linux-kernel - Re: Whats the purpose of get_cycles

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Message-ID: <20071031102346.GE29700@8bytes.org>
Date:	Wed, 31 Oct 2007 11:23:46 +0100
From:	Joerg Roedel <joro@...tes.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()

Hi Vojtech,

On Tue, Oct 30, 2007 at 11:02:09PM +0100, Vojtech Pavlik wrote:
> The K8's still guarantee that subsequent RDTSCs return increasing
> values, even if the processor reorders them.
> 
> 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.
> 
> 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.

Can you tell me more about the problems you have seen or give me a
pointer to a mail discussion regarding that problems? Can you also
provide your test program to me please? I want to understand these
problems a bit better.

Joerg
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