Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 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 However, actually setting this config option to "y" results in the system 14 running the test immediately upon boot, and ending only when the system 15 is taken down. Normally, one will instead want to build the system 16 with CONFIG_RCU_TORTURE_TEST=m and to use modprobe and rmmod to control 17 the test, perhaps using a script similar to the one shown at the end of 18 this document. Note that you will need CONFIG_MODULE_UNLOAD in order 19 to be able to end the test. 20 21 22 MODULE PARAMETERS 23 24 This module has the following parameters: 25 26 nreaders This is the number of RCU reading threads supported. 27 The default is twice the number of CPUs. Why twice? 28 To properly exercise RCU implementations with preemptible 29 read-side critical sections. 30 31 nfakewriters This is the number of RCU fake writer threads to run. Fake 32 writer threads repeatedly use the synchronous "wait for 33 current readers" function of the interface selected by 34 torture_type, with a delay between calls to allow for various 35 different numbers of writers running in parallel. 36 nfakewriters defaults to 4, which provides enough parallelism 37 to trigger special cases caused by multiple writers, such as 38 the synchronize_srcu() early return optimization. 39 40 stat_interval The number of seconds between output of torture 41 statistics (via printk()). Regardless of the interval, 42 statistics are printed when the module is unloaded. 43 Setting the interval to zero causes the statistics to 44 be printed -only- when the module is unloaded, and this 45 is the default. 46 47 shuffle_interval 48 The number of seconds to keep the test threads affinitied 49 to a particular subset of the CPUs, defaults to 5 seconds. 50 Used in conjunction with test_no_idle_hz. 51 52 test_no_idle_hz Whether or not to test the ability of RCU to operate in 53 a kernel that disables the scheduling-clock interrupt to 54 idle CPUs. Boolean parameter, "1" to test, "0" otherwise. 55 Defaults to omitting this test. 56 57 torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API, 58 "rcu_sync" for rcu_read_lock() with synchronous reclamation, 59 "rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for 60 rcu_read_lock_bh() with synchronous reclamation, "srcu" for 61 the "srcu_read_lock()" API, and "sched" for the use of 62 preempt_disable() together with synchronize_sched(). 63 64 verbose Enable debug printk()s. Default is disabled. 65 66 67 OUTPUT 68 69 The statistics output is as follows: 70 71 rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0 72 rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915 73 rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0 74 rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0 75 rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0 76 rcu-torture: --- End of test 77 78 The command "dmesg | grep torture:" will extract this information on 79 most systems. On more esoteric configurations, it may be necessary to 80 use other commands to access the output of the printk()s used by 81 the RCU torture test. The printk()s use KERN_ALERT, so they should 82 be evident. ;-) 83 84 The entries are as follows: 85 86 o "rtc": The hexadecimal address of the structure currently visible 87 to readers. 88 89 o "ver": The number of times since boot that the rcutw writer task 90 has changed the structure visible to readers. 91 92 o "tfle": If non-zero, indicates that the "torture freelist" 93 containing structure to be placed into the "rtc" area is empty. 94 This condition is important, since it can fool you into thinking 95 that RCU is working when it is not. :-/ 96 97 o "rta": Number of structures allocated from the torture freelist. 98 99 o "rtaf": Number of allocations from the torture freelist that have 100 failed due to the list being empty. 101 102 o "rtf": Number of frees into the torture freelist. 103 104 o "Reader Pipe": Histogram of "ages" of structures seen by readers. 105 If any entries past the first two are non-zero, RCU is broken. 106 And rcutorture prints the error flag string "!!!" to make sure 107 you notice. The age of a newly allocated structure is zero, 108 it becomes one when removed from reader visibility, and is 109 incremented once per grace period subsequently -- and is freed 110 after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. 111 112 The output displayed above was taken from a correctly working 113 RCU. If you want to see what it looks like when broken, break 114 it yourself. ;-) 115 116 o "Reader Batch": Another histogram of "ages" of structures seen 117 by readers, but in terms of counter flips (or batches) rather 118 than in terms of grace periods. The legal number of non-zero 119 entries is again two. The reason for this separate view is that 120 it is sometimes easier to get the third entry to show up in the 121 "Reader Batch" list than in the "Reader Pipe" list. 122 123 o "Free-Block Circulation": Shows the number of torture structures 124 that have reached a given point in the pipeline. The first element 125 should closely correspond to the number of structures allocated, 126 the second to the number that have been removed from reader view, 127 and all but the last remaining to the corresponding number of 128 passes through a grace period. The last entry should be zero, 129 as it is only incremented if a torture structure's counter 130 somehow gets incremented farther than it should. 131 132 Different implementations of RCU can provide implementation-specific 133 additional information. For example, SRCU provides the following: 134 135 srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0 136 srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0 137 srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0 138 srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0 139 srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) 140 141 The first four lines are similar to those for RCU. The last line shows 142 the per-CPU counter state. The numbers in parentheses are the values 143 of the "old" and "current" counters for the corresponding CPU. The 144 "idx" value maps the "old" and "current" values to the underlying array, 145 and is useful for debugging. 146 147 148 USAGE 149 150 The following script may be used to torture RCU: 151 152 #!/bin/sh 153 154 modprobe rcutorture 155 sleep 100 156 rmmod rcutorture 157 dmesg | grep torture: 158 159 The output can be manually inspected for the error flag of "!!!". 160 One could of course create a more elaborate script that automatically 161 checked for such errors. The "rmmod" command forces a "SUCCESS" or 162 "FAILURE" indication to be printk()ed.