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

[Rob Clark]

On Fri, Feb 24, 2023 at 7:27 AM Luben Tuikov <luben.tuikov at amd.com> 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 at linux.intel.com> wrote:
> >>
> >>> On 24/02/2023 10:24, Pekka Paalanen wrote:
> >>>> On Fri, 24 Feb 2023 09:41:46 +0000
> >>>> Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> wrote:
> >>>>
> >>>>> On 24/02/2023 09:26, Pekka Paalanen wrote:
> >>>>>> On Thu, 23 Feb 2023 10:51:48 -0800
> >>>>>> Rob Clark <robdclark at gmail.com> wrote:
> >>>>>>
> >>>>>>> On Thu, Feb 23, 2023 at 1:38 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >>>>>>>>
> >>>>>>>> On Wed, 22 Feb 2023 07:37:26 -0800
> >>>>>>>> Rob Clark <robdclark at gmail.com> wrote:
> >>>>>>>>
> >>>>>>>>> On Wed, Feb 22, 2023 at 1:49 AM Pekka Paalanen <ppaalanen at gmail.com> 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.

BR,
-R

> --
> Regards,
> Luben
>