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

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

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