[Intel-gfx] [PATCH 2/2] drm/i915/gt: Shrink the RPS evalution intervals

[Chris Wilson]

Quoting Francisco Jerez (2020-04-14 20:39:48)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Quoting Chris Wilson (2020-04-14 17:14:23)
> >> Try to make RPS dramatically more responsive by shrinking the evaluation
> >> intervales by a factor of 100! The issue is as we now park the GPU
> >> rapidly upon idling, a short or bursty workload such as the composited
> >> desktop never sustains enough work to fill and complete an evaluation
> >> window. As such, the frequency we program remains stuck. This was first
> >> reported as once boosted, we never relinquished the boost [see commit
> >> 21abf0bf168d ("drm/i915/gt: Treat idling as a RPS downclock event")] but
> >> it equally applies in the order direction for bursty workloads that
> >> *need* low latency, like desktop animations.
> >> 
> >> What we could try is preserve the incomplete EI history across idling,
> >> it is not clear whether that would be effective, nor whether the
> >> presumption of continuous workloads is accurate. A clearer path seems to
> >> treat it as symptomatic that we fail to handle bursty workload with the
> >> current EI, and seek to address that by shrinking the EI so the
> >> evaluations are run much more often.
> >> 
> >> This will likely entail more frequent interrupts, and by the time we
> >> process the interrupt in the bottom half [from inside a worker], the
> >> workload on the GPU has changed. To address the changeable nature, in
> >> the previous patch we compared the previous complete EI with the
> >> interrupt request and only up/down clock if both agree. The impact of
> >> asking for, and presumably, receiving more interrupts is still to be
> >> determined and mitigations sought. The first idea is to differentiate
> >> between up/down responsivity and make upclocking more responsive than
> >> downlocking. This should both help thwart jitter on bursty workloads by
> >> making it easier to increase than it is to decrease frequencies, and
> >> reduce the number of interrupts we would need to process.
> >
> > Another worry I'd like to raise, is that by reducing the EI we risk
> > unstable evaluations. I'm not sure how accurate the HW is, and I worry
> > about borderline workloads (if that is possible) but mainly the worry is
> > how the HW is sampling.
> >
> > The other unmentioned unknown is the latency in reprogramming the
> > frequency. At what point does it start to become a significant factor?
> > I'm presuming the RPS evaluation itself is free, until it has to talk
> > across the chip to send an interrupt.
> > -Chris
> 
> At least on ICL the problem which this patch and 21abf0bf168d were
> working around seems to have to do with RPS interrupt delivery being
> inadvertently blocked for extended periods of time.  Looking at the GPU
> utilization and RPS events on a graph I could see the GPU being stuck at
> low frequency without any RPS interrupts firing, for a time interval
> orders of magnitude greater than the EI we're theoretically programming
> today.  IOW it seems like the real problem isn't that our EIs are too
> long, but that we're missing a bunch of them.
> 
> The solution I was suggesting for this on IRC during the last couple of
> days wouldn't have any of the drawbacks you mention above, I'll send it
> to this list in a moment if the general approach seems okay to you:
> 
> https://github.com/curro/linux/commit/f7bc31402aa727a52d957e62d985c6dae6be4b86

Confirmed that the PMINTRMSK is sufficiently delayed to cause problems.
 
> That said it *might* be helpful to reduce the EIs we use right now in
> addition, but a factor of 100 seems over the top since that will cause
> the evaluation interval to be roughly two orders of magnitude shorter
> than the rendering time of a typical frame, which can lead to massive
> oscillations even in workloads that use a small fraction of the GPU time
> to render a single frame.  Maybe we want something in between?

And confirmed that both are problems :) The EI are just too large to
handle the bursty workload. That is with the 10+ms EI, we do not see any
interrupts in the simple animations as we park within an EI.
-Chris