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

[Rob Clark]

On Mon, Sep 21, 2020 at 8:16 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> 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?

Looks like kthread goes out of it's way to *not* be owned by users (to
avoid fork, and such complications?)

But maybe we could modify the kthread_worker's task->real_cred?  I
didn't see any examples of anything else doing something similar, so
I'm not sure what sorts of dragons there lie..

> 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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> > > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch