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 n_barrier_cbs   If this is nonzero, RCU barrier testing will be conducted,
  51                 in which case n_barrier_cbs specifies the number of
  52                 RCU callbacks (and corresponding kthreads) to use for
  53                 this testing.  The value cannot be negative.  If you
  54                 specify this to be non-zero when torture_type indicates a
  55                 synchronous RCU implementation (one for which a member of
  56                 the synchronize_rcu() rather than the call_rcu() family is
  57                 used -- see the documentation for torture_type below), an
  58                 error will be reported and no testing will be carried out.
  59
  60 nfakewriters    This is the number of RCU fake writer threads to run.  Fake
  61                 writer threads repeatedly use the synchronous "wait for
  62                 current readers" function of the interface selected by
  63                 torture_type, with a delay between calls to allow for various
  64                 different numbers of writers running in parallel.
  65                 nfakewriters defaults to 4, which provides enough parallelism
  66                 to trigger special cases caused by multiple writers, such as
  67                 the synchronize_srcu() early return optimization.
  68
  69 nreaders        This is the number of RCU reading threads supported.
  70                 The default is twice the number of CPUs.  Why twice?
  71                 To properly exercise RCU implementations with preemptible
  72                 read-side critical sections.
  73
  74 onoff_interval
  75                 The number of seconds between each attempt to execute a
  76                 randomly selected CPU-hotplug operation.  Defaults to
  77                 zero, which disables CPU hotplugging.  In HOTPLUG_CPU=n
  78                 kernels, rcutorture will silently refuse to do any
  79                 CPU-hotplug operations regardless of what value is
  80                 specified for onoff_interval.
  81
  82 onoff_holdoff   The number of seconds to wait until starting CPU-hotplug
  83                 operations.  This would normally only be used when
  84                 rcutorture was built into the kernel and started
  85                 automatically at boot time, in which case it is useful
  86                 in order to avoid confusing boot-time code with CPUs
  87                 coming and going.
  88
  89 shuffle_interval
  90                 The number of seconds to keep the test threads affinitied
  91                 to a particular subset of the CPUs, defaults to 3 seconds.
  92                 Used in conjunction with test_no_idle_hz.
  93
  94 shutdown_secs   The number of seconds to run the test before terminating
  95                 the test and powering off the system.  The default is
  96                 zero, which disables test termination and system shutdown.
  97                 This capability is useful for automated testing.
  98
  99 stall_cpu       The number of seconds that a CPU should be stalled while
 100                 within both an rcu_read_lock() and a preempt_disable().
 101                 This stall happens only once per rcutorture run.
 102                 If you need multiple stalls, use modprobe and rmmod to
 103                 repeatedly run rcutorture.  The default for stall_cpu
 104                 is zero, which prevents rcutorture from stalling a CPU.
 105
 106                 Note that attempts to rmmod rcutorture while the stall
 107                 is ongoing will hang, so be careful what value you
 108                 choose for this module parameter!  In addition, too-large
 109                 values for stall_cpu might well induce failures and
 110                 warnings in other parts of the kernel.  You have been
 111                 warned!
 112
 113 stall_cpu_holdoff
 114                 The number of seconds to wait after rcutorture starts
 115                 before stalling a CPU.  Defaults to 10 seconds.
 116
 117 stat_interval   The number of seconds between output of torture
 118                 statistics (via printk()).  Regardless of the interval,
 119                 statistics are printed when the module is unloaded.
 120                 Setting the interval to zero causes the statistics to
 121                 be printed -only- when the module is unloaded, and this
 122                 is the default.
 123
 124 stutter         The length of time to run the test before pausing for this
 125                 same period of time.  Defaults to "stutter=5", so as
 126                 to run and pause for (roughly) five-second intervals.
 127                 Specifying "stutter=0" causes the test to run continuously
 128                 without pausing, which is the old default behavior.
