linux-kernel - Re: [patch] x86, tsc: fix SMI induced variation in quick_pit

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Date:	Mon, 16 Jan 2012 16:41:09 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	asit.k.mallick@...el.com
Subject: Re: [patch] x86, tsc: fix SMI induced variation in
 quick_pit_calibrate()

On Mon, 2012-01-16 at 16:18 -0800, Linus Torvalds wrote:
> So I think your patch is a total hack that just compares the two last
> TSC deltas, but it's actually close to the "correct' thing in that it
> does start taking the time to see the last of the previous MSB into
> account.

yeah, it was a hack but your alternative sounds good.

> Now the delta is twice as big as before (it covers four PIT accesses,
> roughly 4us), so the comments might have to be updated to match, but
> the rest of the code should "just work" (except it might loop a bit
> longer, and maybe it gives closer to 250 ppm precision).
> 
> Does this fix it for you? I have NOT tested it in any way.
> 

also may be we should change the correction 

from: delta += (long)(d2 - d1)/2;
to:   delta += (long)(d2 - d1)/4;

I will give this patch a try.

thanks,
suresh



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