linux-kernel - Re: Whats the purpose of get_cycles

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Date:	Tue, 30 Oct 2007 23:42:34 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Vojtech Pavlik <vojtech@...e.cz>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Joerg Roedel <joerg.roedel@....com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	benjamin.serebrin@....com
Subject: Re: Whats the purpose of get_cycles_sync()

On Tue, Oct 30, 2007 at 11:02:09PM +0100, Vojtech Pavlik wrote:
> > He can give details on the test.
> > 
> > I suspect the reason was because the CPU reordered the RDTSCs so that
> > a later RDTSC could return a value before an earlier one. This can
> > happen because gettimeofday() is so fast that a tight loop calling it can
> > fit more than one iteration into the CPU's reordering window.
> 
> The K8's still guarantee that subsequent RDTSCs return increasing
> values, even if the processor reorders them.

Ah didn't realize this

> 
> What could have been happening then was that the RDTSC instruction might
> have been reordered by the CPU out of the seqlock, causing trouble in
> the calculation.

Ok anyways it fixed that problem.  So it cannot be taken out.
> 
> Anyway, adding the CPUID didn't solve all the problems we've seen back
> then, and so far none of the approaches for using TSC without acquiring
> a spinlock on multi-socket AMD boxes worked 100% correctly.

The code is not used on multi-core anyways currently (without Jiri's
patch). It should just work correctly on single core.

-Andi
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