Documentation/RCU/torture.txt

   1 RCU Torture Test Operation
   2
   3
   4 CONFIG_RCU_TORTURE_TEST
   5
   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.
  12
  13 CONFIG_RCU_TORTURE_TEST_RUNNABLE
  14
  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
  22 CONFIG_RCU_TORTURE_TEST_RUNNABLE config option.
  23
  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.
  27
  28
  29 MODULE PARAMETERS
  30
  31 This module has the following parameters:
  32
  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.
  38
  39 fqs_holdoff     Holdoff time (in microseconds) between consecutive calls
  40                 to force_quiescent_state() within a burst.
  41
  42 fqs_stutter     Wait time (in seconds) between consecutive bursts
  43                 of calls to force_quiescent_state().
  44
  45 irqreader       Says to invoke RCU readers from irq level.  This is currently
  46                 done via timers.  Defaults to "1" for variants of RCU that
  47                 permit this.  (Or, more accurately, variants of RCU that do
  48                 -not- permit this know to ignore this variable.)
  49
  50 nfakewriters    This is the number of RCU fake writer threads to run.  Fake
  51                 writer threads repeatedly use the synchronous "wait for
  52                 current readers" function of the interface selected by
  53                 torture_type, with a delay between calls to allow for various
  54                 different numbers of writers running in parallel.
  55                 nfakewriters defaults to 4, which provides enough parallelism
  56                 to trigger special cases caused by multiple writers, such as
  57                 the synchronize_srcu() early return optimization.
  58
  59 nreaders        This is the number of RCU reading threads supported.
  60                 The default is twice the number of CPUs.  Why twice?
  61                 To properly exercise RCU implementations with preemptible
  62                 read-side critical sections.
  63
  64 onoff_interval
  65                 The number of seconds between each attempt to execute a
  66                 randomly selected CPU-hotplug operation.  Defaults to
  67                 zero, which disables CPU hotplugging.  In HOTPLUG_CPU=n
  68                 kernels, rcutorture will silently refuse to do any
  69                 CPU-hotplug operations regardless of what value is
  70                 specified for onoff_interval.
  71
  72 shuffle_interval
  73                 The number of seconds to keep the test threads affinitied
  74                 to a particular subset of the CPUs, defaults to 3 seconds.
  75                 Used in conjunction with test_no_idle_hz.
  76
  77 shutdown_secs   The number of seconds to run the test before terminating
  78                 the test and powering off the system.  The default is
  79                 zero, which disables test termination and system shutdown.
  80                 This capability is useful for automated testing.
  81
  82 stat_interval   The number of seconds between output of torture
  83                 statistics (via printk()).  Regardless of the interval,
  84                 statistics are printed when the module is unloaded.
  85                 Setting the interval to zero causes the statistics to
  86                 be printed -only- when the module is unloaded, and this
  87                 is the default.
  88
  89 stutter         The length of time to run the test before pausing for this
  90                 same period of time.  Defaults to "stutter=5", so as
  91                 to run and pause for (roughly) five-second intervals.
  92                 Specifying "stutter=0" causes the test to run continuously
  93                 without pausing, which is the old default behavior.
  94
  95 test_boost      Whether or not to test the ability of RCU to do priority
  96                 boosting.  Defaults to "test_boost=1", which performs
  97                 RCU priority-inversion testing only if the selected
  98                 RCU implementation supports priority boosting.  Specifying
  99                 "test_boost=0" never performs RCU priority-inversion
 100                 testing.  Specifying "test_boost=2" performs RCU
 101                 priority-inversion testing even if the selected RCU
 102                 implementation does not support RCU priority boosting,
 103                 which can be used to test rcutorture's ability to
 104                 carry out RCU priority-inversion testing.
 105
 106 test_boost_interval
 107                 The number of seconds in an RCU priority-inversion test
 108                 cycle.  Defaults to "test_boost_interval=7".  It is
 109                 usually wise for this value to be relatively prime to
 110                 the value selected for "stutter".
 111
 112 test_boost_duration
 113                 The number of seconds to do RCU priority-inversion testing
 114                 within any given "test_boost_interval".  Defaults to
 115                 "test_boost_duration=4".
 116
 117 test_no_idle_hz Whether or not to test the ability of RCU to operate in
 118                 a kernel that disables the scheduling-clock interrupt to
 119                 idle CPUs.  Boolean parameter, "1" to test, "0" otherwise.
 120                 Defaults to omitting this test.
 121
 122 torture_type    The type of RCU to test, with string values as follows:
 123
 124                 "rcu":  rcu_read_lock(), rcu_read_unlock() and call_rcu().
 125
 126                 "rcu_sync":  rcu_read_lock(), rcu_read_unlock(), and
 127                         synchronize_rcu().
 128
 129                 "rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and
 130                         synchronize_rcu_expedited().
 131
 132                 "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
 133                         call_rcu_bh().
 134
 135                 "rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(),
 136                         and synchronize_rcu_bh().
 137
 138                 "rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(),
 139                         and synchronize_rcu_bh_expedited().
 140
 141                 "srcu": srcu_read_lock(), srcu_read_unlock() and
 142                         synchronize_srcu().
 143
 144                 "srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
 145                         synchronize_srcu_expedited().
 146
 147                 "sched": preempt_disable(), preempt_enable(), and
 148                         call_rcu_sched().
