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

[Qais Yousef <qais.yousef@arm.com>]

On 09/30/20 14:17, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.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.

For me thees 2 properties

	1. Run ASAP
	2. Finish the work un-interrupted

Scream the workers need to be SCHED_FIFO by default. CFS can't give you these
guarantees.

IMO using sched_set_fifo() for these workers is the right thing.

> 
> 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.

I'm not sure I agree with this. I think it's better to characterize tasks based
on their properties/requirements rather than what the rest of the userspace is
using.

I do appreciate that maybe some of these tasks have varying requirements during
their life time. e.g: they have RT property during specific critical section
but otherwise are CFS tasks. I think the UI thread in Android behaves like
that.

It's worth IMO trying that approach I pointed out earlier to see if making RT
try to pick an idle CPU rather than preempt CFS helps. Not sure if it'd be
accepted but IMHO it's a better direction to consider and discuss.

Or maybe you can wrap userspace pipeline critical section lock such that any
task holding it will automatically be promoted to SCHED_FIFO and then demoted
to CFS once it releases it.

Haven't worked with display pipelines before, so hopefully this makes sense :-)

Thanks

--
Qais Yousef

> 
> 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.
> 
> 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
>