[PATCH 4/4] perf lock contention: Increase default stack skip to 4

[Namhyung Kim]

In most configurations, it works well with skipping 4 entries by
default.  If some systems still have 3 BPF internal stack frames,
the next frame should be in a lock function which will be skipped
later when it tries to find a caller.  So increasing to 4 won't
affect such systems too.

With --stack-skip=0, I can see something like this:

    24     49.84 us      7.41 us      2.08 us        mutex   bpf_prog_e1b85959d520446c_contention_begin+0x12e
                    0xffffffffc045040e  bpf_prog_e1b85959d520446c_contention_begin+0x12e
                    0xffffffffc045040e  bpf_prog_e1b85959d520446c_contention_begin+0x12e
                    0xffffffff82ea2071  bpf_trace_run2+0x51
                    0xffffffff82de775b  __bpf_trace_contention_begin+0xb
                    0xffffffff82c02045  __mutex_lock+0x245
                    0xffffffff82c019e3  __mutex_lock_slowpath+0x13
                    0xffffffff82c019c0  mutex_lock+0x20
                    0xffffffff830a083c  kernfs_iop_permission+0x2c

Cc: Song Liu <song@xxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 tools/perf/util/lock-contention.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/lock-contention.h b/tools/perf/util/lock-contention.h
index b8cb8830b7bc..e3c061b1795b 100644
--- a/tools/perf/util/lock-contention.h
+++ b/tools/perf/util/lock-contention.h
@@ -91,7 +91,7 @@ struct thread_stat {
  * Number of stack trace entries to skip when finding callers.
  * The first few entries belong to the locking implementation itself.
  */
-#define CONTENTION_STACK_SKIP  3
+#define CONTENTION_STACK_SKIP  4
 
 /*
  * flags for lock:contention_begin
-- 
2.38.1.273.g43a17bfeac-goog