perf lock contention: Increase default stack skip to 4

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 <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

0 files changed, 0 insertions, 0 deletions