 149
 150                 "sched_sync": preempt_disable(), preempt_enable(), and
 151                         synchronize_sched().
 152
 153                 "sched_expedited": preempt_disable(), preempt_enable(), and
 154                         synchronize_sched_expedited().
 155
 156                 Defaults to "rcu".
 157
 158 verbose         Enable debug printk()s.  Default is disabled.
 159
 160
 161 OUTPUT
 162
 163 The statistics output is as follows:
 164
 165         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
 166         rcu-torture: rtc:           (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767
 167         rcu-torture: Reader Pipe:  727860534 34213 0 0 0 0 0 0 0 0 0
 168         rcu-torture: Reader Batch:  727877838 17003 0 0 0 0 0 0 0 0 0
 169         rcu-torture: Free-Block Circulation:  155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
 170         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
 171
 172 The command "dmesg | grep torture:" will extract this information on
 173 most systems.  On more esoteric configurations, it may be necessary to
 174 use other commands to access the output of the printk()s used by
 175 the RCU torture test.  The printk()s use KERN_ALERT, so they should
 176 be evident.  ;-)
 177
 178 The first and last lines show the rcutorture module parameters, and the
 179 last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
 180 automatic determination as to whether RCU operated correctly.
 181
 182 The entries are as follows:
 183
 184 o       "rtc": The hexadecimal address of the structure currently visible
 185         to readers.
 186
 187 o       "ver": The number of times since boot that the RCU writer task
 188         has changed the structure visible to readers.
 189
 190 o       "tfle": If non-zero, indicates that the "torture freelist"
 191         containing structures to be placed into the "rtc" area is empty.
 192         This condition is important, since it can fool you into thinking
 193         that RCU is working when it is not.  :-/
 194
 195 o       "rta": Number of structures allocated from the torture freelist.
 196
 197 o       "rtaf": Number of allocations from the torture freelist that have
 198         failed due to the list being empty.  It is not unusual for this
 199         to be non-zero, but it is bad for it to be a large fraction of
 200         the value indicated by "rta".
 201
 202 o       "rtf": Number of frees into the torture freelist.
 203
 204 o       "rtmbe": A non-zero value indicates that rcutorture believes that
 205         rcu_assign_pointer() and rcu_dereference() are not working
 206         correctly.  This value should be zero.
 207
 208 o       "rtbke": rcutorture was unable to create the real-time kthreads
 209         used to force RCU priority inversion.  This value should be zero.
 210
 211 o       "rtbre": Although rcutorture successfully created the kthreads
 212         used to force RCU priority inversion, it was unable to set them
 213         to the real-time priority level of 1.  This value should be zero.
 214
 215 o       "rtbf": The number of times that RCU priority boosting failed
 216         to resolve RCU priority inversion.
 217
 218 o       "rtb": The number of times that rcutorture attempted to force
 219         an RCU priority inversion condition.  If you are testing RCU
 220         priority boosting via the "test_boost" module parameter, this
 221         value should be non-zero.
 222
 223 o       "nt": The number of times rcutorture ran RCU read-side code from
 224         within a timer handler.  This value should be non-zero only
 225         if you specified the "irqreader" module parameter.
 226
 227 o       "Reader Pipe": Histogram of "ages" of structures seen by readers.
 228         If any entries past the first two are non-zero, RCU is broken.
 229         And rcutorture prints the error flag string "!!!" to make sure
 230         you notice.  The age of a newly allocated structure is zero,
 231         it becomes one when removed from reader visibility, and is
 232         incremented once per grace period subsequently -- and is freed
 233         after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
 234
 235         The output displayed above was taken from a correctly working
 236         RCU.  If you want to see what it looks like when broken, break
 237         it yourself.  ;-)
 238
 239 o       "Reader Batch": Another histogram of "ages" of structures seen
 240         by readers, but in terms of counter flips (or batches) rather
 241         than in terms of grace periods.  The legal number of non-zero
 242         entries is again two.  The reason for this separate view is that
 243         it is sometimes easier to get the third entry to show up in the
 244         "Reader Batch" list than in the "Reader Pipe" list.
 245
 246 o       "Free-Block Circulation": Shows the number of torture structures
 247         that have reached a given point in the pipeline.  The first element
 248         should closely correspond to the number of structures allocated,
 249         the second to the number that have been removed from reader view,
 250         and all but the last remaining to the corresponding number of
 251         passes through a grace period.  The last entry should be zero,
 252         as it is only incremented if a torture structure's counter
 253         somehow gets incremented farther than it should.
 254
 255 Different implementations of RCU can provide implementation-specific
 256 additional information.  For example, SRCU provides the following
 257 additional line:
 258
 259         srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
 260
 261 This line shows the per-CPU counter state.  The numbers in parentheses are
 262 the values of the "old" and "current" counters for the corresponding CPU.
 263 The "idx" value maps the "old" and "current" values to the underlying
 264 array, and is useful for debugging.
 265
 266
 267 USAGE
 268
 269 The following script may be used to torture RCU:
 270
 271         #!/bin/sh
 272
 273         modprobe rcutorture
 274         sleep 100
 275         rmmod rcutorture
 276         dmesg | grep torture:
 277
 278 The output can be manually inspected for the error flag of "!!!".
 279 One could of course create a more elaborate script that automatically
 280 checked for such errors.  The "rmmod" command forces a "SUCCESS" or
 281 "FAILURE" indication to be printk()ed.