linux-kernel - Re: [patch] x86_64: fix sched

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Message-Id: <1180321967.29566.2.camel@localhost.localdomain>
Date:	Mon, 28 May 2007 13:12:47 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Satyam Sharma <satyam.sharma@...il.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch] x86_64: fix sched_clock()

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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