linux-kernel - Re: x86: tsc: make TSC calibration more immune to interrupts

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Date:	Wed, 20 Apr 2011 12:15:59 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Kasper Pedersen <kernel@...perkp.dk>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: x86: tsc: make TSC calibration more immune to interrupts

On Wed, 2011-04-20 at 20:52 +0200, Kasper Pedersen wrote:
> When a SMI or plain interrupt occurs during the delayed part
> of TSC calibration, and the SMI/irq handler is good and fast
> so that is does not exceed SMI_TRESHOLD, tsc_khz can be a bit
> off (10-30ppm).
> 
> We should not depend on interrupts being longer than 50000
> clocks, so always do the 5 tries, and use the best sample we
> get.
> This should work always for any four periodic or rate-limited
> interrupt sources. If we get 5 interrupts with 500ns gaps in
> a row, behaviour should be as without this patch.
> 
> This costs us 20-100 microseconds in startup time, as
> tsc_read_refs is called 8 times.
> 
> measurements:
> On a 700MHz P3 I see t2-t1=~22000, and 31ppm error.
> A Core2 is similar: http://n1.taur.dk/tscdeviat.png
> (while mostly t2-t1=~1000, in about 1 of 3000 tests
> I see t2-t1=~20000 for both machines.)
> vmware ESX4 has t2-t1=~8000 and up.


I guess I'm curious how useful this is with the refined TSC calibration
that was added not too long ago:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=08ec0c58fb8a05d3191d5cb6f5d6f81adb419798

Are you saying that you see the same 10-30ppm variance in the dmesg
line: "Refined TSC clocksource calibration: XXXX.XXX MHz" ?

thanks
-john


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