linux-kernel - Re: Can Linux kernel handle unsynced TSC?

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Message-Id: <1204295241.6243.108.camel@lappy>
Date:	Fri, 29 Feb 2008 15:27:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Zhao Forrest <forrest.zhao@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm-devel@...ts.sourceforge.net
Subject: Re: Can Linux kernel handle unsynced TSC?


On Fri, 2008-02-29 at 22:20 +0800, Zhao Forrest wrote:
> On 2/29/08, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Fri, 2008-02-29 at 16:55 +0800, Zhao Forrest wrote:
> > > Sorry for reposting it.
> > >
> > > For example,
> > > 1 rdtsc() is invoked on CPU0
> > > 2 process is migrated to CPU1, and rdtsc() is invoked on CPU1
> > > 3 if TSC on CPU1 is slower than TSC on CPU0, can kernel guarantee
> > > that the second rdtsc() doesn't return a value smaller than the one
> > > returned by the first rdtsc()?
> >
> > No, rdtsc() goes directly to the hardware. You need a (preferably cheap)
> > clock abstraction layer on top if you need this.
> 
> Thank you for the clarification. I think gettimeofday() is such kind
> of clock abstraction layer, am I right?

Yes, gtod is one such a layer, however it fails the 'cheap' test for
many definitions of cheap.

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