linux-kernel - Re: [PATCH 2/2] drm/sched: Add FIXME detailing potential hang

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Message-ID: <aQEc5eUR8bq+XNG1@lstrano-desk.jf.intel.com>
Date: Tue, 28 Oct 2025 12:43:33 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Philipp Stanner <phasta@...nel.org>
CC: Danilo Krummrich <dakr@...nel.org>, Christian König
	<ckoenig.leichtzumerken@...il.com>, 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>, <tursulin@...ulin.net>,
	<dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] drm/sched: Add FIXME detailing potential hang

On Tue, Oct 28, 2025 at 02:46:02PM +0100, Philipp Stanner wrote:
> If a job from a ready entity needs more credits than are currently
> available, drm_sched_run_job_work() (a work item) simply returns and
> doesn't reschedule itself. The scheduler is only woken up again when the
> next job gets pushed with drm_sched_entity_push_job().
> 
> If someone submits a job that needs too many credits and doesn't submit
> more jobs afterwards, this would lead to the scheduler never pulling the
> too-expensive job, effectively hanging forever.
> 
> Document this problem as a FIXME.
> 
> Signed-off-by: Philipp Stanner <phasta@...nel.org>
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 492e8af639db..eaf8d17b2a66 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1237,6 +1237,16 @@ static void drm_sched_run_job_work(struct work_struct *w)
>  
>  	/* Find entity with a ready job */
>  	entity = drm_sched_select_entity(sched);
> +	/*
> +	 * FIXME:
> +	 * The entity can be NULL when the scheduler currently has no capacity
> +	 * (credits) for more jobs. If that happens, the work item terminates
> +	 * itself here, without rescheduling itself.
> +	 *
> +	 * It only gets started again in drm_sched_entity_push_job(). IOW, the
> +	 * scheduler might hang forever if a job that needs too many credits
> +	 * gets submitted to an entity and no other, subsequent jobs are.
> +	 */

drm_sched_job_done frees the credits, which triggers
drm_sched_free_job_work, and that in turn triggers
drm_sched_run_job_work.

This flow could be refined a bit, but I do believe it works—unless I'm
missing something. I'm pretty sure we have tests in Xe that exhaust the
credits, though it might be continuous submissions; I'd have to check.

Matt 

>  	if (!entity) {
>  		/*
>  		 * Either no more work to do, or the next ready job needs more
> -- 
> 2.49.0
>