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

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Date:	Mon, 16 Jan 2012 20:06:04 -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 17:30 -0800, Linus Torvalds wrote:
> On Mon, Jan 16, 2012 at 4:52 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Hmm. I get the feeling that we should remove that line entirely.
> 
> Yeah, I really cannot come up with a single good reason to keep that
> line, and suspect that I was a bit loopy when I wrote it.
> 
> So here's the suggested trivially updated patch. Does this work for people?

It does work. But it is taking almost 105 iterations or so to stabilize.
And if I boot the system in power-save mode (lowest p-state), it takes
almost 115 iterations to stabilize (close to the 25msec limit). This is
on the latest gen platform.

So I suspect we can either go back to 500ppm error tolerance:

                        if ((d1+d2) >= delta >> 10)
                                continue;

or increase the MAX_QUICK_PIT_MS bit more.

thanks,
suresh

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