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 13Module parameters are prefixed by "rcutorture." in 14Documentation/admin-guide/kernel-parameters.txt. 15 16OUTPUT 17 18The statistics output is as follows: 19 20 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 21 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 22 rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 23 rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 24 rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 25 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 26 27The command "dmesg | grep torture:" will extract this information on 28most systems. On more esoteric configurations, it may be necessary to 29use other commands to access the output of the printk()s used by 30the RCU torture test. The printk()s use KERN_ALERT, so they should 31be evident. ;-) 32 33The first and last lines show the rcutorture module parameters, and the 34last line shows either "SUCCESS" or "FAILURE", based on rcutorture's 35automatic determination as to whether RCU operated correctly. 36 37The entries are as follows: 38 39o "rtc": The hexadecimal address of the structure currently visible 40 to readers. 41 42o "ver": The number of times since boot that the RCU writer task 43 has changed the structure visible to readers. 44 45o "tfle": If non-zero, indicates that the "torture freelist" 46 containing structures to be placed into the "rtc" area is empty. 47 This condition is important, since it can fool you into thinking 48 that RCU is working when it is not. :-/ 49 50o "rta": Number of structures allocated from the torture freelist. 51 52o "rtaf": Number of allocations from the torture freelist that have 53 failed due to the list being empty. It is not unusual for this 54 to be non-zero, but it is bad for it to be a large fraction of 55 the value indicated by "rta". 56 57o "rtf": Number of frees into the torture freelist. 58 59o "rtmbe": A non-zero value indicates that rcutorture believes that 60 rcu_assign_pointer() and rcu_dereference() are not working 61 correctly. This value should be zero. 62 63o "rtbe": A non-zero value indicates that one of the rcu_barrier() 64 family of functions is not working correctly. 65 66o "rtbke": rcutorture was unable to create the real-time kthreads 67 used to force RCU priority inversion. This value should be zero. 68 69o "rtbre": Although rcutorture successfully created the kthreads 70 used to force RCU priority inversion, it was unable to set them 71 to the real-time priority level of 1. This value should be zero. 72 73o "rtbf": The number of times that RCU priority boosting failed 74 to resolve RCU priority inversion. 75 76o "rtb": The number of times that rcutorture attempted to force 77 an RCU priority inversion condition. If you are testing RCU 78 priority boosting via the "test_boost" module parameter, this 79 value should be non-zero. 80 81o "nt": The number of times rcutorture ran RCU read-side code from 82 within a timer handler. This value should be non-zero only 83 if you specified the "irqreader" module parameter. 84 85o "Reader Pipe": Histogram of "ages" of structures seen by readers. 86 If any entries past the first two are non-zero, RCU is broken. 87 And rcutorture prints the error flag string "!!!" to make sure 88 you notice. The age of a newly allocated structure is zero, 89 it becomes one when removed from reader visibility, and is 90 incremented once per grace period subsequently -- and is freed 91 after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. 92 93 The output displayed above was taken from a correctly working 94 RCU. If you want to see what it looks like when broken, break 95 it yourself. ;-) 96 97o "Reader Batch": Another histogram of "ages" of structures seen 98 by readers, but in terms of counter flips (or batches) rather 99 than in terms of grace periods. The legal number of non-zero 100 entries is again two. The reason for this separate view is that 101 it is sometimes easier to get the third entry to show up in the 102 "Reader Batch" list than in the "Reader Pipe" list. 103 104o "Free-Block Circulation": Shows the number of torture structures 105 that have reached a given point in the pipeline. The first element 106 should closely correspond to the number of structures allocated, 107 the second to the number that have been removed from reader view, 108 and all but the last remaining to the corresponding number of 109 passes through a grace period. The last entry should be zero, 110 as it is only incremented if a torture structure's counter 111 somehow gets incremented farther than it should. 112 113Different implementations of RCU can provide implementation-specific 114additional information. For example, Tree SRCU provides the following 115additional line: 116 117 srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6) 118 119This line shows the per-CPU counter state, in this case for Tree SRCU 120using a dynamically allocated srcu_struct (hence "srcud-" rather than 121"srcu-"). The numbers in parentheses are the values of the "old" and 122"current" counters for the corresponding CPU. The "idx" value maps the 123"old" and "current" values to the underlying array, and is useful for 124debugging. The final "T" entry contains the totals of the counters. 125 126 127USAGE 128 129The following script may be used to torture RCU: 130 131 #!/bin/sh 132 133 modprobe rcutorture 134 sleep 3600 135 rmmod rcutorture 136 dmesg | grep torture: 137 138The output can be manually inspected for the error flag of "!!!". 139One could of course create a more elaborate script that automatically 140checked for such errors. The "rmmod" command forces a "SUCCESS", 141"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first 142two are self-explanatory, while the last indicates that while there 143were no RCU failures, CPU-hotplug problems were detected. 144 145However, the tools/testing/selftests/rcutorture/bin/kvm.sh script 146provides better automation, including automatic failure analysis. 147It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd. 148See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions 149on setting up such an initrd. 150