 129
 130 test_boost      Whether or not to test the ability of RCU to do priority
 131                 boosting.  Defaults to "test_boost=1", which performs
 132                 RCU priority-inversion testing only if the selected
 133                 RCU implementation supports priority boosting.  Specifying
 134                 "test_boost=0" never performs RCU priority-inversion
 135                 testing.  Specifying "test_boost=2" performs RCU
 136                 priority-inversion testing even if the selected RCU
 137                 implementation does not support RCU priority boosting,
 138                 which can be used to test rcutorture's ability to
 139                 carry out RCU priority-inversion testing.
 140
 141 test_boost_interval
 142                 The number of seconds in an RCU priority-inversion test
 143                 cycle.  Defaults to "test_boost_interval=7".  It is
 144                 usually wise for this value to be relatively prime to
 145                 the value selected for "stutter".
 146
 147 test_boost_duration
 148                 The number of seconds to do RCU priority-inversion testing
 149                 within any given "test_boost_interval".  Defaults to
 150                 "test_boost_duration=4".
 151
 152 test_no_idle_hz Whether or not to test the ability of RCU to operate in
 153                 a kernel that disables the scheduling-clock interrupt to
 154                 idle CPUs.  Boolean parameter, "1" to test, "0" otherwise.
 155                 Defaults to omitting this test.
 156
 157 torture_type    The type of RCU to test, with string values as follows:
 158
 159                 "rcu":  rcu_read_lock(), rcu_read_unlock() and call_rcu().
 160
 161                 "rcu_sync":  rcu_read_lock(), rcu_read_unlock(), and
 162                         synchronize_rcu().
 163
 164                 "rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and
 165                         synchronize_rcu_expedited().
 166
 167                 "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and
 168                         call_rcu_bh().
 169
 170                 "rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(),
 171                         and synchronize_rcu_bh().
 172
 173                 "rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(),
 174                         and synchronize_rcu_bh_expedited().
 175
 176                 "srcu": srcu_read_lock(), srcu_read_unlock() and
 177                         call_srcu().
 178
 179                 "srcu_sync": srcu_read_lock(), srcu_read_unlock() and
 180                         synchronize_srcu().
 181
 182                 "srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
 183                         synchronize_srcu_expedited().
 184
 185                 "srcu_raw": srcu_read_lock_raw(), srcu_read_unlock_raw(),
 186                         and call_srcu().
 187
 188                 "srcu_raw_sync": srcu_read_lock_raw(), srcu_read_unlock_raw(),
 189                         and synchronize_srcu().
 190
 191                 "sched": preempt_disable(), preempt_enable(), and
 192                         call_rcu_sched().
 193
 194                 "sched_sync": preempt_disable(), preempt_enable(), and
 195                         synchronize_sched().
 196
 197                 "sched_expedited": preempt_disable(), preempt_enable(), and
 198                         synchronize_sched_expedited().
 199
 200                 Defaults to "rcu".
 201
 202 verbose         Enable debug printk()s.  Default is disabled.
 203
 204
 205 OUTPUT
 206
 207 The statistics output is as follows:
 208
 209         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
 210         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
 211         rcu-torture: Reader Pipe:  727860534 34213 0 0 0 0 0 0 0 0 0
 212         rcu-torture: Reader Batch:  727877838 17003 0 0 0 0 0 0 0 0 0
 213         rcu-torture: Free-Block Circulation:  155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0
 214         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
 215
 216 The command "dmesg | grep torture:" will extract this information on
 217 most systems.  On more esoteric configurations, it may be necessary to
 218 use other commands to access the output of the printk()s used by
 219 the RCU torture test.  The printk()s use KERN_ALERT, so they should
 220 be evident.  ;-)
 221
 222 The first and last lines show the rcutorture module parameters, and the
 223 last line shows either "SUCCESS" or "FAILURE", based on rcutorture's
 224 automatic determination as to whether RCU operated correctly.
 225
 226 The entries are as follows:
 227
 228 o       "rtc": The hexadecimal address of the structure currently visible
 229         to readers.
 230
 231 o       "ver": The number of times since boot that the RCU writer task
 232         has changed the structure visible to readers.
 233
 234 o       "tfle": If non-zero, indicates that the "torture freelist"
 235         containing structures to be placed into the "rtc" area is empty.
