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SubjectRe: [PATCH] perf stat: Poll for monitored tasks being alive in fork mode
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On 1/4/2019 8:54 PM, Jiri Olsa wrote:
> On Fri, Jan 04, 2019 at 10:28:17AM +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.
>>
>> Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
>> ---
>> tools/perf/builtin-stat.c | 7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
>> index 63a3afc..71f3bc8 100644
>> --- a/tools/perf/builtin-stat.c
>> +++ b/tools/perf/builtin-stat.c
>> @@ -553,6 +553,13 @@ 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) &&
>> + (child_pid != -1)) {
>
> do we need that child_pid check? we just returned from waitpid
> so we should be ok.. we just make the race window smaller
>
> could we just do:
>
> if (!is_target_alive(&target, evsel_list->threads)) {
> kill(child_pid, SIGTERM);
> break;
> }
>

I think this code should be OK and I have tested yet. I have a question
about the race condition, we really don't need a lock to protect the
child_pid?

skip_signal()
{
/*
* render child_pid harmless
* won't send SIGTERM to a random
* process in case of race condition
* and fast PID recycling
*/
child_pid = -1;
}

__run_perf_stat()
{
....
kill(child_pid, SIGTERM);
}

If child_pid is set by -1 in a small window between checking of
child_pid and kill(), then kill(-1, SIGTERM) may happen. All processes
except the kill process itself and init would receive SIGTERM.

Is this case possible?

> also I'm not sure we should do this only under new option,
> as it might break people's scripts.. thoughts?
>
> jirka
>

In current behavior, for non fork mode, if we terminate the monitored
task, the perf stat would return immediately. So I think this patch
should be OK.

Thanks
Jin Yao

>> + kill(child_pid, SIGTERM);
>> + break;
>> + }
>> +
>> nanosleep(&ts, NULL);
>> if (timeout)
>> break;
>> --
>> 2.7.4
>>

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