Re: [PATCH v2 0/3] drm: commit_work scheduling

[Rob Clark]

On Thu, Oct 1, 2020 at 12:25 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Wed, Sep 30, 2020 at 11:16 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> >
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > The android userspace treats the display pipeline as a realtime problem.
> > And arguably, if your goal is to not miss frame deadlines (ie. vblank),
> > it is.  (See https://lwn.net/Articles/809545/ for the best explaination
> > that I found.)
> >
> > But this presents a problem with using workqueues for non-blocking
> > atomic commit_work(), because the SCHED_FIFO userspace thread(s) can
> > preempt the worker.  Which is not really the outcome you want.. once
> > the required fences are scheduled, you want to push the atomic commit
> > down to hw ASAP.
> >
> > But the decision of whether commit_work should be RT or not really
> > depends on what userspace is doing.  For a pure CFS userspace display
> > pipeline, commit_work() should remain SCHED_NORMAL.
> >
> > To handle this, convert non-blocking commit_work() to use per-CRTC
> > kthread workers, instead of system_unbound_wq.  Per-CRTC workers are
> > used to avoid serializing commits when userspace is using a per-CRTC
> > update loop.  And the last patch exposes the task id to userspace as
> > a CRTC property, so that userspace can adjust the priority and sched
> > policy to fit it's needs.
> >
> >
> > v2: Drop client cap and in-kernel setting of priority/policy in
> >     favor of exposing the kworker tid to userspace so that user-
> >     space can set priority/policy.
>
> Yeah I think this looks more reasonable. Still a bit irky interface,
> so I'd like to get some kworker/rt ack on this. Other opens:
> - needs userspace, the usual drill

fwiw, right now the userspace is "modetest + chrt".. *probably* the
userspace will become a standalone helper or daemon, mostly because
the chrome gpu-process sandbox does not allow setting SCHED_FIFO.  I'm
still entertaining the possibility of switching between rt and cfs
depending on what is in the foreground (ie. only do rt for android
apps).

> - we need this also for vblank workers, otherwise this wont work for
> drivers needing those because of another priority inversion.

I have a thought on that, see below..

> - we probably want some indication of whether this actually does
> something useful, not all drivers use atomic commit helpers. Not sure
> how to do that.

I'm leaning towards converting the other drivers over to use the
per-crtc kwork, and then dropping the 'commit_work` from atomic state.
I can add a patch to that, but figured I could postpone that churn
until there is some by-in on this whole idea.

> - not sure whether the vfunc is an awesome idea, I'd frankly just
> open-code this inline. We have similar special cases already for e.g.
> dpms (and in multiple places), this isn't the worst.

I could introduce a "PID" property type.  This would be useful if we
wanted to also expose the vblank workers.

> - still feeling we could at least change the default to highpriority niceness.

AFAIU this would still be preempted by something that is SCHED_FIFO.
Also, with cfs/SCHED_NORMAL, you can still be preempted by lower
priority things that haven't had a chance to run for a while.

> - there's still the problem that commit works can overlap, and a
> single worker can't do that anymore. So rolling that out for everyone
> as-is feels a bit risky.

That is why it is per-CRTC..  I'm not sure there is a need to overlap
work for a single CRTC?

We could OFC make this a driver knob, and keep the old 'commit_work'
option, but that doesn't really feel like the right direction

BR,
-R

> Cheers, Daniel
>
> >
> > Rob Clark (3):
> >   drm/crtc: Introduce per-crtc kworker
> >   drm/atomic: Use kthread worker for nonblocking commits
> >   drm: Expose CRTC's kworker task id
> >
> >  drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++++----
> >  drivers/gpu/drm/drm_crtc.c          | 14 +++++++++++++
> >  drivers/gpu/drm/drm_mode_config.c   | 14 +++++++++++++
> >  drivers/gpu/drm/drm_mode_object.c   |  4 ++++
> >  include/drm/drm_atomic.h            | 31 +++++++++++++++++++++++++++++
> >  include/drm/drm_crtc.h              |  8 ++++++++
> >  include/drm/drm_mode_config.h       |  9 +++++++++
> >  include/drm/drm_property.h          |  9 +++++++++
> >  8 files changed, 98 insertions(+), 4 deletions(-)
> >
> > --
> > 2.26.2
> >
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch