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

[Luben Tuikov <ltuikov89@gmail.com>]

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On 2024-01-26 11:29, Matthew Brost wrote:
> On Fri, Jan 26, 2024 at 11:32:57AM +0100, Christian König wrote:
>> 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

Hi Matt,

Thanks for the patch. Under close review, let's use "checked" instead of "drained",
to read as follows,

All entities must be checked in the DRM scheduler run job worker to ...

>>>>> avoid the following case. An entity found that is ready, no job found

Continue with the example given by using a colon, as follows,

... avoid the following case: an entity is found which is ready, yet
no job is returned for that entity when calling drm_sched_entity_pop_job(entity).
This causes the job worker to go idle. The correct behaviour is to loop
over all ready entities, until drm_sched_entity_pop_job(entity) returns non-NULL,
or there are no more ready entities.

>>>>> 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)

You see here how "drain" isn't clear enough, and you clarify in parenthesis
that we in fact "loop over all ready entities". So, perhaps let's not use the
verb "drain" and simply use the sentence in the paragraph I've redacted above.

Also, let's please not use "drain" in the title, as it is confusing and makes me
think of capacitors, transistors, or buckets with water and Archimedes screws and siphons,
and instead say,

[PATCH]: drm/sched: Really find a ready entity and job in DRM sched run-job worker

Which makes it really simple and accessible a description. :-)

>>>>> 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.
>>
> 
> Right, that what this patch does - the run worker does not go idle until
> no ready entities are found. That was incorrect in the original patch
> and fixed here. Do you have any issues with this fix? It has been tested
> 3x times and clearly fixes the issue.

Thanks for following up with Christian.

I agree, the fix makes sense and achieves the original intention as described
by Christian. Also, thanks to all who tested it. Good work, thanks!

With the above changes to the patch title and text addressed, this patch would be then,

Reviewed-by: Luben Tuikov <ltuikov89@gmail.com>

-- 
Regards,
Luben

 
> 
> Matt
> 
>> 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;
>>

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