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
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20221028180128.3311491-5-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
* 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