On 09/30/20 14:17, Rob Clark 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. 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 >
