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Diffstat (limited to 'Documentation/RCU/torture.txt')
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diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt new file mode 100644 index 00000000..375d3fb7 --- /dev/null +++ b/Documentation/RCU/torture.txt @@ -0,0 +1,308 @@ +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. + +CONFIG_RCU_TORTURE_TEST_RUNNABLE + +It is also possible to specify CONFIG_RCU_TORTURE_TEST=y, which will +result in the tests being loaded into the base kernel. In this case, +the CONFIG_RCU_TORTURE_TEST_RUNNABLE config option is used to specify +whether the RCU torture tests are to be started immediately during +boot or whether the /proc/sys/kernel/rcutorture_runnable file is used +to enable them. This /proc file can be used to repeatedly pause and +restart the tests, regardless of the initial state specified by the +CONFIG_RCU_TORTURE_TEST_RUNNABLE config option. + +You will normally -not- want to start the RCU torture tests during boot +(and thus the default is CONFIG_RCU_TORTURE_TEST_RUNNABLE=n), but doing +this can sometimes be useful in finding boot-time bugs. + + +MODULE PARAMETERS + +This module has the following parameters: + +fqs_duration Duration (in microseconds) of artificially induced bursts + of force_quiescent_state() invocations. In RCU + implementations having force_quiescent_state(), these + bursts help force races between forcing a given grace + period and that grace period ending on its own. + +fqs_holdoff Holdoff time (in microseconds) between consecutive calls + to force_quiescent_state() within a burst. + +fqs_stutter Wait time (in seconds) between consecutive bursts + of calls to force_quiescent_state(). + +irqreader Says to invoke RCU readers from irq level. This is currently + done via timers. Defaults to "1" for variants of RCU that + permit this. (Or, more accurately, variants of RCU that do + -not- permit this know to ignore this variable.) + +nfakewriters This is the number of RCU fake writer threads to run. Fake + writer threads repeatedly use the synchronous "wait for + current readers" function of the interface selected by + torture_type, with a delay between calls to allow for various + different numbers of writers running in parallel. + nfakewriters defaults to 4, which provides enough parallelism + to trigger special cases caused by multiple writers, such as + the synchronize_srcu() early return optimization. + +nreaders This is the number of RCU reading threads supported. + The default is twice the number of CPUs. Why twice? + To properly exercise RCU implementations with preemptible + read-side critical sections. + +onoff_interval + The number of seconds between each attempt to execute a + randomly selected CPU-hotplug operation. Defaults to + zero, which disables CPU hotplugging. In HOTPLUG_CPU=n + kernels, rcutorture will silently refuse to do any + CPU-hotplug operations regardless of what value is + specified for onoff_interval. + +onoff_holdoff The number of seconds to wait until starting CPU-hotplug + operations. This would normally only be used when + rcutorture was built into the kernel and started + automatically at boot time, in which case it is useful + in order to avoid confusing boot-time code with CPUs + coming and going. + +shuffle_interval + The number of seconds to keep the test threads affinitied + to a particular subset of the CPUs, defaults to 3 seconds. + Used in conjunction with test_no_idle_hz. + +shutdown_secs The number of seconds to run the test before terminating + the test and powering off the system. The default is + zero, which disables test termination and system shutdown. + This capability is useful for automated testing. + +stall_cpu The number of seconds that a CPU should be stalled while + within both an rcu_read_lock() and a preempt_disable(). + This stall happens only once per rcutorture run. + If you need multiple stalls, use modprobe and rmmod to + repeatedly run rcutorture. The default for stall_cpu + is zero, which prevents rcutorture from stalling a CPU. + + Note that attempts to rmmod rcutorture while the stall + is ongoing will hang, so be careful what value you + choose for this module parameter! In addition, too-large + values for stall_cpu might well induce failures and + warnings in other parts of the kernel. You have been + warned! + +stall_cpu_holdoff + The number of seconds to wait after rcutorture starts + before stalling a CPU. Defaults to 10 seconds. + +stat_interval The number of seconds between output of torture + statistics (via printk()). Regardless of the interval, + statistics are printed when the module is unloaded. + Setting the interval to zero causes the statistics to + be printed -only- when the module is unloaded, and this + is the default. + +stutter The length of time to run the test before pausing for this + same period of time. Defaults to "stutter=5", so as + to run and pause for (roughly) five-second intervals. + Specifying "stutter=0" causes the test to run continuously + without pausing, which is the old default behavior. + +test_boost Whether or not to test the ability of RCU to do priority + boosting. Defaults to "test_boost=1", which performs + RCU priority-inversion testing only if the selected + RCU implementation supports priority boosting. Specifying + "test_boost=0" never performs RCU priority-inversion + testing. Specifying "test_boost=2" performs RCU + priority-inversion testing even if the selected RCU + implementation does not support RCU priority boosting, + which can be used to test rcutorture's ability to + carry out RCU priority-inversion testing. + +test_boost_interval + The number of seconds in an RCU priority-inversion test + cycle. Defaults to "test_boost_interval=7". It is + usually wise for this value to be relatively prime to + the value selected for "stutter". + +test_boost_duration + The number of seconds to do RCU priority-inversion testing + within any given "test_boost_interval". Defaults to + "test_boost_duration=4". + +test_no_idle_hz Whether or not to test the ability of RCU to operate in + a kernel that disables the scheduling-clock interrupt to + idle CPUs. Boolean parameter, "1" to test, "0" otherwise. + Defaults to omitting this test. + +torture_type The type of RCU to test, with string values as follows: + + "rcu": rcu_read_lock(), rcu_read_unlock() and call_rcu(). + + "rcu_sync": rcu_read_lock(), rcu_read_unlock(), and + synchronize_rcu(). + + "rcu_expedited": rcu_read_lock(), rcu_read_unlock(), and + synchronize_rcu_expedited(). + + "rcu_bh": rcu_read_lock_bh(), rcu_read_unlock_bh(), and + call_rcu_bh(). + + "rcu_bh_sync": rcu_read_lock_bh(), rcu_read_unlock_bh(), + and synchronize_rcu_bh(). + + "rcu_bh_expedited": rcu_read_lock_bh(), rcu_read_unlock_bh(), + and synchronize_rcu_bh_expedited(). + + "srcu": srcu_read_lock(), srcu_read_unlock() and + synchronize_srcu(). + + "srcu_expedited": srcu_read_lock(), srcu_read_unlock() and + synchronize_srcu_expedited(). + + "sched": preempt_disable(), preempt_enable(), and + call_rcu_sched(). + + "sched_sync": preempt_disable(), preempt_enable(), and + synchronize_sched(). + + "sched_expedited": preempt_disable(), preempt_enable(), and + synchronize_sched_expedited(). + + Defaults to "rcu". + +verbose Enable debug printk()s. Default is disabled. + + +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 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 "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, SRCU provides the following +additional line: + + srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) + +This line shows the per-CPU counter state. 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. + + +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. |