Re: [PATCH 2/3] drm/atomic: Use kthread worker for nonblocking commits

[Rob Clark]

On Mon, Sep 21, 2020 at 2:23 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Sat, Sep 19, 2020 at 12:37:25PM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > This will allow us to more easily switch scheduling rules based on what
> > userspace wants.
> >
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> I still think switching to the highpriority systemwq as a start (like i915
> already does) would be a good first step no matter what we end up doing
> for the android thing.

highpri wq is probably better than the current state, but it doesn't
really address the problem.  You'll still end up with surfaceflinger
preempting commit_work..

And with non-RT priority, you'll still occasionally get lower priority
threads which haven't had a chance to run for a while preempting you.

BR,
-R


> -Daniel
>
> > ---
> >  drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++++----
> >  include/drm/drm_atomic.h            | 31 +++++++++++++++++++++++++++++
> >  2 files changed, 40 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index 9e1ad493e689..75eeec5e7b10 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1659,11 +1659,11 @@ static void commit_tail(struct drm_atomic_state *old_state)
> >       drm_atomic_state_put(old_state);
> >  }
> >
> > -static void commit_work(struct work_struct *work)
> > +static void commit_work(struct kthread_work *work)
> >  {
> >       struct drm_atomic_state *state = container_of(work,
> >                                                     struct drm_atomic_state,
> > -                                                   commit_work);
> > +                                                   commit_kwork);
> >       commit_tail(state);
> >  }
> >
> > @@ -1797,6 +1797,7 @@ int drm_atomic_helper_commit(struct drm_device *dev,
> >                            struct drm_atomic_state *state,
> >                            bool nonblock)
> >  {
> > +     struct kthread_worker *worker = NULL;
> >       int ret;
> >
> >       if (state->async_update) {
> > @@ -1814,7 +1815,7 @@ int drm_atomic_helper_commit(struct drm_device *dev,
> >       if (ret)
> >               return ret;
> >
> > -     INIT_WORK(&state->commit_work, commit_work);
> > +     kthread_init_work(&state->commit_kwork, commit_work);
> >
> >       ret = drm_atomic_helper_prepare_planes(dev, state);
> >       if (ret)
> > @@ -1857,8 +1858,12 @@ int drm_atomic_helper_commit(struct drm_device *dev,
> >        */
> >
> >       drm_atomic_state_get(state);
> > +
> >       if (nonblock)
> > -             queue_work(system_unbound_wq, &state->commit_work);
> > +             worker = drm_atomic_pick_worker(state);
> > +
> > +     if (worker)
> > +             kthread_queue_work(worker, &state->commit_kwork);
> >       else
> >               commit_tail(state);
> >
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index d07c851d255b..8d0ee19953df 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
> > @@ -373,8 +373,18 @@ struct drm_atomic_state {
> >        *
> >        * Work item which can be used by the driver or helpers to execute the
> >        * commit without blocking.
> > +      *
> > +      * This is deprecated, use commit_kwork.
> >        */
> >       struct work_struct commit_work;
> > +
> > +     /**
> > +      * @commit_kwork:
> > +      *
> > +      * Work item which can be used by the driver or helpers to execute the
> > +      * commit without blocking.
> > +      */
> > +     struct kthread_work commit_kwork;
> >  };
> >
> >  void __drm_crtc_commit_free(struct kref *kref);
> > @@ -954,6 +964,27 @@ void drm_state_dump(struct drm_device *dev, struct drm_printer *p);
> >                     (new_obj_state) = (__state)->private_objs[__i].new_state, 1); \
> >            (__i)++)
> >
> > +/**
> > + * drm_atomic_pick_worker - helper to get kworker to use for nonblocking commit
> > + * @state: the &drm_atomic_state for the commit
> > + *
> > + * Pick an appropriate worker for a given atomic update.  The first CRTC
> > + * invovled in the atomic update is used to pick the worker, to prevent
> > + * serializing multiple pageflips / atomic-updates on indenpendent CRTCs.
> > + */
> > +static inline struct kthread_worker *
> > +drm_atomic_pick_worker(const struct drm_atomic_state *state)
> > +{
> > +     struct drm_crtc_state *crtc_state;
> > +     struct drm_crtc *crtc;
> > +     unsigned i;
> > +
> > +     for_each_new_crtc_in_state(state, crtc, crtc_state, i)
> > +             return crtc->worker;
> > +
> > +     return NULL;
> > +}
> > +
> >  /**
> >   * drm_atomic_crtc_needs_modeset - compute combined modeset need
> >   * @state: &drm_crtc_state for the CRTC
> > --
> > 2.26.2
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch