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

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Message-ID: <20190107090522.GA3383@krava>
Date:   Mon, 7 Jan 2019 10:05:22 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Jin Yao <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, 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

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
>