linux-kernel - Re: [PATCH] drm/sched: Discourage usage of separate workqueues

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Message-ID: <aEB6GOTlC_Z_Rq8b@cassiopeiae>
Date: Wed, 4 Jun 2025 18:53:44 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Simona Vetter <simona.vetter@...ll.ch>,
	Christian König <christian.koenig@....com>,
	Philipp Stanner <phasta@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sched: Discourage usage of separate workqueues

On Wed, Jun 04, 2025 at 09:45:00AM -0700, Matthew Brost wrote:
> On Wed, Jun 04, 2025 at 05:07:15PM +0200, Simona Vetter wrote:
> > We should definitely document this trick better though, I didn't find any
> > place where that was documented.
> 
> This is a good idea.

I think - and I also mentioned this a few times in the patch series that added
the workqueue support - we should also really document the pitfalls of this.

If the scheduler shares a workqueue with the driver, the driver needs to take
special care when submitting work that it's not possible to prevent run_job and
free_job work from running by doing this.

For instance, if it's a single threaded workqueue and the driver submits work
that allocates with GFP_KERNEL, this is a deadlock condition.

More generally, if the driver submits N work that, for instance allocates with
GFP_KERNEL, it's also a deadlock condition if N == max_active.