linux-kernel - Re: [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks

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Message-ID: <20110330183656.GA2564@mgebm.net>
Date:	Wed, 30 Mar 2011 14:36:56 -0400
From:	Eric B Munson <emunson@...bm.net>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	a.p.zijlstra@...llo.nl, paulus@...ba.org, mingo@...e.hu,
	acme@...stprotocols.net, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, anton@...ba.org
Subject: Re: [PATCH] POWER: perf_event: Skip updating kernel counters if
 register value shrinks

On Wed, 30 Mar 2011, Benjamin Herrenschmidt wrote:

> On Tue, 2011-03-29 at 10:25 -0400, Eric B Munson wrote:
> > Here I made the assumption that the hardware would never remove more events in
> > a speculative roll back than it had added.  This is not a situation I
> > encoutered in my limited testing, so I didn't think underflow was possible.  I
> > will send out a V2 using the signed 32 bit delta and remeber to CC stable
> > this time. 
> 
> I'm not thinking about underflow but rollover... or that isn't possible
> with those counters ? IE. They don't wrap back to 0 after hitting
> ffffffff ?
> 

They do roll over to 0 after ffffffff, but I thought that case was already
covered by the perf_event_interrupt.  Are you concerned that we will reset a
counter and speculative roll back will underflow that counter?

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