linux-kernel - Re: [PATCH v1 2/2] drm/sched: limit sched score update to jobs change

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Message-ID: <5183ed88-1634-42c2-9bd8-ed0e626f877b@damsy.net>
Date: Fri, 26 Sep 2025 10:20:38 +0200
From: Pierre-Eric Pelloux-Prayer <pierre-eric@...sy.net>
To: Tvrtko Ursulin <tursulin@...ulin.net>,
 Tomeu Vizoso <tomeu@...euvizoso.net>, Oded Gabbay <ogabbay@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 Simona Vetter <simona@...ll.ch>,
 Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>,
 Danilo Krummrich <dakr@...nel.org>,
 Christian König <ckoenig.leichtzumerken@...il.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Thomas Zimmermann <tzimmermann@...e.de>, Philipp Stanner <phasta@...nel.org>
Subject: Re: [PATCH v1 2/2] drm/sched: limit sched score update to jobs change

Hi,

Le 01/09/2025 à 11:20, Tvrtko Ursulin a écrit :
> 
> 
> + Tomeu and Oded
> 
> On 22/08/2025 14:43, Pierre-Eric Pelloux-Prayer wrote:
>> Currently, the scheduler score is incremented when a job is pushed to an
>> entity and when an entity is attached to the scheduler.
>>
>> This leads to some bad scheduling decision where the score value is
>> largely made of idle entities.
>>
>> For instance, a scenario with 2 schedulers and where 10 entities submit
>> a single job, then do nothing, each scheduler will probably end up with
>> a score of 5.
>> Now, 5 userspace apps exit, so their entities will be dropped. In
>> the worst case, these apps' entities where all attached to the same
>> scheduler and we end up with score=5 (the 5 remaining entities) and
>> score=0, despite the 2 schedulers being idle.
>> When new entities show up, they will all select the second scheduler
>> based on its low score value, instead of alternating between the 2.
>>
>> Some amdgpu rings depended on this feature, but the previous commit
>> implemented the same thing in amdgpu directly so it can be safely
>> removed from drm/sched.
>>
>> Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/ 
>> scheduler/sched_main.c
>> index 5a550fd76bf0..e6d232a8ec58 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -206,7 +206,6 @@ void drm_sched_rq_add_entity(struct drm_sched_rq *rq,
>>       if (!list_empty(&entity->list))
>>           return;
>> -    atomic_inc(rq->sched->score);
>>       list_add_tail(&entity->list, &rq->entities);
>>   }
>> @@ -228,7 +227,6 @@ void drm_sched_rq_remove_entity(struct drm_sched_rq *rq,
>>       spin_lock(&rq->lock);
>> -    atomic_dec(rq->sched->score);
>>       list_del_init(&entity->list);
>>       if (rq->current_entity == entity)
> 
> LGTM.
> 
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>
> 

Thanks.

> Only detail is, I did a revisit of the scheduler users and it looks like the new 
> rocket driver is the only one other than amdgpu which passes a list of more than 
> one scheduler to drm_sched_entity_init. I don't *think* it would be affected 
> though. It would still pick the least loaded (based on active jobs) scheduler at 
> job submit time. Unless there is some hidden behaviour in that driver where it 
> would be important to consider number of entities too. Anyway, it would be good 
> for rocket driver to double-check and ack.
> 

Tomeu, Oded: any concerns about this change?