linux-kernel - Re: [patch] i386, numaq: enable TSCs again

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Message-Id: <1180117406.6345.4.camel@localhost.localdomain>
Date:	Fri, 25 May 2007 11:23:26 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Dave Hansen <hansendc@...ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	William Lee Irwin III <wli@...omorphy.com>,
	Andi Kleen <andi@...stfloor.org>,
	Satyam Sharma <satyam.sharma@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch] i386, numaq: enable TSCs again

On Fri, 2007-05-25 at 11:16 -0700, Dave Hansen wrote:
> On Fri, 2007-05-25 at 10:41 +0200, Ingo Molnar wrote:
> > * William Lee Irwin III <wli@...omorphy.com> wrote:
> > > > yes, that's what i meant under 'slightly async'. Some AMD CPUs are 
> > > > like that too and sched_clock() now handles that fine. So we should 
> > > > try my patch.
> > > 
> > > Sorry, then. I took slight to mean something else. In any event I was 
> > > only quantifying things. I've no opinion whatsoever on the impact of 
> > > the code on NUMA-Q, only some recall of its operating characteristics.
> > 
> > there's no need to apologize at all! Thanks for reminding us about the 
> > time-scale and nature of the TSC drift on NUMAQ. I was worried that 
> > maybe the TSC was totally unusable for some reason - but that's 
> > fortunately not the case. So we now have one quirk less, hopefully :-)
> 
> Last I remember, it was totally useless for timekeeping, but was useful
> for cpu-local time measurements.
> 
> John, it's still useless for time, right?  Does sched_clock() really fix
> it?

Yea, on multi-node NUMAQ the TSC shouldn't be used for timekeeping.

However it should be fine for sched_clock(), or other cpu-local
measurements as the TSCs are constant (no cpufreq, no deep sleep
states).

-john

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