From: Eric B Munson <emunson@mgebm.net>
To: David Laight <David.Laight@ACULAB.COM>
Cc: a.p.zijlstra@chello.nl, linux-kernel@vger.kernel.org,
paulus@samba.org, anton@samba.org, acme@ghostprotocols.net,
mingo@elte.hu, linuxppc-dev@lists.ozlabs.org, stable@kernel.org
Subject: Re: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks
Date: Wed, 27 Apr 2011 09:08:22 -0400 [thread overview]
Message-ID: <20110427130822.GC2936@mgebm.net> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6D8AD04@saturn3.aculab.com>
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On Wed, 27 Apr 2011, David Laight wrote:
>
> > + if (prev > val && (prev - val) < 256)
> > + delta = 0;
> > +
> > + return delta;
>
> Also, 'prev' is a true 64bit value, but 'val' is only ever 32bits.
> So once the 64bit 'prev' exceeds 2^32+256 both 'prev > val'
> and 'prev - val' are true regardless of the value of 'val'.
> This will lead to jumps in the value ....
prev and val are both 64 bit variables holding 32 bit numbers, we do not
accumulate in either, they are both replaced by values directly from the
registers. So prev > val will not always be true.
Eric
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next prev parent reply other threads:[~2011-04-27 13:08 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-15 18:12 [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks Eric B Munson
2011-04-27 7:40 ` David Laight
2011-04-27 12:19 ` Eric B Munson
2011-04-27 12:34 ` David Laight
2011-04-27 12:59 ` Eric B Munson
2011-04-27 13:04 ` David Laight
2011-04-27 12:42 ` David Laight
2011-04-27 13:08 ` Eric B Munson [this message]
2011-04-27 13:13 ` David Laight
2011-04-27 13:20 ` Eric B Munson
2011-04-27 13:26 ` Benjamin Herrenschmidt
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