1RCU Torture Test Operation 2 3 4CONFIG_RCU_TORTURE_TEST 5 6The CONFIG_RCU_TORTURE_TEST config option is available for all RCU 7implementations. It creates an rcutorture kernel module that can 8be loaded to run a torture test. The test periodically outputs 9status messages via printk(), which can be examined via the dmesg 10command (perhaps grepping for "torture"). The test is started 11when the module is loaded, and stops when the module is unloaded. 12 13CONFIG_RCU_TORTURE_TEST_RUNNABLE 14 15It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will 16result in the tests being loaded into the base kernel. In this case, 17the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify 18whether the RCU torture tests are to be started immediately during 19boot or whether the /proc/sys/kernel/rcutorture_runnable file is used 20to enable them. This /proc file can be used to repeatedly pause and 21restart the tests, regardless of the initial state specified by the 22CONFIG_RCU_TORTURE_TEST_RUNNABLE config option. 23 24You 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 26this can sometimes be useful in finding boot-time bugs. 27 28 29MODULE PARAMETERS 30 31This module has the following parameters: 32 33fqs_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 39fqs_holdoff Holdoff time (in microseconds) between consecutive calls 40 to force_quiescent_state() within a burst. 41 42fqs_stutter Wait time (in seconds) between consecutive bursts 43 of calls to force_quiescent_state(). 44 45gp_normal Make the fake writers use normal synchronous grace-period 46 primitives. 47 48gp_exp Make the fake writers use expedited synchronous grace-period 49 primitives. If both gp_normal and gp_exp are set, or 50 if neither gp_normal nor gp_exp are set, then randomly 51 choose the primitive so that about 50% are normal and 52 50% expedited. By default, neither are set, which 53 gives best overall test coverage. 54 55irqreader Says to invoke RCU readers from irq level. This is currently 56 done via timers. Defaults to "1" for variants of RCU that 57 permit this. (Or, more accurately, variants of RCU that do 58 -not- permit this know to ignore this variable.) 59 60n_barrier_cbs If this is nonzero, RCU barrier testing will be conducted, 61 in which case n_barrier_cbs specifies the number of 62 RCU callbacks (and corresponding kthreads) to use for 63 this testing. The value cannot be negative. If you 64 specify this to be non-zero when torture_type indicates a 65 synchronous RCU implementation (one for which a member of 66 the synchronize_rcu() rather than the call_rcu() family is 67 used -- see the documentation for torture_type below), an 68 error will be reported and no testing will be carried out. 69 70nfakewriters This is the number of RCU fake writer threads to run. Fake 71 writer threads repeatedly use the synchronous "wait for 72 current readers" function of the interface selected by 73 torture_type, with a delay between calls to allow for various 74 different numbers of writers running in parallel. 75 nfakewriters defaults to 4, which provides enough parallelism 76 to trigger special cases caused by multiple writers, such as 77 the synchronize_srcu() early return optimization. 78 79nreaders This is the number of RCU reading threads supported. 80 The default is twice the number of CPUs. Why twice? 81 To properly exercise RCU implementations with preemptible 82 read-side critical sections. 83 84onoff_interval 85 The number of seconds between each attempt to execute a 86 randomly selected CPU-hotplug operation. Defaults to 87 zero, which disables CPU hotplugging. In HOTPLUG_CPU=n 88 kernels, rcutorture will silently refuse to do any 89 CPU-hotplug operations regardless of what value is 90 specified for onoff_interval. 91 92onoff_holdoff The number of seconds to wait until starting CPU-hotplug 93 operations. This would normally only be used when 94 rcutorture was built into the kernel and started 95 automatically at boot time, in which case it is useful 96 in order to avoid confusing boot-time code with CPUs 97 coming and going. 98 99shuffle_interval 100 The number of seconds to keep the test threads affinitied 101 to a particular subset of the CPUs, defaults to 3 seconds. 102 Used in conjunction with test_no_idle_hz. 103 104shutdown_secs The number of seconds to run the test before terminating 105 the test and powering off the system. The default is 106 zero, which disables test termination and system shutdown. 107 This capability is useful for automated testing. 108 109stall_cpu The number of seconds that a CPU should be stalled while 110 within both an rcu_read_lock() and a preempt_disable(). 