Subject: Re: [patch] x86_64: fix sched_clock()
From: Rusty Russell <rusty@rustcorp.com.au>
To: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Satyam Sharma <satyam.sharma@gmail.com>, linux-kernel@vger.kernel.org,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
In-Reply-To: <20070525110543.GM8094@one.firstfloor.org>
References: <20070525075446.GA20140@elte.hu>
	 <20070525080209.GE8094@one.firstfloor.org> <20070525080415.GB21446@elte.hu>
	 <20070525082018.GF8094@one.firstfloor.org> <20070525083430.GB27236@elte.hu>
	 <20070525084109.GI8094@one.firstfloor.org> <20070525084426.GA29973@elte.hu>
	 <20070525084547.GJ8094@one.firstfloor.org>
	 <20070525015544.bce3cb99.akpm@linux-foundation.org>
	 <20070525090315.GK8094@one.firstfloor.org>
	 <20070525110543.GM8094@one.firstfloor.org>
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Date: Mon, 28 May 2007 13:12:47 +1000
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On Fri, 2007-05-25 at 13:05 +0200, Andi Kleen wrote:
> On Fri, May 25, 2007 at 11:03:15AM +0200, Andi Kleen wrote:
> > > This?    I sent it to you earlier this week:
> > 
> > Sorry haven't processed those yet.
> > 
> > Ah. The correct fix here is to clear the tsc flag in boot_cpu_data
> > when the option is set. Will do that.
> 
> Hmm I double checked this now; tsc_disable indeed clears
> X86_FEATURE_TSC in identify_cpu and that should be always
> called before anything sched_clock related runs on a CPU.

Yes, agreed.

> Also the only possibly faulting division is protected
> by a cpu_has_tsc which even checks boot_cpu_data. this means
> even if the resync frequency code was called for
> some reason before the identify_cpu of a AP it should
> still work. For the BP this definitely cannot happen.
> 
> I also tried it with qemu myself (both one and two cpus) and it worked
> 
> Rusty, was this really on a standard kernel? Was it with multiple
> CPUs? 

No, will re-check.  Drop it for now, if I can reproduce I'll produce the
real fix or a decent explanation.

Thanks,
Rusty.


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