Re: A concern about overflow ring buffer mode

Liang, Kan
2018-10-26T19:07:47Z

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@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"),
>