[Intel-gfx] freedesktop bug id: 100548, bisected to sched/clock commit

[Peter Zijlstra]

On Wed, Apr 12, 2017 at 04:40:11PM +0300, Martin Peres wrote:

> > > So, why is this only affecting the Core 2 Duo?
> > 
> > Core2 doesn't have a usable TSC and would revert to the slow path. I'll
> > have another look at that patch.
> > 
> 
> So, by default, it is using the hpet clock source. FYI, I tried the only
> other available clock source (acpi_pm) and got the same result.

So because HPET is unbearably slow we've cobbled together something that
takes the HPET (or rather get-time-of-day CLOCK_MONOTONIC) value at tick
time and uses TSC to add per-cpu increments to that. Using windowing to
keep the TSC maddness at bay.

The patch in question affects the windowing.. clearly something went
amiss.