Based on kernel version 3.9. Page generated on 2013-05-02 23:13 EST.
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.