On Tue, Sep 22, 2020 at 07:48:10AM -0700, Rob Clark wrote: > On Mon, Sep 21, 2020 at 11:59 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark <robdclark@xxxxxxxxx> wrote: > > > > > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, 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. > > > > > > > > > > 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. > > > > Userspace already needs that for the SCHED_FIFO for surface-flinger. > > Or is the problem that CAP_SYS_NICE is only good for your own > > processes? > > tbh, I'm not completely sure offhand what gives surfaceflinger > permission to set itself SCHED_FIFO > > (But on CrOS there are a few more pieces to the puzzle) > > > Other question I have for this is whether there's any recommendations > > for naming the kthreads (since I guess that name is what becomes the > > uapi for userspace to control this)? > > > > Otherwise I think "userspace calls sched_setscheduler on the right > > kthreads" sounds like a good interface, since it lets userspace decide > > how it all needs to fit together and compose. Anything we hard-code in > > an ioctl is kinda lost cause. And we can choose the default values to > > work reasonably well when the compositor runs at normal priority > > (lowest niceness or something like that for the commit work). > > I don't really like the naming convention approach.. what is to stop > some unrelated process to name it's thread the same thing to get a > SCHED_FIFO boost.. > > But we can stick with my idea to expose the thread id as a read-only > CRTC property, for userspace to find the things to call > sched_setscheduler() on. If for whatever reason the drm master is not > privileged (or is running in a sandbox, etc), a small helper that has > the necessary permissions could open the drm device to find the CRTC > thread-ids and call sched_setscheduler().. Hm thread ids don't translate too well across PID namespaces I think ... So that's another can of worms. And pidfd doesn't really work as a property. I also thought kernel threads can be distinguished from others, so userspace shouldn't be able to sneak in and get elevated by accident. -Daniel > > BR, > -R > > > -Daniel > > > > > 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? > > > > > > 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-devel@xxxxxxxxxxxxxxxxxxxxx > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > > > > > > > -- > > > > Daniel Vetter > > > > Software Engineer, Intel Corporation > > > > http://blog.ffwll.ch > > > _______________________________________________ > > > dri-devel mailing list > > > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > > > > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
