linux-kernel - Re: Why is PERF_FORMAT_GROUP incompatible with inherited events?

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Message-ID: <20100215045644.GK13769@brick.ozlabs.ibm.com>
Date:	Mon, 15 Feb 2010 15:56:44 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, Dave Wootton <dwootton@...ibm.com>
Subject: Re: Why is PERF_FORMAT_GROUP incompatible with inherited events?

On Sun, Feb 14, 2010 at 01:38:27PM +0100, Peter Zijlstra wrote:
> On Sun, 2010-02-14 at 22:33 +1100, Paul Mackerras wrote:
> > 
> > But we don't go and collect the count delta from children without
> > PERF_FORMAT_GROUP, so why would we with it? 
> 
> Yes we do, see perf_event_read_value().

Ah, true, I should have read the code more carefully.

> But now that I look at it we don't seem to do so in
> perf_output_read_one()... I guess we should fix that.

I suppose it should give the same value as read() would, but the
possibly unbounded interrupt latency is a bit of a worry.  I can't
think of a way to avoid it, though (other than not using
PERF_SAMPLE_READ with inherited sampling events :).

> There is of course the lock inversion in the .read() code reported by
> stephane, but other than that is seems to actually support inherited &&
> group just fine.
> 
> So I think if we fix that lock inversion and make the PERF_SAMPLE_READ
> code look like the .read() code it should all work out.

Cool.

Paul.
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