linux-kernel - Re: [PATCH] Turn rdmsr, rdtsc into inline functions, clarify names

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Date:	Mon, 7 Aug 2006 15:18:34 +0200
From:	Vojtech Pavlik <vojtech@...e.cz>
To:	Andi Kleen <ak@....de>
Cc:	Dmitry Torokhov <dtor@...ightbb.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Turn rdmsr, rdtsc into inline functions, clarify names

On Mon, Aug 07, 2006 at 02:56:39PM +0200, Andi Kleen wrote:
> On Mon, Aug 07, 2006 at 02:48:55PM +0200, Vojtech Pavlik wrote:
> > On Mon, Aug 07, 2006 at 02:28:45PM +0200, Andi Kleen wrote:
> > > On Mon, Aug 07, 2006 at 01:09:31PM +0200, Vojtech Pavlik wrote:
> > > > On Mon, Aug 07, 2006 at 10:48:50AM +0200, Andi Kleen wrote:
> > > > > On Sun, Aug 06, 2006 at 10:43:44PM -0400, Dmitry Torokhov wrote:
> > > > > > On Saturday 05 August 2006 23:16, Andi Kleen wrote:
> > > > > > > This whole thing is broken, e.g. on a preemptive kernel when the
> > > > > > > code can switch CPUs 
> > > > > > > 
> > > > > > 
> > > > > > Would not preempt_disable fix that?
> > > > > 
> > > > > Partially, but you still have other problems. Please just get rid
> > > > > of it. Why do we have timer code in the kernel if you then chose
> > > > > not to use it?
> > > >  
> > > > The problem is that gettimeofday() is not always fast. 
> > > 
> > > When it is not fast that means it is not reliable and then you're
> > > also not well off using it anyways.
> > 
> > I assume you wanted to say "When gettimeofday() is slow, it means TSC is
> > not reliable", which I agree with. 
> > 
> > But I need, in the driver, in the no-TSC case use i/o counting, not a
> > slow but reliable method. And I can't say, from outside the timing
> > subsystem, whether gettimeofday() is fast or slow.
> 
> Hmm if that is the only obstacle I can export a "slow gettimeofday" flag.

That would help.

> However it would be some work to implement it for all architectures.
> 
> > I assume we could make it work with the monotonic timer instead. 
> 
> The monotonic timer is the right thing to use to make you independent
> of ntpd, but it's normally not faster or slower than gettimeofday.
 
Yup.

-- 
Vojtech Pavlik
Director SuSE Labs
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