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SubjectRe: [PATCH] perf stat: Poll for monitored tasks being alive in fork mode
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On 1/6/2019 9:25 PM, Jiri Olsa wrote:
> On Sat, Jan 05, 2019 at 11:16:40AM +0800, Jin, Yao wrote:
>>
>>
>> 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.
>
> ah right, -1 is special.. however that can still happen also
> in the orginal patch.. how about we do something like below
>
> jirka
>
>
> ---
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index acfd48db52dd..c322cb271180 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -583,6 +583,14 @@ 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;
>

Hi Jiri,

I think your patch is good. At least, we can avoid the case of kill(-1,
SIGTERM).

BTW, you post this patch or I re-post it, both fine for me. :)

Thanks
Jin Yao

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