linux-kernel - Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance

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Message-ID: <20090508121521.GC18758@elte.hu>
Date:	Fri, 8 May 2009 14:15:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	George Spelvin <linux@...izon.com>,
	ulrich.windl@...uni-regensburg.de, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, Clark Williams <williams@...hat.com>,
	zippel@...ux-m68k.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration
	variance


* john stultz <johnstul@...ibm.com> wrote:

> All, 
> 
> Despite recent tweaking, TSC calibration variance is still biting 
> users who care about keeping close sync with NTP servers over 
> reboots.
> 
> Here's a recent example: 
> http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02061.html
> 
> The problem is, each reboot, we have to calibrate the TSC, and any 
> error, regardless of how small, in the calibrated freq has to be 
> corrected for by NTP. Assuming the error is within 500ppm NTP can 
> correct this, but until it finds the proper correction value for 
> the new TSC freq, users may see time offsets from the NTP server.
> 
> In my experience, its fairly easy to see 100khz variance from 
> reboot to reboot with 2.6.30-rc.
> 
> While I think its worth trying to improve the calibration further, 
> there will likely be a trade-off between very accurate calibration 
> and fast boot times.
> 
> To mitigate this, I wanted to provide a tsc_khz= boot option. This 
> would allow users to set the tsc_khz value at boot-up, assuming 
> they are within 1Mhz of the calibrated value (to protect against 
> bad values). Once the tsc_khz value is set in grub, the box will 
> always boot with the same value, so the NTP drift value prior to 
> reboot will still be correct after rebooting.
> 
> Thanks to George Spelvin for the idea:
> 	http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02807.html
> 
> Thoughts or feedback?
> 
> Signed-off-by: John Stultz <johnstul@...ibm.com>

Wouldnt it be a lot more flexible to have a sysctl for this, which 
would be set before ntpd is started? (or which would be set by ntpd)

The mechanism and semantics would be similar: we would _not_ expose 
cpu_khz directly, we'd have a boot_cpu_khz value saved for sure, and 
we'd allow the sysctl to set the cpu_khz to within 1MHz of cpu_khz - 
and we'd re-scale the timer irq and other calibrated values 
accordingly.

Alternatively, a much simpler method: why doesnt ntpd save its own 
notion of cpu_khz once it has reached stability, and reads cpu_khz 
(from /proc/cpuinfo) during bootup and re-scales its initial offset 
and phase shift accordingly, compensating for that noise? (if it's 
within 1MHz)

	Ingo
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