linux-kernel - Re: [RFC PATCH 5/5] perf: Implement perf_output

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Date:	Wed, 19 May 2010 18:34:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	rostedt@...dmis.org
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/5] perf: Implement perf_output_addr()

On Wed, 2010-05-19 at 12:27 -0400, Steven Rostedt wrote:

> OK, so I would let them evaluate separately. If they do have two
> different results, then that's fine, because the view of an event could
> possible be different. The &bar may change in the two instances, but how
> much does that matter? Which version of &bar is correct anyway?

Right, can do, but again, that sucks for filters -- I'm starting to
think we never should have accepted that stuff.

> How do you handle the multiple readers then? The call to record the
> event copies to each buffer that is registered for that event?

Yeah, each perf_event has its own buffer (usually) so we simply generate
multiple events, one for each buffer. The readers, task A and B will get
wakeups once in a while to empty the buffer.

> If more than one buffer is attached to an event, you could also work to
> directly write to one, and then copy directly from that buffer to the
> others.

Yeah, I mentioned this a few emails back, but since you need the keep
the commit open until you've copied it the code can get quite ugly.
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