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 45irqreaders 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 50nfakewriters 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 59nreaders 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 64shuffle_interval 65 The number of seconds to keep the test threads affinitied 66 to a particular subset of the CPUs, defaults to 3 seconds. 67 Used in conjunction with test_no_idle_hz. 68 69stat_interval The number of seconds between output of torture 70 statistics (via printk()). Regardless of the interval, 71 statistics are printed when the module is unloaded. 72 Setting the interval to zero causes the statistics to 73 be printed -only- when the module is unloaded, and this 74 is the default. 75 76stutter The length of time to run the test before pausing for this 77 same period of time. Defaults to "stutter=5", so as 78 to run and pause for (roughly) five-second intervals. 79 Specifying "stutter=0" causes the test to run continuously 80 without pausing, which is the old default behavior. 81 82test_no_idle_hz Whether or not to test the ability of RCU to operate in 83 a kernel that disables the scheduling-clock interrupt to 84 idle CPUs. Boolean parameter, "1" to test, "0" otherwise. 85 Defaults to omitting this test. 86 87torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API, 88 "rcu_sync" for rcu_read_lock() with synchronous reclamation, 89 "rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for 90 rcu_read_lock_bh() with synchronous reclamation, "srcu" for 91 the "srcu_read_lock()" API, "sched" for the use of 92 preempt_disable() together with synchronize_sched(), 93 and "sched_expedited" for the use of preempt_disable() 94 with synchronize_sched_expedited(). 95 96verbose Enable debug printk()s. Default is disabled. 97 98 99OUTPUT 100 101The statistics output is as follows: 102 103 rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0 104 rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915 105 rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0 106 rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0 107 rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0 108 rcu-torture: --- End of test 109 110The command "dmesg | grep torture:" will extract this information on 111most systems. On more esoteric configurations, it may be necessary to 112use other commands to access the output of the printk()s used by 113the RCU torture test. The printk()s use KERN_ALERT, so they should 114be evident. ;-) 115 116The entries are as follows: 117 118o "rtc": The hexadecimal address of the structure currently visible 119 to readers. 120 121o "ver": The number of times since boot that the rcutw writer task 122 has changed the structure visible to readers. 123 124o "tfle": If non-zero, indicates that the "torture freelist" 125 containing structure to be placed into the "rtc" area is empty. 126 This condition is important, since it can fool you into thinking 127 that RCU is working when it is not. :-/ 128 129o "rta": Number of structures allocated from the torture freelist. 130 131o "rtaf": Number of allocations from the torture freelist that have 132 failed due to the list being empty. 133 134o "rtf": Number of frees into the torture freelist. 135 136o "Reader Pipe": Histogram of "ages" of structures seen by readers. 137 If any entries past the first two are non-zero, RCU is broken. 138 And rcutorture prints the error flag string "!!!" to make sure 139 you notice. The age of a newly allocated structure is zero, 140 it becomes one when removed from reader visibility, and is 141 incremented once per grace period subsequently -- and is freed 142 after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. 143 144 The output displayed above was taken from a correctly working 145 RCU. If you want to see what it looks like when broken, break 146 it yourself. ;-) 147 148o "Reader Batch": Another histogram of "ages" of structures seen 149 by readers, but in terms of counter flips (or batches) rather 150 than in terms of grace periods. The legal number of non-zero 151 entries is again two. The reason for this separate view is that 152 it is sometimes easier to get the third entry to show up in the 153 "Reader Batch" list than in the "Reader Pipe" list. 154 155o "Free-Block Circulation": Shows the number of torture structures 156 that have reached a given point in the pipeline. The first element 157 should closely correspond to the number of structures allocated, 158 the second to the number that have been removed from reader view, 159 and all but the last remaining to the corresponding number of 160 passes through a grace period. The last entry should be zero, 161 as it is only incremented if a torture structure's counter 162 somehow gets incremented farther than it should. 163 164Different implementations of RCU can provide implementation-specific 165additional information. For example, SRCU provides the following: 166 167 srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0 168 srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0 169 srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0 170 srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0 171 srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) 172 173The first four lines are similar to those for RCU. The last line shows 174the per-CPU counter state. The numbers in parentheses are the values 175of the "old" and "current" counters for the corresponding CPU. The 176"idx" value maps the "old" and "current" values to the underlying array, 177and is useful for debugging. 178 179Similarly, sched_expedited RCU provides the following: 180 181 sched_expedited-torture: rtc: d0000000016c1880 ver: 1090796 tfle: 0 rta: 1090796 rtaf: 0 rtf: 1090787 rtmbe: 0 nt: 27713319 182 sched_expedited-torture: Reader Pipe: 12660320201 95875 0 0 0 0 0 0 0 0 0 183 sched_expedited-torture: Reader Batch: 12660424885 0 0 0 0 0 0 0 0 0 0 184 sched_expedited-torture: Free-Block Circulation: 1090795 1090795 1090794 1090793 1090792 1090791 1090790 1090789 1090788 1090787 0 185 186 187USAGE 188 189The following script may be used to torture RCU: 190 191 #!/bin/sh 192 193 modprobe rcutorture 194 sleep 100 195 rmmod rcutorture 196 dmesg | grep torture: 197 198The output can be manually inspected for the error flag of "!!!". 199One could of course create a more elaborate script that automatically 200checked for such errors. The "rmmod" command forces a "SUCCESS" or 201"FAILURE" indication to be printk()ed. 202