linux-kernel - Re: [PATCH] perf: Prevent buildid collection running forever

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Date:	Thu, 2 Dec 2010 23:32:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	LKML <linux-kernel@...r.kernel.org>
cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] perf: Prevent buildid collection running forever

On Thu, 2 Dec 2010, Thomas Gleixner wrote:

> Collecting the buildids after a long perf run can result in running
> forever.
> 
> The reason is that the perf_ops which buildids uses are not having the
> .comm function set. Sounds weird, but the way how perf fork/mmap
> handling works explains it:
> 
> On fork the new task in perf userspace inherits the registered memory
> maps of the parent. This is almost correct (it's ignoring
> MADV_DONTFORK) for threads and forks. For new processes the extra
> "comm" event is generated which then removes the parent mappings from
> the child. The buildid code ignores the comm event and therefor the
> parents mappings are kept. When tracing e.g. a kernel compile then the
> inherited process chain can be pretty long and every child gets the
> old and partially replaced mappings of the various ancestors, which
> leads to an endless shuffling in map_groups__fixup_overlappings()
> 
> Add the default comm event for now to "fix" this.

<REMOVE THIS>

> The real fix is to transport this information at clone/fork time with
> the event, which is easy as the kernel knows it already. That would
> avoid that whole inherit/clone/remove heuristics in the postprocessing
> tools.

</REMOVE THIS>

Gah. I confused myself. Grr. commit 4385d580f2 explains the magic, but
we really should have done the explicit PERF_RECORD_EXEC or at least
made the .comm thing mandatory.

It's so fricking non obvious that the map flush happens in
thread__set_comm(). 

Yours grumpy

      tglx
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