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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 "rcutorture"). The test is started
when the module is loaded, and stops when the module is unloaded.
However, actually setting this config option to "y" results in the system
running the test immediately upon boot, and ending only when the system
is taken down. Normally, one will instead want to build the system
with CONFIG_RCU_TORTURE_TEST=m and to use modprobe and rmmod to control
the test, perhaps using a script similar to the one shown at the end of
this document. Note that you will need CONFIG_MODULE_UNLOAD in order
to be able to end the test.
MODULE PARAMETERS
This module has the following parameters:
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.
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.
verbose Enable debug printk()s. Default is disabled.
OUTPUT
The statistics output is as follows:
rcutorture: --- Start of test: nreaders=16 stat_interval=0 verbose=0
rcutorture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915
rcutorture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0
rcutorture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0
rcutorture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0
rcutorture: --- End of test
The command "dmesg | grep rcutorture:" 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 "ggp": The number of counter flips (or batches) since boot.
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 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.
USAGE
The following script may be used to torture RCU:
#!/bin/sh
modprobe rcutorture
sleep 100
rmmod rcutorture
dmesg | grep rcutorture:
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.
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