[PATCH] Documentation: drm: Add entry for removing spsc_queue to TODO list

[PATCH] Documentation: drm: Add entry for removing spsc_queue to TODO list

[Philipp Stanner]

drm_sched contains a lockless queue (spsc_queue) that seems to be
useless and potentially unsound.

Add a TODO list entry for replacing spsc_queue with a locked list.

Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
 Documentation/gpu/todo.rst | 41 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
index cdddf8db35f5..87e082b0eb48 100644
--- a/Documentation/gpu/todo.rst
+++ b/Documentation/gpu/todo.rst
@@ -948,6 +948,47 @@ Contact: Philipp Stanner <phasta@kernel.org>
 
 Level: Intermediate
 
+Replace the lockless queue with a locked list
+---------------------------------------------
+
+drm_sched is the only user in the entire kernel of a special lockless queue, the
+spsc_queue. This queue utilizes:
+
+- preempt_disable()
+- atomic instructions
+- memory barriers
+- ACCESS_ONCE()
+
+whereas a conventional spinlock utilizes:
+
+- preempt_disable()
+- 1 atomic instruction for taking / releasing the lock
+- memory barriers
+
+Moreover, drm_sched_entity_push_job(), the only user of spsc_queue_push(), has
+to take a lock in some situations anyways and calls to it are often serialized
+with a driver lock.
+
+It is, thus, highly questionable whether the lockless queue grants any advantage
+at all. Considering that its internals are not well documented and its correctness
+is not formally proven, it seems desirable to replace the queue with a mere list
+or hlist that is protected by a spinlock.
+
+Tasks:
+
+- Replace the spsc_queue in drm/sched (and those who might access the scheduler's
+  internal queue) with a spinlock + (h)list.
+- Ideally, check with some micro benchmarks and real world tests (preferably
+  with amdgpu) for relevant performance regressions.
+- Remove the spsc_queue from the kernel altogether.
+
+Contact:
+
+- Philipp Stanner <phasta@kernel.org>
+- Christian König <christian.koenig@amd.com>
+
+Level: Beginner
+
 Outside DRM
 ===========
 
-- 
2.54.0

Re: [PATCH] Documentation: drm: Add entry for removing spsc_queue to TODO list

[Christian König]

On 5/29/26 23:52, Philipp Stanner wrote:
> drm_sched contains a lockless queue (spsc_queue) that seems to be
> useless and potentially unsound.
> 
> Add a TODO list entry for replacing spsc_queue with a locked list.
> 
> Signed-off-by: Philipp Stanner <phasta@kernel.org>

Reviewed-by: Christian König <christian.koenig@amd.com>

> ---
>  Documentation/gpu/todo.rst | 41 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index cdddf8db35f5..87e082b0eb48 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -948,6 +948,47 @@ Contact: Philipp Stanner <phasta@kernel.org>
>  
>  Level: Intermediate
>  
> +Replace the lockless queue with a locked list
> +---------------------------------------------
> +
> +drm_sched is the only user in the entire kernel of a special lockless queue, the
> +spsc_queue. This queue utilizes:
> +
> +- preempt_disable()
> +- atomic instructions
> +- memory barriers
> +- ACCESS_ONCE()
> +
> +whereas a conventional spinlock utilizes:
> +
> +- preempt_disable()
> +- 1 atomic instruction for taking / releasing the lock
> +- memory barriers
> +
> +Moreover, drm_sched_entity_push_job(), the only user of spsc_queue_push(), has
> +to take a lock in some situations anyways and calls to it are often serialized
> +with a driver lock.
> +
> +It is, thus, highly questionable whether the lockless queue grants any advantage
> +at all. Considering that its internals are not well documented and its correctness
> +is not formally proven, it seems desirable to replace the queue with a mere list
> +or hlist that is protected by a spinlock.
> +
> +Tasks:
> +
> +- Replace the spsc_queue in drm/sched (and those who might access the scheduler's
> +  internal queue) with a spinlock + (h)list.
> +- Ideally, check with some micro benchmarks and real world tests (preferably
> +  with amdgpu) for relevant performance regressions.
> +- Remove the spsc_queue from the kernel altogether.
> +
> +Contact:
> +
> +- Philipp Stanner <phasta@kernel.org>
> +- Christian König <christian.koenig@amd.com>
> +
> +Level: Beginner
> +
>  Outside DRM
>  ===========
>  

Re: [PATCH] Documentation: drm: Add entry for removing spsc_queue to TODO list

[Philipp Stanner]

On Mon, 2026-06-01 at 10:30 +0200, Christian König wrote:
> 
> 
> On 5/29/26 23:52, Philipp Stanner wrote:
> > drm_sched contains a lockless queue (spsc_queue) that seems to be
> > useless and potentially unsound.
> > 
> > Add a TODO list entry for replacing spsc_queue with a locked list.
> > 
> > Signed-off-by: Philipp Stanner <phasta@kernel.org>
> 
> Reviewed-by: Christian König <christian.koenig@amd.com>

Pushed to drm-misc-next

Thx
P.

> 
> > ---
> >  Documentation/gpu/todo.rst | 41 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 41 insertions(+)
> > 
> > diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> > index cdddf8db35f5..87e082b0eb48 100644
> > --- a/Documentation/gpu/todo.rst
> > +++ b/Documentation/gpu/todo.rst
> > @@ -948,6 +948,47 @@ Contact: Philipp Stanner <phasta@kernel.org>
> >  
> >  Level: Intermediate
> >  
> > +Replace the lockless queue with a locked list
> > +---------------------------------------------
> > +
> > +drm_sched is the only user in the entire kernel of a special lockless queue, the
> > +spsc_queue. This queue utilizes:
> > +
> > +- preempt_disable()
> > +- atomic instructions
> > +- memory barriers
> > +- ACCESS_ONCE()
> > +
> > +whereas a conventional spinlock utilizes:
> > +
> > +- preempt_disable()
> > +- 1 atomic instruction for taking / releasing the lock
> > +- memory barriers
> > +
> > +Moreover, drm_sched_entity_push_job(), the only user of spsc_queue_push(), has
> > +to take a lock in some situations anyways and calls to it are often serialized
> > +with a driver lock.
> > +
> > +It is, thus, highly questionable whether the lockless queue grants any advantage
> > +at all. Considering that its internals are not well documented and its correctness
> > +is not formally proven, it seems desirable to replace the queue with a mere list
> > +or hlist that is protected by a spinlock.
> > +
> > +Tasks:
> > +
> > +- Replace the spsc_queue in drm/sched (and those who might access the scheduler's
> > +  internal queue) with a spinlock + (h)list.
> > +- Ideally, check with some micro benchmarks and real world tests (preferably
> > +  with amdgpu) for relevant performance regressions.
> > +- Remove the spsc_queue from the kernel altogether.
> > +
> > +Contact:
> > +
> > +- Philipp Stanner <phasta@kernel.org>
> > +- Christian König <christian.koenig@amd.com>
> > +
> > +Level: Beginner
> > +
> >  Outside DRM
> >  ===========
> >