200 lines
8.6 KiB
Plaintext
200 lines
8.6 KiB
Plaintext
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:
|
|
|
|
irqreaders 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.
|
|
|
|
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.
|
|
|
|
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_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: "rcu" for the rcu_read_lock() API,
|
|
"rcu_sync" for rcu_read_lock() with synchronous reclamation,
|
|
"rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for
|
|
rcu_read_lock_bh() with synchronous reclamation, "srcu" for
|
|
the "srcu_read_lock()" API, "sched" for the use of
|
|
preempt_disable() together with synchronize_sched(),
|
|
and "sched_expedited" for the use of preempt_disable()
|
|
with synchronize_sched_expedited().
|
|
|
|
verbose Enable debug printk()s. Default is disabled.
|
|
|
|
|
|
OUTPUT
|
|
|
|
The statistics output is as follows:
|
|
|
|
rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
|
|
rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
|
|
rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0
|
|
rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0
|
|
rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
|
|
rcu-torture: --- End of test
|
|
|
|
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 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 rcutw writer task
|
|
has changed the structure visible to readers.
|
|
|
|
o "tfle": If non-zero, indicates that the "torture freelist"
|
|
containing structure 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.
|
|
|
|
o "rtf": Number of frees into the torture freelist.
|
|
|
|
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:
|
|
|
|
srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0
|
|
srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0
|
|
srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0
|
|
srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0
|
|
srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
|
|
|
|
The first four lines are similar to those for RCU. The last 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.
|
|
|
|
Similarly, sched_expedited RCU provides the following:
|
|
|
|
sched_expedited-torture: rtc: d0000000016c1880 ver: 1090796 tfle: 0 rta: 1090796 rtaf: 0 rtf: 1090787 rtmbe: 0 nt: 27713319
|
|
sched_expedited-torture: Reader Pipe: 12660320201 95875 0 0 0 0 0 0 0 0 0
|
|
sched_expedited-torture: Reader Batch: 12660424885 0 0 0 0 0 0 0 0 0 0
|
|
sched_expedited-torture: Free-Block Circulation: 1090795 1090795 1090794 1090793 1090792 1090791 1090790 1090789 1090788 1090787 0
|
|
state: -1 / 0:0 3:0 4:0
|
|
|
|
As before, the first four lines are similar to those for RCU.
|
|
The last line shows the task-migration state. The first number is
|
|
-1 if synchronize_sched_expedited() is idle, -2 if in the process of
|
|
posting wakeups to the migration kthreads, and N when waiting on CPU N.
|
|
Each of the colon-separated fields following the "/" is a CPU:state pair.
|
|
Valid states are "0" for idle, "1" for waiting for quiescent state,
|
|
"2" for passed through quiescent state, and "3" when a race with a
|
|
CPU-hotplug event forces use of the synchronize_sched() primitive.
|
|
|
|
|
|
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.
|