[PATCH v3 01/20] drm/sched: entity->rq selection cannot fail

[Christian König]

Am 09.07.21 um 09:14 schrieb Daniel Vetter:
> On Fri, Jul 9, 2021 at 8:53 AM Christian König <christian.koenig at amd.com> wrote:
>> Am 08.07.21 um 19:37 schrieb Daniel Vetter:
>>> If it does, someone managed to set up a sched_entity without
>>> schedulers, which is just a driver bug.
>> NAK, it is perfectly valid for rq selection to fail.
> There isn't a better way to explain stuff to someone who's new to the
> code and tries to improve it with docs than to NAK stuff with
> incomplete explanations?

Well as far as I understand it a NAK means that the author has missed 
something important and needs to re-iterate.

It's just to say that we absolutely can't merge a patch or something 
will break.

>> See drm_sched_pick_best():
>>
>>                   if (!sched->ready) {
>>                           DRM_WARN("scheduler %s is not ready, skipping",
>>                                    sched->name);
>>                           continue;
>>                   }
>>
>> This can happen when a device reset fails for some engine.
> Well yeah I didn't expect amdgpu to just change this directly, so I
> didn't find it. Getting an ENOENT on a hw failure instead of an EIO is
> a bit interesting semantics I guess, also what happens with the jobs
> which raced against the scheduler not being ready? I'm not seeing any
> checks for ready in the main scheduler logic so this at least looks
> somewhat accidental as a side effect, also no other driver than amdgpu
> communitcates that reset failed back to drm/sched like this. They seem
> to just not, and I guess timeout on the next request will get us into
> an endless reset loop?

Correct. Key point is that there aren't any jobs which are currently 
scheduled.

When the ready flag is changed the scheduler is paused, e.g. the main 
thread is not running any more.

I'm pretty sure that all of this is horrible racy, but nobody really 
looked into the design from a higher level as far as I know.

Christian.



> -Daniel
>
>
>> Regards,
>> Christian.
>>
>>> We BUG_ON() here because in the next patch drm_sched_job_init() will
>>> be split up, with drm_sched_job_arm() never failing. And that's the
>>> part where the rq selection will end up in.
>>>
>>> Note that if having an empty sched_list set on an entity is indeed a
>>> valid use-case, we can keep that check in job_init even after the split
>>> into job_init/arm.
>>>
>>> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>>> Cc: "Christian König" <christian.koenig at amd.com>
>>> Cc: Luben Tuikov <luben.tuikov at amd.com>
>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>> Cc: Steven Price <steven.price at arm.com>
>>> Cc: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>> Cc: Boris Brezillon <boris.brezillon at collabora.com>
>>> Cc: Jack Zhang <Jack.Zhang1 at amd.com>
>>> ---
>>>    drivers/gpu/drm/scheduler/sched_entity.c | 2 +-
>>>    drivers/gpu/drm/scheduler/sched_main.c   | 3 +--
>>>    2 files changed, 2 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
>>> index 79554aa4dbb1..6fc116ee7302 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>>> @@ -45,7 +45,7 @@
>>>     * @guilty: atomic_t set to 1 when a job on this queue
>>>     *          is found to be guilty causing a timeout
>>>     *
>>> - * Note: the sched_list should have at least one element to schedule
>>> + * Note: the sched_list must have at least one element to schedule
>>>     *       the entity
>>>     *
>>>     * Returns 0 on success or a negative error code on failure.
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 33c414d55fab..01dd47154181 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -586,8 +586,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>>>        struct drm_gpu_scheduler *sched;
>>>
>>>        drm_sched_entity_select_rq(entity);
>>> -     if (!entity->rq)
>>> -             return -ENOENT;
>>> +     BUG_ON(!entity->rq);
>>>
>>>        sched = entity->rq->sched;
>>>
>