Re: [PATCH] drm/sched: Simplify idle entity check

[Philipp Stanner <phasta@mailbox.org>]

On Fri, 2026-01-09 at 14:06 +0000, Tvrtko Ursulin wrote:
> \
> On 07/01/2026 14:11, Philipp Stanner wrote:
> > Happy 2026, Tvrtko!
> > 
> > 

[…]

> > Not to long ago we discussed that the spsc_queue should be removed and
> > replaced by some sort of list, with proper locks. Christian has agreed
> > that this should fly.
> > 
> > The spsc queue has only 1 user in the kernel and it's super hard to
> > understand how it's supposed to work and when any why barriers and
> > READ_ONCE's are necessary (that's, of course, also not documented in
> > the queue).
> > 
> > Until that is done I don't really want to touch any of those memory
> > barriers..
> 
> (I had a branch with spsc gone more than a year ago but I abandoned it 
> for now since it contained some other too ambitious changes. So no 
> complaints from me. Who is doing it BTW?)

No one is working on it.

But I think the discussion has succeeded. The only objector was
Christian because he was worried about some cache-line performance
regression. I can't remember for sure, but I think Christian realized
that cache lines are not an issue (with a hlist?).

If you want to pick up removing SPSC-queue, be my guest, but make sure
to discuss it with Christian before investing too much of your time.

> 
> Back to the point - this patch can wait, no problem. To explain the 
> context though.
> 
> I wanted to get rid of looking at the list_empty here because I have a 
> branch which improves the flow for the 1:1 sched:entity drivers.
> 
> Why are the two related? If you remember in the fair scheduler series
> all the run-queue stuff is nicely grouped in sched_rq.c and encapsulated 
> in the rq API, which made it possible to follow up with virtualizing the 
> rq operations.
> 
> The yet another relevant thing is the patch I sent this week which 
> removes the special case where entity can be initialized with no schedulers.
> 
> If we combined all these three pre-requisites, my branch allows the 
> fully invariant sched:entity and 1:1:1 sched:rq:entity. Run-queue vfuncs 

Hm, wouldn't the CFS series annihilate multiple RQs anyways? This
sounds as if there are several series' floating around, cleaning up
similar things.


P.