RE: x86 TSC time warp puzzle

["Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>]

At what point are you seeing these delays? During early boot or after
boot? 
It can be SMI happening in the platform. Typically BIOS uses some SMI
polling 
to handle some devices during early boot. Though 500 microseconds sounds
a 
bit too high.

Thanks,
Venki

>-----Original Message-----
>From: linux-kernel-owner@vger.kernel.org 
>[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 
>Jonathan Lundell
>Sent: Friday, April 01, 2005 5:43 PM
>To: LKML
>Subject: x86 TSC time warp puzzle
>
>Well, not actually a time warp, though it feels like one.
>
>I'm doing some real-time bit-twiddling in a driver, using the TSC to 
>measure out delays on the order of hundreds of nanoseconds. Because I 
>want an upper limit on the delay, I disable interrupts around it.
>
>The logic is something like:
>
>	local_irq_save
>	out(set a bit)
>	t0 = TSC
>	wait while (t = (TSC - t0)) < delay_time
>	out(clear the bit)
>	local_irq_restore
>
> From time to time, when I exit the delay, t is *much* bigger than 
>delay_time. If delay_time is, say, 300ns, t is usually no more than 
>325ns. But every so often, t can be 2000, or 10000, or even much 
>higher.
>
>The value of t seems to depend on the CPU involved, The worst case is 
>with an Intel 915GV chipset, where t approaches 500 microseconds (!).
>
>This is with ACPI and HT disabled, to avoid confounding interactions. 
>I suspected NMI, of course, but I monitored the nmi counter, and 
>mostly saw nothing (from time to time a random hit, but mostly not).
>
>The longer delay is real. I can see the bit being set/cleared in the 
>pseudocode above on a scope, and when the long delay happens, the bit 
>is set for a correspondingly long time.
>
>BTW, the symptom is independent of my IO. I wrote a test case that 
>does diddles nothing but reading TSC, and get the same result.
>
>Finally, on some CPUs, at least, the extra delay appears to be 
>periodic. The 500us delay happens about every second. On a different 
>machine (chipset) it happens at about 5 Hz. And the characteristic 
>delay on each type of machine seems consistent.
>
>Any ideas of where to look? Other lists to inquire on?
>
>Thanks.
>-- 
>/Jonathan Lundell.
>-
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