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

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Date:	Thu, 07 Apr 2011 14:22:57 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Eric B Munson <emunson@...bm.net>
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


> > Doesn't that mean that power_pmu_read() can only ever increase the value of
> > the perf_event and so will essentially -stop- once the counter rolls over ?
> > 
> > Similar comments every where you do this type of comparison.
> > 
> > Cheers,
> > Ben.
> 
> Sorry for the nag, but am I missing something about the way the register and
> the previous values are reset in the overflow interrupt handler?

Well, not all counters get interrupts right ? Some counters are just
free running... I'm not sure when that power_pmu_read() function is
actually used by the core, I'm not that familiar with perf, but I'd say
better safe than sorry. When comparing counter values, doing in a way
that is generally safe vs. wraparounds. Eventually do a helper for that.

Cheers,
Ben.


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