Re: [PATCH] drm/sched: Drain all entities in DRM sched run job worker

["Christian König" <christian.koenig@amd.com>]

Am 25.01.24 um 18:30 schrieb Matthew Brost:
> On Thu, Jan 25, 2024 at 04:12:58PM +0100, Christian König wrote:
>>
>> Am 24.01.24 um 22:08 schrieb Matthew Brost:
>>> All entities must be drained in the DRM scheduler run job worker to
>>> avoid the following case. An entity found that is ready, no job found
>>> ready on entity, and run job worker goes idle with other entities + jobs
>>> ready. Draining all ready entities (i.e. loop over all ready entities)
>>> in the run job worker ensures all job that are ready will be scheduled.
>> That doesn't make sense. drm_sched_select_entity() only returns entities
>> which are "ready", e.g. have a job to run.
>>
> That is what I thought too, hence my original design but it is not
> exactly true. Let me explain.
>
> drm_sched_select_entity() returns an entity with a non-empty spsc queue
> (job in queue) and no *current* waiting dependecies [1]. Dependecies for
> an entity can be added when drm_sched_entity_pop_job() is called [2][3]
> returning a NULL job. Thus we can get into a scenario where 2 entities
> A and B both have jobs and no current dependecies. A's job is waiting
> B's job, entity A gets selected first, a dependecy gets installed in
> drm_sched_entity_pop_job(), run work goes idle, and now we deadlock.

And here is the real problem. run work doesn't goes idle in that moment.

drm_sched_run_job_work() should restarts itself until there is either no 
more space in the ring buffer or it can't find a ready entity any more.

At least that was the original design when that was all still driven by 
a kthread.

It can perfectly be that we messed this up when switching from kthread 
to a work item.

Regards,
Christian.

>
> The proper solution is to loop over all ready entities until one with a
> job is found via drm_sched_entity_pop_job() and then requeue the run
> job worker. Or loop over all entities until drm_sched_select_entity()
> returns NULL and then let the run job worker go idle. This is what the
> old threaded design did too [4]. Hope this clears everything up.
>
> Matt
>
> [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/scheduler/sched_entity.c#L144
> [2] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/scheduler/sched_entity.c#L464
> [3] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/scheduler/sched_entity.c#L397
> [4] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/scheduler/sched_main.c#L1011
>
>> If that's not the case any more then you have broken something else.
>>
>> Regards,
>> Christian.
>>
>>> Cc: Thorsten Leemhuis <regressions@leemhuis.info>
>>> Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
>>> Closes: https://lore.kernel.org/all/CABXGCsM2VLs489CH-vF-1539-s3in37=bwuOWtoeeE+q26zE+Q@mail.gmail.com/
>>> Reported-and-tested-by: Mario Limonciello <mario.limonciello@amd.com>
>>> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3124
>>> Link: https://lore.kernel.org/all/20240123021155.2775-1-mario.limonciello@amd.com/
>>> Reported-by: Vlastimil Babka <vbabka@suse.cz>
>>> Closes: https://lore.kernel.org/dri-devel/05ddb2da-b182-4791-8ef7-82179fd159a8@amd.com/T/#m0c31d4d1b9ae9995bb880974c4f1dbaddc33a48a
>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>> ---
>>>    drivers/gpu/drm/scheduler/sched_main.c | 15 +++++++--------
>>>    1 file changed, 7 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 550492a7a031..85f082396d42 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -1178,21 +1178,20 @@ static void drm_sched_run_job_work(struct work_struct *w)
>>>    	struct drm_sched_entity *entity;
>>>    	struct dma_fence *fence;
>>>    	struct drm_sched_fence *s_fence;
>>> -	struct drm_sched_job *sched_job;
>>> +	struct drm_sched_job *sched_job = NULL;
>>>    	int r;
>>>    	if (READ_ONCE(sched->pause_submit))
>>>    		return;
>>> -	entity = drm_sched_select_entity(sched);
>>> +	/* Find entity with a ready job */
>>> +	while (!sched_job && (entity = drm_sched_select_entity(sched))) {
>>> +		sched_job = drm_sched_entity_pop_job(entity);
>>> +		if (!sched_job)
>>> +			complete_all(&entity->entity_idle);
>>> +	}
>>>    	if (!entity)
>>> -		return;
>>> -
>>> -	sched_job = drm_sched_entity_pop_job(entity);
>>> -	if (!sched_job) {
>>> -		complete_all(&entity->entity_idle);
>>>    		return;	/* No more work */
>>> -	}
>>>    	s_fence = sched_job->s_fence;