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 33irqreaders Says to invoke RCU readers from irq level. This is currently 34 done via timers. Defaults to "1" for variants of RCU that 35 permit this. (Or, more accurately, variants of RCU that do 36 -not- permit this know to ignore this variable.) 37 38nfakewriters This is the number of RCU fake writer threads to run. Fake 39 writer threads repeatedly use the synchronous "wait for 40 current readers" function of the interface selected by 41 torture_type, with a delay between calls to allow for various 42 different numbers of writers running in parallel. 43 nfakewriters defaults to 4, which provides enough parallelism 44 to trigger special cases caused by multiple writers, such as 45 the synchronize_srcu() early return optimization. 46 47nreaders This is the number of RCU reading threads supported. 48 The default is twice the number of CPUs. Why twice? 49 To properly exercise RCU implementations with preemptible 50 read-side critical sections. 51 52shuffle_interval 53 The number of seconds to keep the test threads affinitied 54 to a particular subset of the CPUs, defaults to 3 seconds. 55 Used in conjunction with test_no_idle_hz. 56 57stat_interval The number of seconds between output of torture 58 statistics (via printk()). Regardless of the interval, 59 statistics are printed when the module is unloaded. 60 Setting the interval to zero causes the statistics to 61 be printed -only- when the module is unloaded, and this 62 is the default. 63 64stutter The length of time to run the test before pausing for this 65 same period of time. Defaults to "stutter=5", so as 66 to run and pause for (roughly) five-second intervals. 67 Specifying "stutter=0" causes the test to run continuously 68 without pausing, which is the old default behavior. 69 70test_no_idle_hz Whether or not to test the ability of RCU to operate in 71 a kernel that disables the scheduling-clock interrupt to 72 idle CPUs. Boolean parameter, "1" to test, "0" otherwise. 73 Defaults to omitting this test. 74 75torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API, 76 "rcu_sync" for rcu_read_lock() with synchronous reclamation, 77 "rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for 78 rcu_read_lock_bh() with synchronous reclamation, "srcu" for 79 the "srcu_read_lock()" API, and "sched" for the use of 80 preempt_disable() together with synchronize_sched(). 81 82verbose Enable debug printk()s. Default is disabled. 83 84 85OUTPUT 86 87The statistics output is as follows: 88 89 rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0 90 rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915 91 rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0 92 rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0 93 rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0 94 rcu-torture: --- End of test 95 96The command "dmesg | grep torture:" will extract this information on 97most systems. On more esoteric configurations, it may be necessary to 98use other commands to access the output of the printk()s used by 99the RCU torture test. The printk()s use KERN_ALERT, so they should 100be evident. ;-) 101 102The entries are as follows: 103 104o "rtc": The hexadecimal address of the structure currently visible 105 to readers. 106 107o "ver": The number of times since boot that the rcutw writer task 108 has changed the structure visible to readers. 109 110o "tfle": If non-zero, indicates that the "torture freelist" 111 containing structure to be placed into the "rtc" area is empty. 112 This condition is important, since it can fool you into thinking 113 that RCU is working when it is not. :-/ 114 115o "rta": Number of structures allocated from the torture freelist. 116 117o "rtaf": Number of allocations from the torture freelist that have 118 failed due to the list being empty. 119 120o "rtf": Number of frees into the torture freelist. 121 122o "Reader Pipe": Histogram of "ages" of structures seen by readers. 123 If any entries past the first two are non-zero, RCU is broken. 124 And rcutorture prints the error flag string "!!!" to make sure 125 you notice. The age of a newly allocated structure is zero, 126 it becomes one when removed from reader visibility, and is 127 incremented once per grace period subsequently -- and is freed 128 after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. 129 130 The output displayed above was taken from a correctly working 131 RCU. If you want to see what it looks like when broken, break 132 it yourself. ;-) 133 134o "Reader Batch": Another histogram of "ages" of structures seen 135 by readers, but in terms of counter flips (or batches) rather 136 than in terms of grace periods. The legal number of non-zero 137 entries is again two. The reason for this separate view is that 138 it is sometimes easier to get the third entry to show up in the 139 "Reader Batch" list than in the "Reader Pipe" list. 140 141o "Free-Block Circulation": Shows the number of torture structures 142 that have reached a given point in the pipeline. The first element 143 should closely correspond to the number of structures allocated, 144 the second to the number that have been removed from reader view, 145 and all but the last remaining to the corresponding number of 146 passes through a grace period. The last entry should be zero, 147 as it is only incremented if a torture structure's counter 148 somehow gets incremented farther than it should. 149 150Different implementations of RCU can provide implementation-specific 151additional information. For example, SRCU provides the following: 152 153 srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0 154 srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0 155 srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0 156 srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0 157 srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) 158 159The first four lines are similar to those for RCU. The last line shows 160the per-CPU counter state. The numbers in parentheses are the values 161of the "old" and "current" counters for the corresponding CPU. The 162"idx" value maps the "old" and "current" values to the underlying array, 163and is useful for debugging. 164 165 166USAGE 167 168The following script may be used to torture RCU: 169 170 #!/bin/sh 171 172 modprobe rcutorture 173 sleep 100 174 rmmod rcutorture 175 dmesg | grep torture: 176 177The output can be manually inspected for the error flag of "!!!". 178One could of course create a more elaborate script that automatically 179checked for such errors. The "rmmod" command forces a "SUCCESS" or 180"FAILURE" indication to be printk()ed. 181