111 This stall happens only once per rcutorture run. 112 If you need multiple stalls, use modprobe and rmmod to 113 repeatedly run rcutorture. The default for stall_cpu 114 is zero, which prevents rcutorture from stalling a CPU. 115 116 Note that attempts to rmmod rcutorture while the stall 117 is ongoing will hang, so be careful what value you 118 choose for this module parameter! In addition, too-large 119 values for stall_cpu might well induce failures and 120 warnings in other parts of the kernel. You have been 121 warned! 122 123stall_cpu_holdoff 124 The number of seconds to wait after rcutorture starts 125 before stalling a CPU. Defaults to 10 seconds. 126 127stat_interval The number of seconds between output of torture 128 statistics (via printk()). Regardless of the interval, 129 statistics are printed when the module is unloaded. 130 Setting the interval to zero causes the statistics to 131 be printed -only- when the module is unloaded, and this 132 is the default. 133 134stutter The length of time to run the test before pausing for this 135 same period of time. Defaults to "stutter=5", so as 136 to run and pause for (roughly) five-second intervals. 137 Specifying "stutter=0" causes the test to run continuously 138 without pausing, which is the old default behavior. 139 140test_boost Whether or not to test the ability of RCU to do priority 141 boosting. Defaults to "test_boost=1", which performs 142 RCU priority-inversion testing only if the selected 143 RCU implementation supports priority boosting. Specifying 144 "test_boost=0" never performs RCU priority-inversion 145 testing. Specifying "test_boost=2" performs RCU 146 priority-inversion testing even if the selected RCU 147 implementation does not support RCU priority boosting, 148 which can be used to test rcutorture's ability to 149 carry out RCU priority-inversion testing. 150 151test_boost_interval 152 The number of seconds in an RCU priority-inversion test 153 cycle. Defaults to "test_boost_interval=7". It is 154 usually wise for this value to be relatively prime to 155 the value selected for "stutter". 156 157test_boost_duration 158 The number of seconds to do RCU priority-inversion testing 159 within any given "test_boost_interval". Defaults to 160 "test_boost_duration=4". 161 162test_no_idle_hz Whether or not to test the ability of RCU to operate in 163 a kernel that disables the scheduling-clock interrupt to 164 idle CPUs. Boolean parameter, "1" to test, "0" otherwise. 165 Defaults to omitting this test. 166 167torture_type The type of RCU to test, with string values as follows: 168 169 "rcu": rcu_read_lock(), rcu_read_unlock() and call_rcu(). 170 171 "rcu_sync": rcu_read_lock(), rcu_read_unlock(), and 172 synchronize_rcu(). 173 174 "rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and 175 synchronize_rcu_expedited(). 176 177 "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and 178 call_rcu_bh(). 179 180 "rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(), 181 and synchronize_rcu_bh(). 182 183 "rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(), 184 and synchronize_rcu_bh_expedited(). 185 186 "srcu": srcu_read_lock(), srcu_read_unlock() and 187 call_srcu(). 188 189 "srcu_sync": srcu_read_lock(), srcu_read_unlock() and 190 synchronize_srcu(). 191 192 "srcu_expedited": srcu_read_lock(), srcu_read_unlock() and 193 synchronize_srcu_expedited(). 194 195 "sched": preempt_disable(), preempt_enable(), and 196 call_rcu_sched(). 197 198 "sched_sync": preempt_disable(), preempt_enable(), and 199 synchronize_sched(). 200 201 "sched_expedited": preempt_disable(), preempt_enable(), and 202 synchronize_sched_expedited(). 203 204 Defaults to "rcu". 205 206verbose Enable debug printk()s. Default is disabled. 207 208 209OUTPUT 210 211The statistics output is as follows: 212 213 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=4 214 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 215 rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 216 rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 217 rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 218 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=4 219 220The command "dmesg | grep torture:" will extract this information on 221most systems. On more esoteric configurations, it may be necessary to 222use other commands to access the output of the printk()s used by 223the RCU torture test. The printk()s use KERN_ALERT, so they should 224be evident. ;-) 225 226The first and last lines show the rcutorture module parameters, and the 227last line shows either "SUCCESS" or "FAILURE", based on rcutorture's 228automatic determination as to whether RCU operated correctly. 