On Mon, Sep 21, 2020 at 8:16 AM Rob Clark <robdcl...@gmail.com> wrote: > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter <dan...@ffwll.ch> wrote: > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote: > > > From: Rob Clark <robdcl...@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. > > > > > > 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. > > > > The part I don't like about this is that it all feels rather hacked > > together, and if we add more stuff (or there's some different thing in the > > system that also needs rt scheduling) then it doesn't compose. > > The ideal thing would be that userspace is in control of the > priorities.. the setclientcap approach seemed like a reasonable way to > give the drm-master a way to opt in. > > I suppose instead userspace could use sched_setscheduler().. but that > would require userspace to be root, and would require some way to find > the tid. > > Is there some way we could arrange for the per-crtc kthread's to be > owned by the drm master? That would solve the "must be root" issue. > And since the target audience is an atomic userspace, I suppose we > could expose the tid as a read-only property on the crtc?
Looks like kthread goes out of it's way to *not* be owned by users (to avoid fork, and such complications?) But maybe we could modify the kthread_worker's task->real_cred? I didn't see any examples of anything else doing something similar, so I'm not sure what sorts of dragons there lie.. > BR, > -R > > > So question to rt/worker folks: What's the best way to let userspace set > > the scheduling mode and priorities of things the kernel does on its > > behalf? Surely we're not the first ones where if userspace runs with some > > rt priority it'll starve out the kernel workers that it needs. Hardcoding > > something behind a subsystem ioctl (which just means every time userspace > > changes what it does, we need a new such flag or mode) can't be the right > > thing. > > > > Peter, Tejun? > > > > Thanks, Daniel > > > > > > > > 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 > > > > > > _______________________________________________ > > > dri-devel mailing list > > > dri-de...@lists.freedesktop.org > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch
