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(). gp_normal Make the fake writers use normal synchronous grace-period primitives. gp_exp Make the fake writers use expedited synchronous grace-period primitives. If both gp_normal and gp_exp are set, or if neither gp_normal nor gp_exp are set, then randomly choose the primitive so that about 50% are normal and 50% expedited. By default, neither are set, which gives best overall test coverage. 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.) n_barrier_cbs If this is nonzero, RCU barrier testing will be conducted, in which case n_barrier_cbs specifies the number of RCU callbacks (and corresponding kthreads) to use for this testing. The value cannot be negative. If you specify this to be non-zero when torture_type indicates a synchronous RCU implementation (one for which a member of the synchronize_rcu() rather than the call_rcu() family is used -- see the documentation for torture_type below), an error will be reported and no testing will be carried out. 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 call_srcu(). "srcu_sync": 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 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, 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.