RCU Torture Test Operation CONFIG_RCU_TORTURE_TEST The CONFIG_RCU_TORTURE_TEST config option is available for all RCU implementations. It creates an rcutorture kernel module that can be loaded to run a torture test. The test periodically outputs status messages via printk(), which can be examined via the dmesg command (perhaps grepping for "torture"). The test is started when the module is loaded, and stops when the module is unloaded. Module parameters are prefixed by "rcutorture." in Documentation/admin-guide/kernel-parameters.txt. OUTPUT The statistics output is as follows: 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 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 rcu-torture: Reader Pipe: 727860534 34213 0 0 0 0 0 0 0 0 0 rcu-torture: Reader Batch: 727877838 17003 0 0 0 0 0 0 0 0 0 rcu-torture: Free-Block Circulation: 155440 155440 155440 155440 155440 155440 155440 155440 155440 155440 0 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 The command "dmesg | grep torture:" will extract this information on most systems. On more esoteric configurations, it may be necessary to use other commands to access the output of the printk()s used by the RCU torture test. The printk()s use KERN_ALERT, so they should be evident. ;-) The first and last lines show the rcutorture module parameters, and the last line shows either "SUCCESS" or "FAILURE", based on rcutorture's automatic determination as to whether RCU operated correctly. The entries are as follows: o "rtc": The hexadecimal address of the structure currently visible to readers. o "ver": The number of times since boot that the RCU writer task has changed the structure visible to readers. o "tfle": If non-zero, indicates that the "torture freelist" containing structures to be placed into the "rtc" area is empty. This condition is important, since it can fool you into thinking that RCU is working when it is not. :-/ o "rta": Number of structures allocated from the torture freelist. o "rtaf": Number of allocations from the torture freelist that have failed due to the list being empty. It is not unusual for this to be non-zero, but it is bad for it to be a large fraction of the value indicated by "rta". o "rtf": Number of frees into the torture freelist. o "rtmbe": A non-zero value indicates that rcutorture believes that rcu_assign_pointer() and rcu_dereference() are not working correctly. This value should be zero. o "rtbe": A non-zero value indicates that one of the rcu_barrier() family of functions is not working correctly. o "rtbke": rcutorture was unable to create the real-time kthreads used to force RCU priority inversion. This value should be zero. o "rtbre": Although rcutorture successfully created the kthreads used to force RCU priority inversion, it was unable to set them to the real-time priority level of 1. This value should be zero. o "rtbf": The number of times that RCU priority boosting failed to resolve RCU priority inversion. o "rtb": The number of times that rcutorture attempted to force an RCU priority inversion condition. If you are testing RCU priority boosting via the "test_boost" module parameter, this value should be non-zero. o "nt": The number of times rcutorture ran RCU read-side code from within a timer handler. This value should be non-zero only if you specified the "irqreader" module parameter. o "Reader Pipe": Histogram of "ages" of structures seen by readers. If any entries past the first two are non-zero, RCU is broken. And rcutorture prints the error flag string "!!!" to make sure you notice. The age of a newly allocated structure is zero, it becomes one when removed from reader visibility, and is incremented once per grace period subsequently -- and is freed after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. The output displayed above was taken from a correctly working RCU. If you want to see what it looks like when broken, break it yourself. ;-) o "Reader Batch": Another histogram of "ages" of structures seen by readers, but in terms of counter flips (or batches) rather than in terms of grace periods. The legal number of non-zero entries is again two. The reason for this separate view is that it is sometimes easier to get the third entry to show up in the "Reader Batch" list than in the "Reader Pipe" list. o "Free-Block Circulation": Shows the number of torture structures that have reached a given point in the pipeline. The first element should closely correspond to the number of structures allocated, the second to the number that have been removed from reader view, and all but the last remaining to the corresponding number of passes through a grace period. The last entry should be zero, as it is only incremented if a torture structure's counter somehow gets incremented farther than it should. Different implementations of RCU can provide implementation-specific additional information. For example, Tree SRCU provides the following additional line: 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) This line shows the per-CPU counter state, in this case for Tree SRCU using a dynamically allocated srcu_struct (hence "srcud-" rather than "srcu-"). The numbers in parentheses are the values of the "old" and "current" counters for the corresponding CPU. The "idx" value maps the "old" and "current" values to the underlying array, and is useful for debugging. The final "T" entry contains the totals of the counters. USAGE The following script may be used to torture RCU: #!/bin/sh modprobe rcutorture sleep 3600 rmmod rcutorture dmesg | grep torture: The output can be manually inspected for the error flag of "!!!". One could of course create a more elaborate script that automatically checked for such errors. The "rmmod" command forces a "SUCCESS", "FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first two are self-explanatory, while the last indicates that while there were no RCU failures, CPU-hotplug problems were detected. However, the tools/testing/selftests/rcutorture/bin/kvm.sh script provides better automation, including automatic failure analysis. It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd. See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions on setting up such an initrd.