linux-kernel - Re: [BUG] perf_event: when events are grouped, the time enabled / running values are incorrect

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Date:	Tue, 11 May 2010 22:23:38 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	eranian@...il.com, Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [BUG] perf_event: when events are grouped, the time enabled / 
	running values are incorrect

On Tue, May 11, 2010 at 10:11 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2010-05-11 at 21:55 +0200, stephane eranian wrote:
>> Events are also grouped in this example. Both this same issue exists
>> also when only
>> one event is used. I suspect an update_event_times() or
>> update_group_times() is also
>> missing on the sampling path in perf_overflow_handler().
>
> Hrmm, you're right, except fixing that is non-trivial.
> update_event_times() wants to be called with ctx->lock held and we can't
> do that from NMI context.
>
Why do you need the lock when the event is running on a CPU in NMI context?
What action issued remotely could modify the data used by this function without
the need for IPI?
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