linux-kernel - Re: A concern about overflow ring buffer mode

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Date:   Fri, 26 Oct 2018 16:12:31 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
        Wang Nan <wangnan0@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Kan Liang <kan.liang@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: A concern about overflow ring buffer mode

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@...hat.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@...el.com>
> > Cc: Andi Kleen <ak@...ux.intel.com>
> > Cc: David Ahern <dsahern@...il.com>
> > Cc: David Miller <davem@...emloft.net>
> > Cc: Jin Yao <yao.jin@...ux.intel.com>
> > Cc: Jiri Olsa <jolsa@...nel.org>
> > Cc: Kan Liang <kan.liang@...el.com>
> > Cc: Namhyung Kim <namhyung@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Wang Nan <wangnan0@...wei.com>
> > Link: https://lkml.kernel.org/n/tip-oqgsz2bq4kgrnnajrafcdhie@git.kernel.org
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.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"),
> >