linux-kernel - Re: [RFC][PATCH] perf

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Message-ID: <20100208074438.GC24721@elte.hu>
Date:	Mon, 8 Feb 2010 08:44:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH] perf_events: Fix FORK events


* Peter Zijlstra <peterz@...radead.org> wrote:

> Hi,
> 
> While looking into a problem reported by Pekka, I noticed that I wasn't
> receiving any FORK events for a workload that did fork (see attachment).

ah yes, my old testcase for forks.

> After making the below change, stuff started working again, thing is, I'm 
> not sure why.

In case it matters: that workload of 4x fork (and the whole fork events 
mechanism) was always very sensitive to the precise timing of when a child 
and a parent does what, in the final dance of wait(), do_exit(), 
release_task(), etc. when a task exits. Especially on SMP systems.

> The main change is sending the FORK event to the parent instead of the 
> child thread, however perf_event_fork() is at the end of copy_process(), 
> which is after perf_event_init_task() which inherits all the counters, so 
> it should all have worked as it was.
> 
> We could of course just slam the commit in and not worry about it, but
> that just doesn't feel right.

Would be nice to figure it out ...

	Ingo
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