Re: A concern about overflow ring buffer mode

["Liang, Kan" <kan.liang@linux.intel.com>]

On 10/26/2018 3:12 PM, Arnaldo Carvalho de Melo wrote:
> 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?
>

Sure.

Thanks,
Kan

> - 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"),
>>>