.  This value should be zero.
253	o	"rtbf": The number of times that RCU priority boosting failed
254		to resolve RCU priority inversion.
256	o	"rtb": The number of times that rcutorture attempted to force
257		an RCU priority inversion condition.  If you are testing RCU
258		priority boosting via the "test_boost" module parameter, this
259		value should be non-zero.
261	o	"nt": The number of times rcutorture ran RCU read-side code from
262		within a timer handler.  This value should be non-zero only
263		if you specified the "irqreader" module parameter.
265	o	"Reader Pipe": Histogram of "ages" of structures seen by readers.
266		If any entries past the first two are non-zero, RCU is broken.
267		And rcutorture prints the error flag string "!!!" to make sure
268		you notice.  The age of a newly allocated structure is zero,
269		it becomes one when removed from reader visibility, and is
270		incremented once per grace period subsequently -- and is freed
271		after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
273		The output displayed above was taken from a correctly working
274		RCU.  If you want to see what it looks like when broken, break
275		it yourself.  ;-)
277	o	"Reader Batch": Another histogram of "ages" of structures seen
278		by readers, but in terms of counter flips (or batches) rather
279		than in terms of grace periods.  The legal number of non-zero
280		entries is again two.  The reason for this separate view is that
281		it is sometimes easier to get the third entry to show up in the
282		"Reader Batch" list than in the "Reader Pipe" list.
284	o	"Free-Block Circulation": Shows the number of torture structures
285		that have reached a given point in the pipeline.  The first element
286		should closely correspond to the number of structures allocated,
287		the second to the number that have been removed from reader view,
288		and all but the last remaining to the corresponding number of
289		passes through a grace period.  The last entry should be zero,
290		as it is only incremented if a torture structure's counter
291		somehow gets incremented farther than it should.
293	Different implementations of RCU can provide implementation-specific
294	additional information.  For example, SRCU provides the following
295	additional line:
297		srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
299	This line shows the per-CPU counter state.  The numbers in parentheses are
300	the values of the "old" and "current" counters for the corresponding CPU.
301	The "idx" value maps the "old" and "current" values to the underlying
302	array, and is useful for debugging.
307	The following script may be used to torture RCU:
309		#!/bin/sh
311		modprobe rcutorture
312		sleep 3600
313		rmmod rcutorture
314		dmesg | grep torture:
316	The output can be manually inspected for the error flag of "!!!".
317	One could of course create a more elaborate script that automatically
318	checked for such errors.  The "rmmod" command forces a "SUCCESS",
319	"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed.  The first
320	two are self-explanatory, while the last indicates that while there
321	were no RCU failures, CPU-hotplug problems were detected.
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