Date:   Thu, 1 Oct 2020 17:25:55 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Tim Murray <timmurray@...gle.com>,
        Qais Yousef <qais.yousef@....com>,
        Rob Clark <robdclark@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] drm: commit_work scheduling

On Thu, Oct 1, 2020 at 5:15 PM Rob Clark <robdclark@...il.com> wrote:
>
> On Thu, Oct 1, 2020 at 12:25 AM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Wed, Sep 30, 2020 at 11:16 PM Rob Clark <robdclark@...il.com> wrote:
> > >
> > > From: Rob Clark <robdclark@...omium.org>
> > >
> > > 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..

Hm, not seeing anything about vblank worker 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.

i915 has its own commit code, it's not even using the current commit
helpers (nor the commit_work). Not sure how much other fun there is.

> > - 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.

Hm right, but I think we need at most 2 for commit thread and vblank
thread (at least with the current design). So open-coded if with two
if (prop == crtc_worker_pid_prop || prop  ==
crtc_vblank_worker_pid_prop) doesn't seem too horrible to me.
Otherwise people start creating really funny stuff in their drivers
with this backend, and I don't want to spend all the time making sure
they don't abuse this :-)

> > - 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.

i915 uses highprio workqueue, so I guess to avoid regressions we need
that (it's also not using the commit helpers right now, but no reason
not to afaics, stuff simply happened in parallel back then.

> > - 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

drm_atomic_helper_commit_hw_done is what unblocks the next worker on
the same set of crtc. It's before we do all the buffer cleanup, which
has a full vblank stall beforehand. Most drivers also have the same
vblank stall in their next commit, plus generally the fb cleanup is
cheap, but neither is a requirement. So yeah you can get overlapping
commit work on the same crtc, and that was kinda intentional. Maybe
was overkill, I guess minimally just something that needs to be in the
commit message.
-Daniel

>
> 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



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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