perf lock contention: Add -J/--inject-delay option

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>

1 files changed, 11 insertions, 0 deletions

diff --git a/tools/perf/Documentation/perf-lock.txt b/tools/perf/Documentation/perf-lock.txt
index 859dc11a7372..2d9aecf63042 100644
--- a/tools/perf/Documentation/perf-lock.txt
+++ b/tools/perf/Documentation/perf-lock.txt
@@ -216,6 +216,17 @@ CONTENTION OPTIONS
 --cgroup-filter=<value>::
 	Show lock contention only in the given cgroups (comma separated list).
 
+-J::
+--inject-delay=<time@function>::
+	Add delays to the given lock.  It's added to the contention-end part so
+	that the (new) owner of the lock will be delayed.  But by slowing down
+	the owner, the waiters will also be delayed as well.  This is working
+	only with -b/--use-bpf.
+
+	The 'time' is specified in nsec but it can have a unit suffix.  Available
+	units are "ms" and "us".  Note that it will busy-wait after it gets the
+	lock.  Please use it at your own risk.
+
 
 SEE ALSO
 --------