Re: drm_sched run_job and scheduling latency

Matthew Brost
2026-03-16T04:15:06Z

On Sun, Mar 15, 2026 at 09:05:20PM -0700, Matthew Brost wrote:
> On Thu, Mar 05, 2026 at 10:47:32AM +0100, Philipp Stanner wrote:
>

Obviously this was intended as a private communication — I hit the wrong
button. I apologize to anyone I offended here.

Matt

> Off the list... I don’t think airing our personal attacks publicly is a
> good look. I’m going to be blunt here in an effort to help you.
>
> > On Thu, 2026-03-05 at 01:10 -0800, Matthew Brost wrote:
> > > On Thu, Mar 05, 2026 at 09:38:16AM +0100, Philipp Stanner wrote:
> > > > On Thu, 2026-03-05 at 09:27 +0100, Boris Brezillon wrote:
> > > >
> > > > >
> >
> > […]
> >
> > > > > Honestly, I'm not thrilled by this fast-path/call-run_job-directly idea
> > > > > you're describing. There's just so many things we can forget that would
> > > > > lead to races/ordering issues that will end up being hard to trigger and
> > > > > debug.
> > > > >
> > > >
> > > > +1
> > > >
> > > > I'm not thrilled either. More like the opposite of thrilled actually.
> > > >
> > > > Even if we could get that to work. This is more of a maintainability
> > > > issue.
> > > >
> > > > The scheduler is full of insane performance hacks for this or that
> > > > driver. Lockless accesses, a special lockless queue only used by that
> > > > one party in the kernel (a lockless queue which is nowadays, after N
> > > > reworks, being used with a lock. Ah well).
> > > >
> > >
> > > This is not relevant to this discussion—see below. In general, I agree
> > > that the lockless tricks in the scheduler are not great, nor is the fact
> > > that the scheduler became a dumping ground for driver-specific features.
> > > But again, that is not what we’re talking about here—see below.
> > >
> > > > In the past discussions Danilo and I made it clear that more major
> > > > features in _new_ patch series aimed at getting merged into drm/sched
> > > > must be preceded by cleanup work to address some of the scheduler's
> > > > major problems.
> > >
> > > Ah, we've moved to dictatorship quickly. Noted.
> >
> > I prefer the term "benevolent presidency" /s
> >
> > Or even better: s/dictatorship/accountability enforcement.
> >
>
> It’s very hard to take this seriously when I reply to threads saying
> something breaks dma-fence rules and the response is, “what are
> dma-fence rules?” Or I read through the jobqueue thread and see you
> asking why a dma-fence would come from anywhere other than your own
> driver — that’s the entire point of dma-fence; it’s a cross-driver
> contract. I could go on, but I’d encourage you to take a hard look at
> your understanding of DRM, and whether your responses — to me and to
> others — are backed by the necessary technical knowledge.
>
> Even better — what first annoyed me was your XDC presentation. You gave
> an example of my driver modifying the pending list without a lock while
> scheduling was stopped, and claimed you fixed a bug. That was not a bug
> - Xe would explode if it was as we test our code. The pending list can
> be modified without a lock if scheduling is stopped. I almost grabbed
> the mic to correct you. Yes, it’s a layering violation, but presenting
> it aa a bug shows a clear lack of understanding.
>
> > How does it come that everyone is here and ready so quickly when it
>
> I’ve suggested ideas to fix DRM sched (refcounting, clear teardown
> flows), but they were immediately met with resistance — typically from
> Christian with you agreeing. My willingness to fight with Christian is
> low; I really don’t need another person to argue with.
>
> > comes to new use cases and features, yet I never saw anyone except for
> > Tvrtko and Maíra investing even 15 minutes to write a simple patch to
> > address some of the *various* significant issues in that code base?
> >
> > You were on CC on all discussions we've had here for the last years
> > afair, but I rarely saw you participate. And you know what it's like:
>
> I’ll admit I’m busy with many other things, so my bandwidth is limited.
> But again, if I chime in and explain how I solved something in Xe (e.g.,
> refcounting) and it’s met with resistance, I’ll likely move on — I’ve
> already solved it, and I’ll just let you fail (see cancel_job).
>
> > who doesn't speak up silently agrees in open source.
> >
> > But tell me one thing, if you can be so kind:
> >
>
> I'm glad you asked this, and it inspired me to fix this, more below [1].
>
> > What is your theory why drm/sched came to be in such horrible shape?
>
> drm/sched was ported from AMDGPU into common code. It carried many
> AMDGPU-specific hacks, had no object-lifetime model thought out as a
> common component, and included teardown nightmares that “worked,” but
> other drivers immediately had to work around. With Christian involved —
> who is notoriously hostile — everyone did their best to paper over
