Re: A concern about overflow ring buffer mode

[Liang, Kan]

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)
{

Thanks,
Kan

>  From 40feb09001c7cc2ba8aeaa0a8f03b6d28fa4ca95 Mon Sep 17 00:00:00 2001
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 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@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: David Ahern <dsahern@xxxxxxxxx>
> Cc: David Miller <davem@xxxxxxxxxxxxx>
> Cc: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Kan Liang <kan.liang@xxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Wang Nan <wangnan0@xxxxxxxxxx>
> Link: https://lkml.kernel.org/n/tip-oqgsz2bq4kgrnnajrafcdhie@xxxxxxxxxxxxxx
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>   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"),