linux-kernel - Re: [PATCH v2] Poll for monitored tasks being alive in fork mode

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Date:   Mon, 21 Jan 2019 09:40:45 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>, acme@...nel.org
Cc:     jolsa@...nel.org, peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v2] Poll for monitored tasks being alive in fork mode

Hi Arnaldo,

Is this patch acceptable?

Thanks
Jin Yao

On 1/7/2019 5:05 PM, Jiri Olsa wrote:
> On Mon, Jan 07, 2019 at 09:00:50AM +0800, Jin Yao wrote:
>> Following test shows the stat keeps running even if no longer
>> task to monitor (mgen exits at ~5s).
>>
>> perf stat -e cycles -p `pgrep mgen` -I1000 -- sleep 10
>>              time             counts unit events
>>       1.000148916      1,308,365,864      cycles
>>       2.000379171      1,297,269,875      cycles
>>       3.000556719      1,297,187,078      cycles
>>       4.000914241        761,261,827      cycles
>>       5.001306091      <not counted>      cycles
>>       6.001676881      <not counted>      cycles
>>       7.002046336      <not counted>      cycles
>>       8.002405651      <not counted>      cycles
>>       9.002766625      <not counted>      cycles
>>      10.001395827      <not counted>      cycles
>>
>> We'd better finish stat immediately if there's no longer task to
>> monitor.
>>
>> After:
>>
>> perf stat -e cycles -p `pgrep mgen` -I1000 -- sleep 10
>>              time             counts unit events
>>       1.000180062      1,236,592,661      cycles
>>       2.000421539      1,223,733,572      cycles
>>       3.000609910      1,297,047,663      cycles
>>       4.000807545      1,297,215,816      cycles
>>       5.001001578      1,297,208,032      cycles
>>       6.001390345        582,343,659      cycles
>> sleep: Terminated
>>
>> Now the stat exits immediately when the monitored tasks ends.
>>
>> v2:
>> ---
>> There is a potential race condition window, that is the
>> child_pid set with -1 and kill(-1, SIGTERM) may happen.
>> Jiri provides a simple method which stores the child_pid to
>> a local variable and check the local variable before kill().
>> At least, we can avoid the kill(-1, SIGTERM) case.
>>
>> Signed-off-by: Jin Yao <yao.jin@...ux.intel.com>
> 
> Reviewed-by: Jiri Olsa <jolsa@...nel.org>
> 
> thanks,
> jirka
> 
>> ---
>>   tools/perf/builtin-stat.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
>> index 63a3afc..bd5fb8e 100644
>> --- a/tools/perf/builtin-stat.c
>> +++ b/tools/perf/builtin-stat.c
>> @@ -553,6 +553,15 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
>>   
>>   		if (interval || timeout) {
>>   			while (!waitpid(child_pid, &status, WNOHANG)) {
>> +				if (!is_target_alive(&target,
>> +					evsel_list->threads)) {
>> +					int pid = child_pid;
>> +
>> +					if (pid != -1)
>> +						kill(pid, SIGTERM);
>> +					break;
>> +				}
>> +
>>   				nanosleep(&ts, NULL);
>>   				if (timeout)
>>   					break;
>> -- 
>> 2.7.4
>>