linux-kernel - Re: BUG: perf top enters loop synthesizing events for existing threads

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Date:	Wed, 8 Apr 2015 10:45:45 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Don Zickus <dzickus@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
	Joe Mario <jmario@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: BUG: perf top enters loop synthesizing events for existing
 threads

Em Wed, Apr 08, 2015 at 10:42:51AM -0300, Arnaldo Carvalho de Melo escreveu:
> Heads up,
> 
> 	While testing some unrelated patches I noticed, in one of my
> machines, that 'perf top', as root, is entering some sort of loop and
> ends up consuming a lot of memory, making the system unresponsive,
> bisected it down to:
> 
> [acme@zoo linux]$ git bisect good
> ca6c41c59b964d362823e80442e9e32c31106b29 is the first bad commit
> commit ca6c41c59b964d362823e80442e9e32c31106b29
> Author: David Ahern <dsahern@...il.com>
> Date:   Mon Mar 30 14:35:58 2015 -0600
> 
>     perf tools: Fix ppid for synthesized fork events
>     
>     363b785f38 added synthesized fork events and set a thread's parent id to
>     itself. Since we are already processing /proc/<pid>/status the ppid can
>     be determined properly. Make it so.
>     
>     Signed-off-by: David Ahern <dsahern@...il.com>
>     Acked-by: Don Zickus <dzickus@...hat.com>
>     Acked-by: Jiri Olsa <jolsa@...nel.org>
>     Cc: Joe Mario <jmario@...hat.com>
>     Link: http://lkml.kernel.org/r/1427747758-18510-2-git-send-email-dsahern@gmail.com
>     Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> 
> :040000 040000 24d479e4afbe95aaba882850473a870b6b3c7e87
> cc86fe0e51b5e8a66e8a23f934fd0c4ff0f8dcd6 M	tools
> [acme@zoo linux]$
> 
> 
> I also noticed that if I set /proc/sys/kernel/perf_event_paranoid to -1
> to run it as !root, then the problem "goes away", which I think probably
> is explained by, as !root, not being able to parse some of the /proc
> files for existing threads and thus not triggering the bug, still
> investigating...
> 
> I might need to revert it for this cycle...

Just reverting it makes the symptoms go away, to works as before. Trying
to understand the problem now.

- Arnaldo
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