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)? -- Regards, Luben
