Re: A concern about overflow ring buffer mode

Arnaldo Carvalho de Melo
2018-10-26T19:12:37Z

Em Fri, Oct 26, 2018 at 03:07:40PM -0400, Liang, Kan escreveu:
> On 10/26/2018 3:02 PM, Arnaldo Carvalho de Melo wrote:
> > So, I'm adding the following to my tree to help in diagnosing problems
> > with this overwrite mode:

> Actually, you can use per-event overwrite term to disable overwrite mode for
> perf top.

> /*
> * Check per-event overwrite term.
> * perf top should support consistent term for all events.
> * - All events don't have per-event term
> * E.g. "cpu/cpu-cycles/,cpu/instructions/"
> * Nothing change, return 0.
> * - All events have same per-event term
> * E.g. "cpu/cpu-cycles,no-overwrite/,cpu/instructions,no-overwrite/
> * Using the per-event setting to replace the opts->overwrite if
> * they are different, then return 0.
> * - Events have different per-event term
> * E.g. "cpu/cpu-cycles,overwrite/,cpu/instructions,no-overwrite/"
> * Return -1
> * - Some of the event set per-event term, but some not.
> * E.g. "cpu/cpu-cycles/,cpu/instructions,no-overwrite/"
> * Return -1
> */
> static int perf_top__overwrite_check(struct perf_top *top)
> {

I see, it will disable that opts->overwrite if it finds the no-overwrite
in the per-event definition, so the equivalent of the option I added
below:

perf top --no-overwrite

is:

perf top -e cycles/no-overwrite/

I checked and both have the same result. But I still think there is
value in having the shorter form, ok?

- Arnaldo

> Thanks,
> Kan
>
> >
> > From 40feb09001c7cc2ba8aeaa0a8f03b6d28fa4ca95 Mon Sep 17 00:00:00 2001
> > From: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Date: Fri, 26 Oct 2018 15:55:23 -0300
> > Subject: [PATCH 1/1] perf top: Allow disabling the overwrite mode
> >
> > In ebebbf082357 ("perf top: Switch default mode to overwrite mode") we
> > forgot to leave a way to disable that new default, add a --overwrite
> > option that can be disabled using --no-overwrite, since the code already
> > in such a way that we can readily disable this mode.
> >
> > This is useful when investigating bugs with this mode like the recent
> > report from David Miller where lots of unknown symbols appear due to
> > disabling the events while processing them which disables all record
> > types, not just PERF_RECORD_SAMPLE, which makes it impossible to resolve
> > maps when we lose PERF_RECORD_MMAP records.
> >
> > This can be easily seen while building a kernel, when there are lots of
> > short lived processes.
> >
> > Cc: Adrian Hunter <adrian.hunter@intel.com>
> > Cc: Andi Kleen <ak@linux.intel.com>
> > Cc: David Ahern <dsahern@gmail.com>
> > Cc: David Miller <davem@davemloft.net>
> > Cc: Jin Yao <yao.jin@linux.intel.com>
> > Cc: Jiri Olsa <jolsa@kernel.org>
> > Cc: Kan Liang <kan.liang@intel.com>
> > Cc: Namhyung Kim <namhyung@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Wang Nan <wangnan0@huawei.com>
> > Link: https://lkml.kernel.org/n/tip-oqgsz2bq4kgrnnajrafcdhie@git.kernel.org
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> > ---
> > tools/perf/Documentation/perf-top.txt | 5 +++++
> > tools/perf/builtin-top.c | 2 ++
> > 2 files changed, 7 insertions(+)
> >
> > diff --git a/tools/perf/Documentation/perf-top.txt b/tools/perf/Documentation/perf-top.txt
> > index 114fda12aa49..d4be6061fe1c 100644
> > --- a/tools/perf/Documentation/perf-top.txt
> > +++ b/tools/perf/Documentation/perf-top.txt
> > @@ -242,6 +242,11 @@ Default is to monitor all CPUS.
> > --hierarchy::
> > Enable hierarchy output.
> > +--overwrite::
> > + This is the default, but for investigating problems with it or any other strange
> > + behaviour like lots of unknown samples, we may want to disable this mode by using
> > + --no-overwrite.
> > +
> > --force::
> > Don't do ownership validation.
> > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> > index d21d8751e749..214fad747b04 100644
> > --- a/tools/perf/builtin-top.c
> > +++ b/tools/perf/builtin-top.c
> > @@ -1372,6 +1372,8 @@ int cmd_top(int argc, const char **argv)
> > "Show raw trace event output (do not use print fmt or plugins)"),
> > OPT_BOOLEAN(0, "hierarchy", &symbol_conf.report_hierarchy,
> > "Show entries in a hierarchy"),
> > + OPT_BOOLEAN(0, "overwrite", &top.record_opts.overwrite,
> > + "Use a backward ring buffer, default: yes"),
> > OPT_BOOLEAN(0, "force", &symbol_conf.force, "don't complain, do it"),
> > OPT_UINTEGER(0, "num-thread-synthesize", &top.nr_threads_synthesize,
> > "number of thread to run event synthesize"),
> >