From: Kasper Pedersen <kernel@kasperkp.dk>
To: john stultz <johnstul@us.ibm.com>
Cc: linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
Suresh Siddha <suresh.b.siddha@intel.com>
Subject: Re: x86: tsc: make TSC calibration more immune to interrupts
Date: Wed, 20 Apr 2011 21:44:52 +0200 [thread overview]
Message-ID: <4DAF37B4.3040408@kasperkp.dk> (raw)
In-Reply-To: <1303326959.2796.136.camel@work-vm>
On 04/20/2011 09:15 PM, john stultz wrote:
> 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).
> 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" ?
>
Yes, I do.
With the delayed workqueue patch, I see a wonderful 0.4ppm
offset _almost_ all of the time.
Very rarely though (about 1 in 3000) the value output by
"Refined TSC .." jumps. It can jump no further than
50000/F_TSC/delayed_time, so on a modern machine it jumps
no further than 20ppm.
This can happen when a short irq occurs between
*p = hpet_readl(HPET_COUNTER) & 0xFFFFFFFF;
and
t2 = get_cycles();
Without the delayed workqueue patch 30ppm is insignificant.
/Kasper Pederen
next prev parent reply other threads:[~2011-04-20 19:46 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-20 18:52 x86: tsc: make TSC calibration more immune to interrupts Kasper Pedersen
2011-04-20 19:15 ` john stultz
2011-04-20 19:44 ` Kasper Pedersen [this message]
2011-04-20 20:28 ` john stultz
2011-04-20 21:22 ` x86: tsc: v2 " Kasper Pedersen
2011-04-20 22:39 ` Josh Triplett
2011-04-21 2:19 ` john stultz
2011-04-21 4:32 ` Josh Triplett
2011-04-21 19:46 ` Kasper Pedersen
2011-04-23 1:38 ` john stultz
2011-04-21 19:52 ` x86: tsc: v3 " Kasper Pedersen
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