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@xxxxxxxxxx>

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

> ---
>  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@xxxxxxxxxx>
>  
>  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@xxxxxxxxxx>
> +- Christian König <christian.koenig@xxxxxxx>
> +
> +Level: Beginner
> +
>  Outside DRM
>  ===========
>