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.

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.

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 100
        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" or
"FAILURE" indication to be printk()ed.