[PATCH] drm/sched: Don't disturb the entity when in RR-mode scheduling

[Danilo Krummrich]

On 11/9/23 20:24, Danilo Krummrich wrote:
> On 11/9/23 07:52, Luben Tuikov wrote:
>> Hi,
>>
>> On 2023-11-07 19:41, Danilo Krummrich wrote:
>>> On 11/7/23 05:10, Luben Tuikov wrote:
>>>> Don't call drm_sched_select_entity() in drm_sched_run_job_queue().  In fact,
>>>> rename __drm_sched_run_job_queue() to just drm_sched_run_job_queue(), and let
>>>> it do just that, schedule the work item for execution.
>>>>
>>>> The problem is that drm_sched_run_job_queue() calls drm_sched_select_entity()
>>>> to determine if the scheduler has an entity ready in one of its run-queues,
>>>> and in the case of the Round-Robin (RR) scheduling, the function
>>>> drm_sched_rq_select_entity_rr() does just that, selects the _next_ entity
>>>> which is ready, sets up the run-queue and completion and returns that
>>>> entity. The FIFO scheduling algorithm is unaffected.
>>>>
>>>> Now, since drm_sched_run_job_work() also calls drm_sched_select_entity(), then
>>>> in the case of RR scheduling, that would result in drm_sched_select_entity()
>>>> having been called twice, which may result in skipping a ready entity if more
>>>> than one entity is ready. This commit fixes this by eliminating the call to
>>>> drm_sched_select_entity() from drm_sched_run_job_queue(), and leaves it only
>>>> in drm_sched_run_job_work().
>>>>
>>>> v2: Rebased on top of Tvrtko's renames series of patches. (Luben)
>>>>       Add fixes-tag. (Tvrtko)
>>>>
>>>> Signed-off-by: Luben Tuikov <ltuikov89 at gmail.com>
>>>> Fixes: f7fe64ad0f22ff ("drm/sched: Split free_job into own work item")
>>>> ---
>>>>    drivers/gpu/drm/scheduler/sched_main.c | 16 +++-------------
>>>>    1 file changed, 3 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 27843e37d9b769..cd0dc3f81d05f0 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -256,10 +256,10 @@ drm_sched_rq_select_entity_fifo(struct drm_sched_rq *rq)
>>>>    }
>>>>    /**
>>>> - * __drm_sched_run_job_queue - enqueue run-job work
>>>> + * drm_sched_run_job_queue - enqueue run-job work
>>>>     * @sched: scheduler instance
>>>>     */
>>>> -static void __drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>>>> +static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>>>>    {
>>>>        if (!READ_ONCE(sched->pause_submit))
>>>>            queue_work(sched->submit_wq, &sched->work_run_job);
>>>> @@ -928,7 +928,7 @@ static bool drm_sched_can_queue(struct drm_gpu_scheduler *sched)
>>>>    void drm_sched_wakeup(struct drm_gpu_scheduler *sched)
>>>>    {
>>>>        if (drm_sched_can_queue(sched))
>>>> -        __drm_sched_run_job_queue(sched);
>>>> +        drm_sched_run_job_queue(sched);
>>>>    }
>>>>    /**
>>>> @@ -1040,16 +1040,6 @@ drm_sched_pick_best(struct drm_gpu_scheduler **sched_list,
>>>>    }
>>>>    EXPORT_SYMBOL(drm_sched_pick_best);
>>>> -/**
>>>> - * drm_sched_run_job_queue - enqueue run-job work if there are ready entities
>>>> - * @sched: scheduler instance
>>>> - */
>>>> -static void drm_sched_run_job_queue(struct drm_gpu_scheduler *sched)
>>>> -{
>>>> -    if (drm_sched_select_entity(sched))
>>>
>>> Hm, now that I rebase my patch to implement dynamic job-flow control I recognize that
>>> we probably need the peek semantics here. If we do not select an entity here, we also
>>> do not check whether the corresponding job fits on the ring.
>>>
>>> Alternatively, we simply can't do this check in drm_sched_wakeup(). The consequence would
>>> be that we don't detect that we need to wait for credits to free up before the run work is
>>> already executing and the run work selects an entity.
>>
>> So I rebased v5 on top of the latest drm-misc-next, and looked around and found out that
>> drm_sched_wakeup() is missing drm_sched_entity_is_ready(). It should look like the following,
> 
> Yeah, but that's just the consequence of re-basing it onto Tvrtko's patch.
> 
> My point is that by removing drm_sched_select_entity() from drm_sched_run_job_queue() we do not
> only loose the check whether the selected entity is ready, but also whether we have enough
> credits to actually run a new job. This can lead to queuing up work that does nothing but calling
> drm_sched_select_entity() and return.

Ok, I see it now.  We don't need to peek, we know the entity at drm_sched_wakeup().

However, the missing drm_sched_entity_is_ready() check should have been added already when
drm_sched_select_entity() was removed. Gonna send a fix for that as well.

- Danilo

> 
> By peeking the entity we could know this *before* scheduling work and hence avoid some CPU scheduler
> overhead.
> 
> However, since this patch already landed and we can fail the same way if the selected entity isn't
> ready I don't consider this to be a blocker for the credit patch, hence I will send out a v6.
> 
>>
>> void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
>>               struct drm_sched_entity *entity)
>> {
>>     if (drm_sched_entity_is_ready(entity))
>>         if (drm_sched_can_queue(sched, entity))
>>             drm_sched_run_job_queue(sched);
>> }
>>
>> See the attached patch. (Currently running with base-commit and the attached patch.)