Re: [PATCH 1/5] Skip timer works.patch

[Andi Kleen <ak@suse.de>]

> 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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