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

[Chris Wilson]

Quoting Francisco Jerez (2020-04-14 22:00:25)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > 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?
> >
> 
> Yeah, it's hard to see how disabling interrupt generation could save any
> additional power in a unit which is powered off -- At least on ICL where
> even the interrupt masking register is powered off...
> 
> > 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.
> >
> 
> Right.  In the workloads I was looking at last week the GPU would often
> be active for periods of time several times greater than the EI, and we
> would still fail to clock up.
> 
> > They sound like two different problems. (Underclocking for animations is
> > not icl specific.)
> >
> 
> Sure.  But it seems like the underclocking problem has been greatly
> exacerbated by 21abf0bf168d, which may have been mitigating the same ICL
> problem I was looking at leading to RPS interrupt loss.  Maybe
> 21abf0bf168d wouldn't be necessary with working RPS interrupt delivery?
> And without 21abf0bf168d platforms earlier than ICL wouldn't have as
> much of an underclocking problem either.

21abf0bf168d ("drm/i915/gt: Treat idling as a RPS downclock event")

is necessary due to that we can set a boost frequency and then never run
the RPS worker due to short activity cycles. See
igt/i915_pm_rc6_residency for rc6-idle, the bg_load is essentially just

for (;;) {
	execbuf(&nop);
	sleep(.1);
}

Without 21abf0bf168d if you trigger a waitboost just before, it never
recovers and power utilisation is measurably higher.

> >> 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.)
> 
> 
> How about we stop ramping down after the rendering of a single frame?
> It's not like we save any power by doing that, since the GPU seems to be
> forced to the minimum frequency for as long as it remains parked anyway.
> If the GPU remains idle long enough for the RPS utilization counters to
> drop below the threshold and qualify for a ramp-down the RPS should send
> us an interrupt, at which point we will ramp down the frequency.

Because it demonstrably and quite dramatically reduces power consumption
for very light desktop workloads.

> Unconditionally ramping down on parking seems to disturb the accuracy of
> that RPS feedback loop, which then needs to be worked around by reducing
> the averaging window of the RPS to a tiny fraction of the oscillation
> period of any typical GPU workload, which is going to prevent the RPS
> from seeing a whole oscillation period before it reacts, which is almost
> guaranteed to have a serious energy-efficiency cost.

There is no feedback loop in these workloads. There are no completed RPS
workers, they are all cancelled if they were even scheduled. (Now we
might be tempted to look at the results if they scheduled and take that
into consideration instead of unconditionally downclocking.)

> > As for steady state workloads, I'm optimistic the smoothing helps. (It's
> > harder to find steady state, unthrottled workloads!)
> 
> I'm curious, how is that smoothing you do in PATCH 1 better than simply
> setting 2x the EIs? (Which would also mean half the interrupt processing
> overhead as in this series)

I'm anticipating where the RPS worker may not run until the next jiffie
or two, by which point the iir is stale. But yes, there's only a subtle
difference between comparing the last 320us every 160us and comparing
the last 320us every 320us.
-Chris