Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

[Rodrigo Vivi]

On Mon, Feb 27, 2023 at 02:20:04PM -0800, Rob Clark wrote:
> On Mon, Feb 27, 2023 at 1:36 PM Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx> wrote:
> >
> > On Fri, Feb 24, 2023 at 09:59:57AM -0800, Rob Clark wrote:
> > > On Fri, Feb 24, 2023 at 7:27 AM Luben Tuikov <luben.tuikov@xxxxxxx> wrote:
> > > >
> > > > On 2023-02-24 06:37, Tvrtko Ursulin wrote:
> > > > >
> > > > > On 24/02/2023 11:00, Pekka Paalanen wrote:
> > > > >> On Fri, 24 Feb 2023 10:50:51 +0000
> > > > >> Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> > > > >>
> > > > >>> On 24/02/2023 10:24, Pekka Paalanen wrote:
> > > > >>>> On Fri, 24 Feb 2023 09:41:46 +0000
> > > > >>>> Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> > > > >>>>
> > > > >>>>> On 24/02/2023 09:26, Pekka Paalanen wrote:
> > > > >>>>>> On Thu, 23 Feb 2023 10:51:48 -0800
> > > > >>>>>> Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > > >>>>>>
> > > > >>>>>>> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > > >>>>>>>>
> > > > >>>>>>>> On Wed, 22 Feb 2023 07:37:26 -0800
> > > > >>>>>>>> Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > > >>>>>>>>
> > > > >>>>>>>>> On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > > >>>>>>
> > > > >>>>>> ...
> > > > >>>>>>
> > > > >>>>>>>>>> On another matter, if the application uses SET_DEADLINE with one
> > > > >>>>>>>>>> timestamp, and the compositor uses SET_DEADLINE on the same thing with
> > > > >>>>>>>>>> another timestamp, what should happen?
> > > > >>>>>>>>>
> > > > >>>>>>>>> The expectation is that many deadline hints can be set on a fence.
> > > > >>>>>>>>> The fence signaller should track the soonest deadline.
> > > > >>>>>>>>
> > > > >>>>>>>> You need to document that as UAPI, since it is observable to userspace.
> > > > >>>>>>>> It would be bad if drivers or subsystems would differ in behaviour.
> > > > >>>>>>>>
> > > > >>>>>>>
> > > > >>>>>>> It is in the end a hint.  It is about giving the driver more
> > > > >>>>>>> information so that it can make better choices.  But the driver is
> > > > >>>>>>> even free to ignore it.  So maybe "expectation" is too strong of a
> > > > >>>>>>> word.  Rather, any other behavior doesn't really make sense.  But it
> > > > >>>>>>> could end up being dictated by how the hw and/or fw works.
> > > > >>>>>>
> > > > >>>>>> It will stop being a hint once it has been implemented and used in the
> > > > >>>>>> wild long enough. The kernel userspace regression rules make sure of
> > > > >>>>>> that.
> > > > >>>>>
> > > > >>>>> Yeah, tricky and maybe a gray area in this case. I think we eluded
> > > > >>>>> elsewhere in the thread that renaming the thing might be an option.
> > > > >>>>>
> > > > >>>>> So maybe instead of deadline, which is a very strong word, use something
> > > > >>>>> along the lines of "present time hint", or "signalled time hint"? Maybe
> > > > >>>>> reads clumsy. Just throwing some ideas for a start.
> > > > >>>>
> > > > >>>> You can try, but I fear that if it ever changes behaviour and
> > > > >>>> someone notices that, it's labelled as a kernel regression. I don't
> > > > >>>> think documentation has ever been the authoritative definition of UABI
> > > > >>>> in Linux, it just guides drivers and userspace towards a common
> > > > >>>> understanding and common usage patterns.
> > > > >>>>
> > > > >>>> So even if the UABI contract is not documented (ugh), you need to be
> > > > >>>> prepared to set the UABI contract through kernel implementation.
> > > > >>>
> > > > >>> To be the devil's advocate it probably wouldn't be an ABI regression but
> > > > >>> just an regression. Same way as what nice(2) priorities mean hasn't
> > > > >>> always been the same over the years, I don't think there is a strict
> > > > >>> contract.
> > > > >>>
> > > > >>> Having said that, it may be different with latency sensitive stuff such
> > > > >>> as UIs though since it is very observable and can be very painful to users.
> > > > >>>
> > > > >>>> If you do not document the UABI contract, then different drivers are
> > > > >>>> likely to implement it differently, leading to differing behaviour.
> > > > >>>> Also userspace will invent wild ways to abuse the UABI if there is no
> > > > >>>> documentation guiding it on proper use. If userspace or end users
> > > > >>>> observe different behaviour, that's bad even if it's not a regression.
> > > > >>>>
> > > > >>>> I don't like the situation either, but it is what it is. UABI stability
> > > > >>>> trumps everything regardless of whether it was documented or not.
> > > > >>>>
> > > > >>>> I bet userspace is going to use this as a "make it faster, make it
> > > > >>>> hotter" button. I would not be surprised if someone wrote a LD_PRELOAD
> > > > >>>> library that stamps any and all fences with an expired deadline to
