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

[Rob Clark]

On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark 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.
> >
> > A client-cap is introduced so that userspace can opt-in to SCHED_FIFO
> > priority commit work.
> >
> > A potential issue is that since 616d91b68cd ("sched: Remove
> > sched_setscheduler*() EXPORTs") we have limited RT priority levels,
> > meaning that commit_work() ends up running at the same priority level
> > as vblank-work.  This shouldn't be a big problem *yet*, due to limited
> > use of vblank-work at this point.  And if it could be arranged that
> > vblank-work is scheduled before signaling out-fences and/or sending
> > pageflip events, it could probably work ok to use a single priority
> > level for both commit-work and vblank-work.
>
> The part I don't like about this is that it all feels rather hacked
> together, and if we add more stuff (or there's some different thing in the
> system that also needs rt scheduling) then it doesn't compose.

The ideal thing would be that userspace is in control of the
priorities.. the setclientcap approach seemed like a reasonable way to
give the drm-master a way to opt in.

I suppose instead userspace could use sched_setscheduler().. but that
would require userspace to be root, and would require some way to find
the tid.

Is there some way we could arrange for the per-crtc kthread's to be
owned by the drm master?  That would solve the "must be root" issue.
And since the target audience is an atomic userspace, I suppose we
could expose the tid as a read-only property on the crtc?

BR,
-R

> So question to rt/worker folks: What's the best way to let userspace set
> the scheduling mode and priorities of things the kernel does on its
> behalf? Surely we're not the first ones where if userspace runs with some
> rt priority it'll starve out the kernel workers that it needs. Hardcoding
> something behind a subsystem ioctl (which just means every time userspace
> changes what it does, we need a new such flag or mode) can't be the right
> thing.
>
> Peter, Tejun?
>
> Thanks, Daniel
>
> >
> > Rob Clark (3):
> >   drm/crtc: Introduce per-crtc kworker
> >   drm/atomic: Use kthread worker for nonblocking commits
> >   drm: Add a client-cap to set scheduling mode
> >
> >  drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++----
> >  drivers/gpu/drm/drm_auth.c          |  4 ++++
> >  drivers/gpu/drm/drm_crtc.c          | 37 +++++++++++++++++++++++++++++
> >  drivers/gpu/drm/drm_ioctl.c         | 13 ++++++++++
> >  include/drm/drm_atomic.h            | 31 ++++++++++++++++++++++++
> >  include/drm/drm_crtc.h              | 10 ++++++++
> >  include/uapi/drm/drm.h              | 13 ++++++++++
> >  7 files changed, 117 insertions(+), 4 deletions(-)
> >
> > --
> > 2.26.2
> >
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>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch