linux-kernel - RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

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Date:	Wed, 27 Apr 2011 08:40:36 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric B Munson" <emunson@...bm.net>, <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>,
	<stable@...nel.org>
Subject: RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

I keep telling Eric that the code below is incorrect
modulo arithimetic...

> +static u64 check_and_compute_delta(u64 prev, u64 val)
> +{
> +	u64 delta = (val - prev) & 0xfffffffful;
> +
> +	/*
> +	 * POWER7 can roll back counter values, if the new value is
smaller
> +	 * than the previous value it will cause the delta and the
counter to
> +	 * have bogus values unless we rolled a counter over.  If a
coutner is
> +	 * rolled back, it will be smaller, but within 256, which is the
maximum
> +	 * number of events to rollback at once.  If we dectect a
rollback
> +	 * return 0.  This can lead to a small lack of precision in the
> +	 * counters.
> +	 */
> +	if (prev > val && (prev - val) < 256)
> +		delta = 0;
> +
> +	return delta;

The code should detect rollback by looking at the value of 'delta'
otherwise there are horrid end effects near 2^32-1.

For instance:
	u32 delta = val - prev;
	return delta & 0x80000000 ? 0 : delta;


   David




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