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

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Date:	Wed, 8 Apr 2015 10:42:51 -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: BUG: perf top enters loop synthesizing events for existing threads

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...

- Arnaldo
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