RCU Torture Test Operation


CONFIG_RCU_TORTURE_TEST

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.

Module parameters are prefixed by "rcutorture." in
Documentation/admin-guide/kernel-parameters.txt.

OUTPUT

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, Tree SRCU provides the following
additional line:

	srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6)

This line shows the per-CPU counter state, in this case for Tree SRCU
using a dynamically allocated srcu_struct (hence "srcud-" rather than
"srcu-").  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 final "T" entry contains the totals of the counters.


USAGE

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.

However, the tools/testing/selftests/rcutorture/bin/kvm.sh script
provides better automation, including automatic failure analysis.
It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd.
See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions
on setting up such an initrd.