On Wed, Sep 30, 2020 at 11:16 PM Rob Clark <robdclark@xxxxxxxxx> 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. 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 - we need this also for vblank workers, otherwise this wont work for drivers needing those because of another priority inversion. - 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. - 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. - still feeling we could at least change the default to highpriority niceness. - 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. 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
