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Subjectx86 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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