[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

We were explicitly told to mask the interrupt generation at source
to conserve power. So I would hope for a statement as to whether that is
still a requirement from the HW architects; but I can't see why we would
not apply the mask and that this is just paper. If the observation about
forcewake tallies, would this not suggest that it is still conserving
power on icl?

I haven't looked at whether I see the same phenomenon as you [missing
interrupts on icl] locally, but I was expecting something like the bug
report since the observation that render times are less than EI was
causing the clocks to stay high. And I noticed your problem statement
and was hopeful for a link.

They sound like two different problems. (Underclocking for animations is
not icl specific.)

> 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?

Probably; as you can guess these were pulled out of nowhere based on the
observation that the frame lengths are much shorter than the current EI
and that in order for us to ramp up to maxclocks in a single frame of
animation would take about 4 samples per frame. Based on the reporter's
observations, we do have to ramp up very quickly for single frame of
rendering in order to hit the vblank, as we are ramping down afterwards.

With a target of 4 samples within say 1ms, 160us isn't too far of the
mark. (We have to allow some extra time to catch up rendering.)

As for steady state workloads, I'm optimistic the smoothing helps. (It's
harder to find steady state, unthrottled workloads!)
-Chris