linux-kernel - Re: [PATCH v4 3/3] drm/sched: Limit sched score update to jobs change

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Message-ID: <38836d5b-9a98-4752-a5a5-f40bd643601a@amd.com>
Date: Fri, 7 Nov 2025 11:33:43 +0100
From: Christian König <christian.koenig@....com>
To: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>,
 Matthew Brost <matthew.brost@...el.com>, Danilo Krummrich <dakr@...nel.org>,
 Philipp Stanner <phasta@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>,
 Tomeu Vizoso <tomeu@...euvizoso.net>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] drm/sched: Limit sched score update to jobs change

On 11/7/25 10:04, 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, let's imagine 5 userspace apps exit, so their entities will be dropped. In
> the worst case, these apps' entities were 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>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>
> Acked-by: Tomeu Vizoso <tomeu@...euvizoso.net>
> Acked-by: Philipp Stanner <phasta@...nel.org>

Reviewed-by: Christian König <christian.koenig@....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 c39f0245e3a9..8a3d99a86090 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)