Re: [PATCH] perf stat: Poll for monitored tasks being alive in fork mode

["Jin, Yao" <yao.jin@linux.intel.com>]

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
>>