 236         This condition is important, since it can fool you into thinking
 237         that RCU is working when it is not.  :-/
 238
 239 o       "rta": Number of structures allocated from the torture freelist.
 240
 241 o       "rtaf": Number of allocations from the torture freelist that have
 242         failed due to the list being empty.  It is not unusual for this
 243         to be non-zero, but it is bad for it to be a large fraction of
 244         the value indicated by "rta".
 245
 246 o       "rtf": Number of frees into the torture freelist.
 247
 248 o       "rtmbe": A non-zero value indicates that rcutorture believes that
 249         rcu_assign_pointer() and rcu_dereference() are not working
 250         correctly.  This value should be zero.
 251
 252 o       "rtbe": A non-zero value indicates that one of the rcu_barrier()
 253         family of functions is not working correctly.
 254
 255 o       "rtbke": rcutorture was unable to create the real-time kthreads
 256         used to force RCU priority inversion.  This value should be zero.
 257
 258 o       "rtbre": Although rcutorture successfully created the kthreads
 259         used to force RCU priority inversion, it was unable to set them
 260         to the real-time priority level of 1.  This value should be zero.
 261
 262 o       "rtbf": The number of times that RCU priority boosting failed
 263         to resolve RCU priority inversion.
 264
 265 o       "rtb": The number of times that rcutorture attempted to force
 266         an RCU priority inversion condition.  If you are testing RCU
 267         priority boosting via the "test_boost" module parameter, this
 268         value should be non-zero.
 269
 270 o       "nt": The number of times rcutorture ran RCU read-side code from
 271         within a timer handler.  This value should be non-zero only
 272         if you specified the "irqreader" module parameter.
 273
 274 o       "Reader Pipe": Histogram of "ages" of structures seen by readers.
 275         If any entries past the first two are non-zero, RCU is broken.
 276         And rcutorture prints the error flag string "!!!" to make sure
 277         you notice.  The age of a newly allocated structure is zero,
 278         it becomes one when removed from reader visibility, and is
 279         incremented once per grace period subsequently -- and is freed
 280         after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods.
 281
 282         The output displayed above was taken from a correctly working
 283         RCU.  If you want to see what it looks like when broken, break
 284         it yourself.  ;-)
 285
 286 o       "Reader Batch": Another histogram of "ages" of structures seen
 287         by readers, but in terms of counter flips (or batches) rather
 288         than in terms of grace periods.  The legal number of non-zero
 289         entries is again two.  The reason for this separate view is that
 290         it is sometimes easier to get the third entry to show up in the
 291         "Reader Batch" list than in the "Reader Pipe" list.
 292
 293 o       "Free-Block Circulation": Shows the number of torture structures
 294         that have reached a given point in the pipeline.  The first element
 295         should closely correspond to the number of structures allocated,
 296         the second to the number that have been removed from reader view,
 297         and all but the last remaining to the corresponding number of
 298         passes through a grace period.  The last entry should be zero,
 299         as it is only incremented if a torture structure's counter
 300         somehow gets incremented farther than it should.
 301
 302 Different implementations of RCU can provide implementation-specific
 303 additional information.  For example, SRCU provides the following
 304 additional line:
 305
 306         srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
 307
 308 This line shows the per-CPU counter state.  The numbers in parentheses are
 309 the values of the "old" and "current" counters for the corresponding CPU.
 310 The "idx" value maps the "old" and "current" values to the underlying
 311 array, and is useful for debugging.
 312
 313
 314 USAGE
 315
 316 The following script may be used to torture RCU:
 317
 318         #!/bin/sh
 319
 320         modprobe rcutorture
 321         sleep 3600
 322         rmmod rcutorture
 323         dmesg | grep torture:
 324
 325 The output can be manually inspected for the error flag of "!!!".
 326 One could of course create a more elaborate script that automatically
 327 checked for such errors.  The "rmmod" command forces a "SUCCESS",
 328 "FAILURE", or "RCU_HOTPLUG" indication to be printk()ed.  The first
 329 two are self-explanatory, while the last indicates that while there
 330 were no RCU failures, CPU-hotplug problems were detected.