linux-kernel - Re: [PATCH 03/10] perf sched timehist: Handle zero sample->tid properly

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Date:   Thu, 8 Dec 2016 23:19:05 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <andi@...stfloor.org>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 03/10] perf sched timehist: Handle zero sample->tid
 properly

On Wed, Dec 07, 2016 at 11:06:29AM +0900, Namhyung Kim wrote:
> On Mon, Dec 05, 2016 at 07:52:57PM -0800, David Ahern wrote:
> > On 12/5/16 7:40 PM, Namhyung Kim wrote:
> > > Sometimes samples have tid of 0 but non-0 pid.  It ends up having a
> > 
> > Any idea how that happens?
> 
> It seems that an exiting task wakes up its parent and the parent might
> call wait(2) concurrently.  So at the time of calling last schedule(),
> its pid (and tgid) link might be unhashed by the parent and can have 0
> sample->tid and/or sample->pid depending on timing IMHO.  Not sure
> anything guarantees that the sample tid/pid is preserved during the
> event.  From a quick look I couldn't find..

I found following line in my data file.

  $ perf script --time 52460.536907,
  swapper     0 [000] 52460.536907:       sched:sched_switch: pool:3775 [120] x ==> at-spi2-registr:1961 [120]
                    7f3963 __schedule (/lib/modules/4.8.3-1-ARCH/build/vmlinux)
                    7f3d7c schedule (/lib/modules/4.8.3-1-ARCH/build/vmlinux)
                    280b14 do_exit (/lib/modules/4.8.3-1-ARCH/build/vmlinux)
                    280fa7 [unknown] (/lib/modules/4.8.3-1-ARCH/build/vmlinux)
                    7f7cf2 entry_SYSCALL_64_fastpath (/lib/modules/4.8.3-1-ARCH/build/vmlinux)
  ...

As you can see task 3775 called schedule() during exit but sample tid
was 0.  In this case prev_pid is still correct (though it's always pid
in the root namespace) since its number was saved in task_struct at the
time of creation and not changed.

I'll change to check prev_pid for idle task.

Thanks,
Namhyung