> > > > >>>> just squeeze out a little more through some weird side-effect.
> > > > >>>>
> > > > >>>> Well, that's hopefully overboard in scaring, but in the end, I would
> > > > >>>> like to see UABI documented so I can have a feeling of what it is for
> > > > >>>> and how it was intended to be used. That's all.
> > > > >>>
> > > > >>> We share the same concern. If you read elsewhere in these threads you
> > > > >>> will notice I have been calling this an "arms race". If the ability to
> > > > >>> make yourself go faster does not required additional privilege I also
> > > > >>> worry everyone will do it at which point it becomes pointless. So yes, I
> > > > >>> do share this concern about exposing any of this as an unprivileged uapi.
> > > > >>>
> > > > >>> Is it possible to limit access to only compositors in some sane way?
> > > > >>> Sounds tricky when dma-fence should be disconnected from DRM..
> > > > >>
> > > > >> Maybe it's not that bad in this particular case, because we are talking
> > > > >> only about boosting GPU clocks which benefits everyone (except
> > > > >> battery life) and it does not penalize other programs like e.g.
> > > > >> job priorities do.
> > > > >
> > > > > Apart from efficiency that you mentioned, which does not always favor
> > > > > higher clocks, sometimes thermal budget is also shared between CPU and
> > > > > GPU. So more GPU clocks can mean fewer CPU clocks. It's really hard to
> > > > > make optimal choices without the full coordination between both schedulers.
> > > > >
> > > > > But that is even not the main point, which is that if everyone sets the
> > > > > immediate deadline then having the deadline API is a bit pointless. For
> > > > > instance there is a reason negative nice needs CAP_SYS_ADMIN.
> > > > >
> > > > > However Rob has also pointed out the existence of uclamp.min via
> > > > > sched_setattr which is unprivileged and can influence frequency
> > > > > selection in the CPU world, so I conceded on that point. If CPU world
> > > > > has accepted it so can we I guess.
> > > > >
> > > > > So IMO we are back to whether we can agree defining it is a hint is good
> > > > > enough, be in via the name of the ioctl/flag itself or via documentation.
> > > > >
> > > > >> Drivers are not going to use the deadline for scheduling priorities,
> > > > >> right? I don't recall seeing any mention of that.
> > > > >>
> > > > >> ...right?
> > > > >
> > > > > I wouldn't have thought it would be beneficial to preclude that, or
> > > > > assume what drivers would do with the info to begin with.
> > > > >
> > > > > For instance in i915 we almost had a deadline based scheduler which was
> > > > > much fairer than the current priority sorted fifo and in an ideal world
> > > > > we would either revive or re-implement that idea. In which case
> > > > > considering the fence deadline would naturally slot in and give true
> > > > > integration with compositor deadlines (not just boost clocks and pray it
> > > > > helps).
> > > > How is user-space to decide whether to use ioctl(SET_DEADLINE) or
> > > > poll(POLLPRI)?
> > >
> > > Implementation of blocking gl/vk/cl APIs, like glFinish() would use
> > > poll(POLLPRI).  It could also set an immediate deadline and then call
> > > poll() without POLLPRI.
> > >
> > > Other than compositors which do frame-pacing I expect the main usage
> > > of either of these is mesa.
> >
> > Okay, so it looks like we already agreed that having a way to bump frequency
> > from userspace is acceptable. either because there are already other ways
> > that you can waste power and because this already acceptable in the CPU
> > world.
> >
> > But why we are doing this in hidden ways then?
> >
> > Why can't we have this hint per context that is getting executed?
> > (either with a boost-context flag or with some low/med/max or '-1' to '1'
> > value like the latency priority)?
> >
> > I don't like the waitboost because this heurisitic fails in some media cases.
> > I don't like the global setting because we might be alternating a top-priority
> > with low-priority cases...
> >
> > So, why not something per context in execution?
> >
> 
> It needs to be finer granularity than per-context, because not all
> waits should trigger boosting.  For example, virglrenderer ends up
> with a thread polling unsignaled fences to know when to signal an
> interrupt to the guest virtgpu.  This alone shouldn't trigger
> boosting.  (We also wouldn't want to completely disable boosting for
> virglrenderer.)  Or the usermode driver could be waiting on a fence to
> know when to do some cleanup.
> 
> That is not to say that there isn't room for per-context flags to
> disable/enable boosting for fences created by that context, meaning it
> could be an AND operation for i915 if it needs to be.

Right. It can be both ways I agree.

> 
> BR,
> -R