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 <[email protected]> > > Reviewed-by: Christian König <[email protected]>
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 <[email protected]> > > > > 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 <[email protected]> > > +- Christian König <[email protected]> > > + > > +Level: Beginner > > + > > Outside DRM > > =========== > >
