Nouveau - May 2025 - [PATCH v2 2/6] drm/sched: Prevent teardown waitque from blocking too long

On Fri, May 16, 2025 at 11:19:50AM +0100, Tvrtko Ursulin
wrote:
> 
> On 16/05/2025 10:53, Danilo Krummrich wrote:
> > On Fri, May 16, 2025 at 10:33:30AM +0100, Tvrtko Ursulin wrote:
> > > On 24/04/2025 10:55, Philipp Stanner wrote:
> > > > +	 * @kill_fence_context: kill the fence context belonging
to this scheduler
> > > 
> > > Which fence context would that be? ;)
> > 
> > There's one one per ring and a scheduler instance represents a
single ring. So,
> > what should be specified here?
> 
> I was pointing out the fact not all drivers are 1:1 sched:entity.
I'm well aware, but how is that relevant? Entities don't have an
associated
fence context, but a GPU Ring (either hardware or software) has, which a
scheduler instance represents.
> Thought it would be obvious from the ";)".
I should read from ";)" that you refer to a 1:N-sched:entity
relationship (which
doesn't seem to be related)?
> > > Also, "fence context" would be a new terminology in
gpu_scheduler.h API
> > > level. You could call it ->sched_fini() or similar to signify
at which point
> > > in the API it gets called and then the fact it takes sched as
parameter
> > > would be natural.
> > 
> > The driver should tear down the fence context in this callback, not
the while
> > scheduler. ->sched_fini() would hence be misleading.
> 
> Not the while what? Not while drm_sched_fini()?
*whole
> Could call it sched_kill()
> or anything. My point is that we dont' have "fence context"
in the API but
> entities so adding a new term sounds sub-optimal.
In the callback the driver should neither tear down an entity, nor the whole
scheduler, hence we shouldn't call it like that. sched_kill() is therefore
misleading as well.

It should be named after what it actually does (or should do). Feel free to
propose a different name that conforms with that.
> > > We also probably want some commentary on the topic of indefinite
(or very
> > > long at least) blocking a thread exit / SIGINT/TERM/KILL time.
> > 
> > You mean in case the driver does implement the callback, but does
*not* properly
> > tear down the fence context? So, you ask for describing potential
consequences
> > of drivers having bugs in the implementation of the callback? Or
something else?
> 
> I was proposing the kerneldoc for the vfunc should document the callback
> must not block, or if blocking is unavoidable, either document a guideline
> on how long is acceptable. Maybe even enforce a limit in the scheduler core
> itself.
Killing the fence context shouldn't block.

On 16/05/2025 11:54, Danilo Krummrich wrote:
> On Fri, May 16, 2025 at 11:19:50AM +0100, Tvrtko Ursulin wrote:
>>
>> On 16/05/2025 10:53, Danilo Krummrich wrote:
>>> On Fri, May 16, 2025 at 10:33:30AM +0100, Tvrtko Ursulin wrote:
>>>> On 24/04/2025 10:55, Philipp Stanner wrote:
>>>>> +	 * @kill_fence_context: kill the fence context belonging
to this scheduler
>>>>
>>>> Which fence context would that be? ;)
>>>
>>> There's one one per ring and a scheduler instance represents a
single ring. So,
>>> what should be specified here?
>>
>> I was pointing out the fact not all drivers are 1:1 sched:entity.
> 
> I'm well aware, but how is that relevant? Entities don't have an
associated
> fence context, but a GPU Ring (either hardware or software) has, which a
> scheduler instance represents.
Aha! Well.. how it is relevant and do entities not have an associated 
fence context? Well, entity->fence_context.. that was my first 
association this whole time. Never it crossed my mind this is talking 
about the hardware fence context. Proof in the pudding naming should be 
improved.

But I also don't think there is a requirement for fences returned from 
->run_job() to have a single context. Which again makes it not the best 
naming.
>> Thought it would be obvious from the ";)".
> 
> I should read from ";)" that you refer to a 1:N-sched:entity
relationship (which
> doesn't seem to be related)?
> 
>>>> Also, "fence context" would be a new terminology in
gpu_scheduler.h API
>>>> level. You could call it ->sched_fini() or similar to
signify at which point
>>>> in the API it gets called and then the fact it takes sched as
parameter
>>>> would be natural.
>>>
>>> The driver should tear down the fence context in this callback, not
the while
>>> scheduler. ->sched_fini() would hence be misleading.
>>
>> Not the while what? Not while drm_sched_fini()?
> 
> *whole
> 
>> Could call it sched_kill()
>> or anything. My point is that we dont' have "fence
context" in the API but
>> entities so adding a new term sounds sub-optimal.
> 
> In the callback the driver should neither tear down an entity, nor the
whole
> scheduler, hence we shouldn't call it like that. sched_kill() is
therefore
> misleading as well.
  ->sched_exit()? ->sched_stop()? ->sched_cleanup()?
> It should be named after what it actually does (or should do). Feel free to
> propose a different name that conforms with that.
> 
>>>> We also probably want some commentary on the topic of
indefinite (or very
>>>> long at least) blocking a thread exit / SIGINT/TERM/KILL time.
>>>
>>> You mean in case the driver does implement the callback, but does
*not* properly
>>> tear down the fence context? So, you ask for describing potential
consequences
>>> of drivers having bugs in the implementation of the callback? Or
something else?
>>
>> I was proposing the kerneldoc for the vfunc should document the
callback
>> must not block, or if blocking is unavoidable, either document a
guideline
>> on how long is acceptable. Maybe even enforce a limit in the scheduler
core
>> itself.
> 
> Killing the fence context shouldn't block.
Cool. And maybe convert the wait_event to wait_event_timeout with a 
warning to be robust.

Mind you, I still think a solution which does not add new state 
machinery should be preferred.

Regards,

Tvrtko