> issues driver-side rather than get into fights and fix things properly.
> Asahi Linux publicly aired grievances about this situation years ago.
>
> > What circumstances, what human behavioral patterns have caused this?
> >
>
> See above.
>
> > The DRM subsystem has a bad reputation regarding stability among Linux
> > users, as far as I have sensed. How can we do better?
> >
>
> Write sane code and test it. fwiw, Google shared a doc with me
> indicating that Xe has unprecedented stability, and to be honest, when I
> first wrote Xe I barely knew what I was doing — but I did know how to
> test. I’ve since cleaned up most of my mistakes though.
>
> So how can we do better... We can [1].
>
> I started on [1] after you asking what the problems in DRM sched - which
> got me thinking about what it would look like if we took the good parts
> (stop/start control plane, dependency tracking, ordering, finished
> fences, etc.), dropped the bad parts (no object-lifetime model, no
> refcounting, overly complex queue teardown, messy fence manipulation,
> hardware-scheduling baggage, lack of annotations, etc.), and wrote
> something that addresses all of these problems from the start
> specifically for firmware-scheduling models.
>
> It turns out pretty good.
>
> Main patch [2].
>
> Xe is fully converted, tested, and working. AMDXNDA and Panthor are
> compiling. Nouveau and PVR seem like good candidates to convert as well.
> Rust bindings are also possible given the clear object model with
> refcounting and well-defined object lifetimes.
>
> Thinking further, hardware schedulers should be able to be implemented
> on top of this by embedding the objects in [2] and layering a
> backend/API on top.
>
> Let me know if you have any feedback (off-list) before I share this
> publicly. So far, Dave, Sima, Danilo, and the other Xe maintainers have
> been looped in.
>
> Matt
>
> [1] https://gitlab.freedesktop.org/mbrost/xe-kernel-driver-svn-perf-6-15-2025/-/tree/local_dev/new_scheduler.post?ref_type=heads
> [2] https://gitlab.freedesktop.org/mbrost/xe-kernel-driver-svn-perf-6-15-2025/-/commit/0538a3bc2a3b562dc0427a5922958189e0be8271
>
> > >
> > > >
> > >
> > > I can't say I agree with either of you here.
> > >
> > > In about an hour, I seemingly have a bypass path working in DRM sched +
> > > Xe, and my diff is:
> > >
> > > 108 insertions(+), 31 deletions(-)
> >
> > LOC is a bad metric for complexity.
> >
> > >
> > > About 40 lines of the insertions are kernel-doc, so I'm not buying that
> > > this is a maintenance issue or a major feature - it is literally a
> > > single new function.
> > >
> > > I understand a bypass path can create issues—for example, on certain
> > > queues in Xe I definitely can't use the bypass path, so Xe simply
> > > wouldn’t use it in those cases. This is the driver's choice to use or
> > > not. If a driver doesn't know how to use the scheduler, well, that’s on
> > > the driver. Providing a simple, documented function as a fast path
> > > really isn't some crazy idea.
> >
> > We're effectively talking about a deviation from the default submission
> > mechanism, and all that seems to be desired for a luxury feature.
> >
> > Then you end up with two submission mechanisms, whose correctness in
> > the future relies on someone remembering what the background was, why
> > it was added, and what the rules are..
> >
> > The current scheduler rules are / were often not even documented, and
> > sometimes even Christian took a few weeks to remember again why
> > something had been added – and whether it can now be removed again or
> > not.
> >
> > >
> > > The alternative—asking for RT workqueues or changing the design to use
> > > kthread_worker—actually is.
> > >
> > > > That's especially true if it's features aimed at performance buffs.
> > > >
> > >
> > > With the above mindset, I'm actually very confused why this series [1]
> > > would even be considered as this order of magnitude greater in
> > > complexity than my suggestion here.
> > >
> > > Matt
> > >
> > > [1] https://patchwork.freedesktop.org/series/159025/
> >
> > The discussions about Tvrtko's CFS series were precisely the point
> > where Danilo brought up that after this can be merged, future rework of
> > the scheduler must focus on addressing some of the pending fundamental
> > issues.
> >
> > The background is that Tvrtko has worked on that series already for
> > well over a year, it actually simplifies some things in the sense of
> > removing unused code (obviously it's a complex series, no argument
> > about that), and we agreed on XDC that this can be merged. So this is a
> > question of fairness to the contributor.
> >
> > But at one point you have to finally draw a line. No one will ever
> > address major scheduler issues unless we demand it. Even very
> > experienced devs usually prefer to hack around the central design
> > issues in their drivers instead of fixing the shared infrastructure.
> >
> >
> > P.