On 17/02/2023 17:00, Rob Clark wrote:
On Fri, Feb 17, 2023 at 8:03 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
[snip]
adapted from your patches.. I think the basic idea of deadlines
(which includes "I want it NOW" ;-)) isn't controversial, but the
original idea got caught up in some bikeshed (what about compositors
that wait on fences in userspace to decide which surfaces to update in
the next frame), plus me getting busy and generally not having a good
plan for how to leverage this from VM guests (which is becoming
increasingly important for CrOS). I think I can build on some ongoing
virtgpu fencing improvement work to solve the latter. But now that we
have a 2nd use-case for this, it makes sense to respin.
Sure, I was looking at the old version already. It is interesting. But
also IMO needs quite a bit more work to approach achieving what is
implied from the name of the feature. It would need proper deadline
based sched job picking, and even then drm sched is mostly just a
frontend. So once past runnable status and jobs handed over to backend,
without further driver work it probably wouldn't be very effective past
very lightly loaded systems.
Yes, but all of that is not part of dma_fence ;-)
:) Okay.
Having said that, do we need a step back to think about whether adding
deadline to dma-fences is not making them something too much different
to what they were? Going from purely synchronisation primitive more
towards scheduling paradigms. Just to brainstorm if there will not be
any unintended consequences. I should mention this in your RFC thread
actually.
A pretty common challenging usecase is still the single fullscreen
game, where scheduling isn't the problem, but landing at an
appropriate GPU freq absolutely is. (UI workloads are perhaps more
interesting from a scheduler standpoint, but they generally aren't
challenging from a load/freq standpoint.)
Challenging as in picking the right operating point? Might be latency
impacted (and so user perceived UI smoothness) due missing waitboost for
anything syncobj related. I don't know if anything to measure that
exists currently though. Assuming it is measurable then the question
would be is it perceivable.
Fwiw, the original motivation of the series was to implement something
akin to i915 pageflip boosting without having to abandon the atomic
helpers. (And, I guess it would also let i915 preserve that feature
if it switched to atomic helpers.. I'm unsure if there are still other
things blocking i915's migration.)
Question for display folks I guess.
Then if we fast forward to a world where schedulers perhaps become fully
deadline aware (we even had this for i915 few years back) then the
question will be does equating waits with immediate deadlines still
works. Maybe not too well because we wouldn't have the ability to
distinguish between the "someone is waiting" signal from the otherwise
propagated deadlines.
Is there any other way to handle a wait boost than expressing it as an
ASAP deadline?
A leading question or just a question? Nothing springs to my mind at the
moment.
Regards,
Tvrtko
