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

[Luben Tuikov]

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