> It doesn't happen often, but it is a possibility that the kernel
> calibrates the delay wrong because of timing glitches caused by CPU
> migration, paging, or other phenomena which are supposed to be
> transparent to the kernel (but cause temporal lapse).
We're supposed to handle those because they happen on real hardware
too with long running SMM handlers. Or at least there was a effort some time ago
to do this. If it wasn't enough we'll likely need to fix the code.
> In that case, the
> kernel may not make enough progress in a spin delay loop to properly
> reach the number of microseconds required for N number of timer ticks to
> occur.
Hmm, mdelay is polling RDTSC and assumes it makes forward progress
and waits until the time that was estimated at the original TSC<->PIT
calibration passed. While there is a spin loop it is definitely
polling a timer that is supposed to tick properly even in virtualization.
You're saying that doesn't work on vmware? Does it have trouble
with RDTSC?
Anyways if polling against TSC doesn't work I suppose we could
change it to poll against some other timer.
> In theory this can happen on a real machine, as SMM mode could
> be active, doing USB device emulation or something that takes a while
> during the lpj calibration and throwing the computation off.
Yep.
> By changing the parameters (N ticks at K Hz in T seconds), it is easy to
> create an unstable measurement that can achieve high failure rates,
> although in practice the Linux parameters appear to be reasonable enough
> that it is not a major problem.
Hmm, why exactly?
-Andi
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