[Intel-gfx] [PATCH 1/2] drm/i915/gt: Try to smooth RPS spikes

[Chris Wilson]

Quoting Chris Wilson (2020-04-15 12:24:59)
> Quoting Andi Shyti (2020-04-15 11:56:59)
> > Hi Chris,
> > 
> > On Tue, Apr 14, 2020 at 05:14:22PM +0100, Chris Wilson wrote:
> > > By the time we respond to the RPS interrupt [inside a worker], the GPU
> > > may be running a different workload. As we look to make the evalution
> > > intervals shorter, these spikes are more likely to okay. Let's try to
> > > smooth over the spikes in the workload by comparing the EI interrupt
> > > [up/down events] with the most recently completed EI; if both say up,
> > > then increase the clocks, if they disagree stay the same. In principle,
> > > this means we now take 2 up EI to go increase into the next bin, and
> > > similary 2 down EI to decrease. However, if the worker runs fast enough,
> > > the previous EI in the registers will be the same as triggered the
> > > interrupt, so responsiveness remains unaffect. [Under the current scheme
> > > where EI are on the order of 10ms, it is likely that this is true and we
> > > compare the interrupt with the EI that caused it.]
> > 
> > looks reasonable to me. Wouldn't it make also sense to evaluate
> > the difference between the current and the previous pm_iir?
> 
> I was considering it... We can't look at IIR since we apply the mask,
> but ISR should be valid. Worth a looksee.

[   98.882025] bcs0: DOWN interrupt not recorded for idle, pm_iir:10, pm_isr:0, prev_down:0, down_threshold:3840, down_ei:5dc0

ISR doesn't seem helpful on first try.
-Chris