linux-kernel - [PATCH 0/3] drm: commit

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Message-Id: <20200919193727.2093945-1-robdclark@gmail.com>
Date:   Sat, 19 Sep 2020 12:37:23 -0700
From:   Rob Clark <robdclark@...il.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
        timmurray@...gle.com, linux-arm-msm@...r.kernel.org,
        Rob Clark <robdclark@...omium.org>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH 0/3] drm: commit_work scheduling

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.

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.

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