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

[Luben Tuikov]

On 2023-02-22 04:49, Pekka Paalanen wrote:
> On Tue, 21 Feb 2023 09:53:56 -0800
> Rob Clark <robdclark@xxxxxxxxx> wrote:
> 
>> On Tue, Feb 21, 2023 at 8:48 AM Luben Tuikov <luben.tuikov@xxxxxxx> wrote:
>>>
>>> On 2023-02-20 11:14, Rob Clark wrote:  
>>>> On Mon, Feb 20, 2023 at 12:53 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:  
>>>>>
>>>>> On Sat, 18 Feb 2023 13:15:49 -0800
>>>>> Rob Clark <robdclark@xxxxxxxxx> wrote:
>>>>>  
>>>>>> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>>>>>>
>>>>>> Allow userspace to use the EPOLLPRI/POLLPRI flag to indicate an urgent
>>>>>> wait (as opposed to a "housekeeping" wait to know when to cleanup after
>>>>>> some work has completed).  Usermode components of GPU driver stacks
>>>>>> often poll() on fence fd's to know when it is safe to do things like
>>>>>> free or reuse a buffer, but they can also poll() on a fence fd when
>>>>>> waiting to read back results from the GPU.  The EPOLLPRI/POLLPRI flag
>>>>>> lets the kernel differentiate these two cases.
>>>>>>
>>>>>> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>  
>>>>>
>>>>> Hi,
>>>>>
>>>>> where would the UAPI documentation of this go?
>>>>> It seems to be missing.  
>>>>
>>>> Good question, I am not sure.  The poll() man page has a description,
>>>> but my usage doesn't fit that _exactly_ (but OTOH the description is a
>>>> bit vague).
>>>>  
>>>>> If a Wayland compositor is polling application fences to know which
>>>>> client buffer to use in its rendering, should the compositor poll with
>>>>> PRI or not? If a compositor polls with PRI, then all fences from all
>>>>> applications would always be PRI. Would that be harmful somehow or
>>>>> would it be beneficial?  
>>>>
>>>> I think a compositor would rather use the deadline ioctl and then poll
>>>> without PRI.  Otherwise you are giving an urgency signal to the fence
>>>> signaller which might not necessarily be needed.
>>>>
>>>> The places where I expect PRI to be useful is more in mesa (things
>>>> like glFinish(), readpix, and other similar sorts of blocking APIs)  
>>> Hi,
>>>
>>> Hmm, but then user-space could do the opposite, namely, submit work as usual--never
>>> using the SET_DEADLINE ioctl, and then at the end, poll using (E)POLLPRI. That seems
>>> like a possible usage pattern, unintended--maybe, but possible. Do we want to discourage
>>> this? Wouldn't SET_DEADLINE be enough? I mean, one can call SET_DEADLINE with the current
>>> time, and then wouldn't that be equivalent to (E)POLLPRI?  
>>
>> Yeah, (E)POLLPRI isn't strictly needed if we have SET_DEADLINE.  It is
>> slightly more convenient if you want an immediate deadline (single
>> syscall instead of two), but not strictly needed.  OTOH it piggy-backs
>> on existing UABI.
> 
> In that case, I would be conservative, and not add the POLLPRI
> semantics. An UAPI addition that is not strictly needed and somewhat
> unclear if it violates any design principles is best not done, until it
> is proven to be beneficial.

That is my sentiment as well. Moreover, on hard-realtime systems,
one would want to set the deadline at the outset and not at poll time.
-- 
Regards,
Luben