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.

  CONFIG_RCU_TORTURE_TEST_RUNNABLE
  
  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
  CONFIG_RCU_TORTURE_TEST_RUNNABLE config option.
  
  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.

  
  
  MODULE PARAMETERS
  
  This module has the following parameters:

  irqreaders	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.)

  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.
  
  shuffle_interval
  		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.

  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_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: "rcu" for the rcu_read_lock() API,
  		"rcu_sync" for rcu_read_lock() with synchronous reclamation,

  		"rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for

  		rcu_read_lock_bh() with synchronous reclamation, "srcu" for
  		the "srcu_read_lock()" API, and "sched" for the use of
  		preempt_disable() together with synchronize_sched().

  verbose		Enable debug printk()s.  Default is disabled.
  
  
  OUTPUT
  
  The statistics output is as follows:

  	rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
  	rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
  	rcu-torture: Reader Pipe:  1466408 9747 0 0 0 0 0 0 0 0 0
  	rcu-torture: Reader Batch:  1464477 11678 0 0 0 0 0 0 0 0
  	rcu-torture: Free-Block Circulation:  1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
  	rcu-torture: --- End of test

  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 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 rcutw writer task
  	has changed the structure visible to readers.
  
  o	"tfle": If non-zero, indicates that the "torture freelist"
  	containing structure 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.
  
  o	"rtf": Number of frees into the torture freelist.
  
  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:
  
  	srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0
  	srcu-torture: Reader Pipe:  559738 939 0 0 0 0 0 0 0 0 0
  	srcu-torture: Reader Batch:  560434 243 0 0 0 0 0 0 0 0
  	srcu-torture: Free-Block Circulation:  355 354 353 352 351 350 349 348 347 346 0
  	srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
  
  The first four lines are similar to those for RCU.  The last 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.

  
  USAGE
  
  The following script may be used to torture RCU:
  
  	#!/bin/sh
  
  	modprobe rcutorture
  	sleep 100
  	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" or
  "FAILURE" indication to be printk()ed.