On Thu, Oct 1, 2020 at 12:25 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > 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 fwiw, right now the userspace is "modetest + chrt".. *probably* the userspace will become a standalone helper or daemon, mostly because the chrome gpu-process sandbox does not allow setting SCHED_FIFO. I'm still entertaining the possibility of switching between rt and cfs depending on what is in the foreground (ie. only do rt for android apps). > - we need this also for vblank workers, otherwise this wont work for > drivers needing those because of another priority inversion. I have a thought on that, see below.. > - 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. I'm leaning towards converting the other drivers over to use the per-crtc kwork, and then dropping the 'commit_work` from atomic state. I can add a patch to that, but figured I could postpone that churn until there is some by-in on this whole idea. > - 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. I could introduce a "PID" property type. This would be useful if we wanted to also expose the vblank workers. > - still feeling we could at least change the default to highpriority niceness. AFAIU this would still be preempted by something that is SCHED_FIFO. Also, with cfs/SCHED_NORMAL, you can still be preempted by lower priority things that haven't had a chance to run for a while. > - 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. That is why it is per-CRTC.. I'm not sure there is a need to overlap work for a single CRTC? We could OFC make this a driver knob, and keep the old 'commit_work' option, but that doesn't really feel like the right direction BR, -R > 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
