Message-ID: <20200924084950.GY438822@phenom.ffwll.local>
Date:   Thu, 24 Sep 2020 10:49:50 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>, Tejun Heo <tj@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH 0/3] drm: commit_work scheduling

On Wed, Sep 23, 2020 at 07:33:17PM -0700, Rob Clark wrote:
> On Wed, Sep 23, 2020 at 8:25 AM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > 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@...ll.ch> wrote:
> > > >
> > > > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark <robdclark@...il.com> wrote:
> > > > >
> > > > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter <daniel@...ll.ch> wrote:
> > > > > >
> > > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > > > > From: Rob Clark <robdclark@...omium.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.
> > > >
> > > > 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.
> 
> hmm, I was kinda hoping there was already a solution for translating
> thread-id's, but hadn't had a chance to dig through it yet

You can translate them, and it happens automatically in process context
(iirc at least). But when we set the read-only prop we don't know which
process namespace the compositor is sitting in, so that translation isn't
doing us any good.

I think there's a root namespace that the kernel uses, but tbh I'm not
sure how this all works.

> > I also thought kernel threads can be distinguished from others, so
> > userspace shouldn't be able to sneak in and get elevated by accident.
> 
> I guess maybe you could look at the parent?  I still would like to
> think that we could come up with something a bit less shaking than
> matching thread names by regexp..

ps marks up kernel threads with [], so there is a way. But I haven't
looked at what it is exactly that tells kernel threads apart from others.

But aside from that sounds like "match right kernel thread with regex and
set its scheduler class" is how this is currently done, if I'm
understanding what Tejun and Peter said correctly.

Not pretty, but also *shrug* ...
-Daniel
 
> BR,
> -R
> 
> > -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@...ts.freedesktop.org
> > > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > > > >
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > > > > _______________________________________________
> > > > > dri-devel mailing list
> > > > > dri-devel@...ts.freedesktop.org
> > > > > 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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