229 230The entries are as follows: 231 232o "rtc": The hexadecimal address of the structure currently visible 233 to readers. 234 235o "ver": The number of times since boot that the RCU writer task 236 has changed the structure visible to readers. 237 238o "tfle": If non-zero, indicates that the "torture freelist" 239 containing structures to be placed into the "rtc" area is empty. 240 This condition is important, since it can fool you into thinking 241 that RCU is working when it is not. :-/ 242 243o "rta": Number of structures allocated from the torture freelist. 244 245o "rtaf": Number of allocations from the torture freelist that have 246 failed due to the list being empty. It is not unusual for this 247 to be non-zero, but it is bad for it to be a large fraction of 248 the value indicated by "rta". 249 250o "rtf": Number of frees into the torture freelist. 251 252o "rtmbe": A non-zero value indicates that rcutorture believes that 253 rcu_assign_pointer() and rcu_dereference() are not working 254 correctly. This value should be zero. 255 256o "rtbe": A non-zero value indicates that one of the rcu_barrier() 257 family of functions is not working correctly. 258 259o "rtbke": rcutorture was unable to create the real-time kthreads 260 used to force RCU priority inversion. This value should be zero. 261 262o "rtbre": Although rcutorture successfully created the kthreads 263 used to force RCU priority inversion, it was unable to set them 264 to the real-time priority level of 1. This value should be zero. 265 266o "rtbf": The number of times that RCU priority boosting failed 267 to resolve RCU priority inversion. 268 269o "rtb": The number of times that rcutorture attempted to force 270 an RCU priority inversion condition. If you are testing RCU 271 priority boosting via the "test_boost" module parameter, this 272 value should be non-zero. 273 274o "nt": The number of times rcutorture ran RCU read-side code from 275 within a timer handler. This value should be non-zero only 276 if you specified the "irqreader" module parameter. 277 278o "Reader Pipe": Histogram of "ages" of structures seen by readers. 279 If any entries past the first two are non-zero, RCU is broken. 280 And rcutorture prints the error flag string "!!!" to make sure 281 you notice. The age of a newly allocated structure is zero, 282 it becomes one when removed from reader visibility, and is 283 incremented once per grace period subsequently -- and is freed 284 after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. 285 286 The output displayed above was taken from a correctly working 287 RCU. If you want to see what it looks like when broken, break 288 it yourself. ;-) 289 290o "Reader Batch": Another histogram of "ages" of structures seen 291 by readers, but in terms of counter flips (or batches) rather 292 than in terms of grace periods. The legal number of non-zero 293 entries is again two. The reason for this separate view is that 294 it is sometimes easier to get the third entry to show up in the 295 "Reader Batch" list than in the "Reader Pipe" list. 296 297o "Free-Block Circulation": Shows the number of torture structures 298 that have reached a given point in the pipeline. The first element 299 should closely correspond to the number of structures allocated, 300 the second to the number that have been removed from reader view, 301 and all but the last remaining to the corresponding number of 302 passes through a grace period. The last entry should be zero, 303 as it is only incremented if a torture structure's counter 304 somehow gets incremented farther than it should. 305 306Different implementations of RCU can provide implementation-specific 307additional information. For example, SRCU provides the following 308additional line: 309 310 srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) 311 312This line shows the per-CPU counter state. The numbers in parentheses are 313the values of the "old" and "current" counters for the corresponding CPU. 314The "idx" value maps the "old" and "current" values to the underlying 315array, and is useful for debugging. 316 317 318USAGE 319 320The following script may be used to torture RCU: 321 322 #!/bin/sh 323 324 modprobe rcutorture 325 sleep 3600 326 rmmod rcutorture 327 dmesg | grep torture: 328 329The output can be manually inspected for the error flag of "!!!". 330One could of course create a more elaborate script that automatically 331checked for such errors. The "rmmod" command forces a "SUCCESS", 332"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first 333two are self-explanatory, while the last indicates that while there 334were no RCU failures, CPU-hotplug problems were detected. 335