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c42e219942
This is to slow down lock acquistion (on contention locks) deliberately.
A possible use case is to estimate impact on application performance by
optimization of kernel locking behavior. By delaying the lock it can
simulate the worse condition as a control group, and then compare with
the current behavior as a optimized condition.
The syntax is 'time@function' and the time can have unit suffix like
"us" and "ms". For example, I ran a simple test like below.
$ sudo perf lock con -abl -L tasklist_lock -- \
sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
92 1.18 ms 199.54 us 12.79 us ffffffff8a806080 tasklist_lock (rwlock)
The contention count was 92 and the average wait time was around 10 us.
But if I add 100 usec of delay to the tasklist_lock,
$ sudo perf lock con -abl -L tasklist_lock -J 100us@tasklist_lock -- \
sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
190 15.67 ms 230.10 us 82.46 us ffffffff8a806080 tasklist_lock (rwlock)
The contention count increased and the average wait time was up closed
to 100 usec. If I increase the delay even more,
$ sudo perf lock con -abl -L tasklist_lock -J 1ms@tasklist_lock -- \
sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
1002 2.80 s 3.01 ms 2.80 ms ffffffff8a806080 tasklist_lock (rwlock)
Now every sleep process had contention and the wait time was more than 1
msec. This is on my 4 CPU laptop so I guess one CPU has the lock while
other 3 are waiting for it mostly.
For simplicity, it only supports global locks for now.
Committer testing:
root@number:~# grep -m1 'model name' /proc/cpuinfo
model name : AMD Ryzen 9 9950X3D 16-Core Processor
root@number:~# perf lock con -abl -L tasklist_lock -- sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
142 453.85 us 25.39 us 3.20 us ffffffffae808080 tasklist_lock (rwlock)
root@number:~# perf lock con -abl -L tasklist_lock -J 100us@tasklist_lock -- sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
1040 2.39 s 3.11 ms 2.30 ms ffffffffae808080 tasklist_lock (rwlock)
root@number:~# perf lock con -abl -L tasklist_lock -J 1ms@tasklist_lock -- sh -c 'for i in $(seq 1000); do sleep 1 & done; wait'
contended total wait max wait avg wait address symbol
1025 24.72 s 31.01 ms 24.12 ms ffffffffae808080 tasklist_lock (rwlock)
root@number:~#
Suggested-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250509171950.183591-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.