linux-kernel - Re: [PATCH] raise tsc clocksource rating

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Message-ID: <20071029231702.GA8506@elte.hu>
Date:	Tue, 30 Oct 2007 00:17:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Zachary Amsden <zach@...are.com>
Cc:	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	rusty@...tcorp.com.au, jeremy@...p.org, --cc@...hat.com,
	avi@...amnet.com, kvm-devel@...ts.sourceforge.net,
	Glauber de Oliveira Costa <glauber@....localdomain>,
	Dan Hecht <dhecht@...are.com>,
	Garrett Smith <garrett@...are.com>
Subject: Re: [PATCH] raise tsc clocksource rating


* Zachary Amsden <zach@...are.com> wrote:

> On Tue, 2007-10-30 at 00:02 +0100, Ingo Molnar wrote:
> > * Zachary Amsden <zach@...are.com> wrote:
> 
> > > Not every guest support paravirt, but for correctness, all guests 
> > > require TSC, which must be exposed all the way up to userspace, no 
> > > matter what the efficiency or accuracy may be.
> > 
> > but if there's a perfect TSC available (there is such hardware) then the 
> > TSC _is_ the best clocksource. Paravirt now turns it off unconditionally 
> > in essence.
> 
> No, if no paravirt clocksource is detected, nothing can override the
> perfect TSC hardware clocksource rating of 400.  And if a paravirt
> clocksource is detected, it is always better than TSC.
> 
> > anyway, that's at most an optimization issue. No strong feelings here, 
> > and we can certainly delay this patch until this gets all sorted out.
> 
> This patch should be nacked, since it is just wrong.  This is not an 
> optimization issue.  It is an accuracy issue for all virtualization 
> environments that affects long term kernel clock stability, which is 
> important to fix, and the best way to do that is to use a paravirt 
> clocksource.

i know it's not an optimization issue. Your current pessimisation of 
even perfect TSCs _is_ an optimization issue.

(and note that if the TSC is unstable the guest will/should notice it 
and will fall back to the hyper clocksource)

	Ingo
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