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Namhyung Kim c940fa276b 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 <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>
History 2022-10-31 11:07:48 -03:00
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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 Restructured Text 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.