Re: [PATCH 0/3] drm: commit_work scheduling
On Thu, Sep 24, 2020 at 05:15:00PM +0100, Qais Yousef wrote: > On 09/24/20 10:49, Daniel Vetter wrote: > > [...] > > > > > 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* ... > > Isn't there a real danger that a sneaky application names its threads to match > this regex and get a free promotion to RT without having the capability to do > so? A sneaky application can't fake being a kernel thread, at least that's what I thought. You need to check for that _and_ that the name matches. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH 0/3] drm: commit_work scheduling
On 09/24/20 10:49, Daniel Vetter wrote: [...] > > > 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* ... Isn't there a real danger that a sneaky application names its threads to match this regex and get a free promotion to RT without having the capability to do so? Cheers -- Qais Yousef ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH 0/3] drm: commit_work scheduling
On Thu, Sep 24, 2020 at 1:49 AM Daniel Vetter wrote:
>
> On Wed, Sep 23, 2020 at 07:33:17PM -0700, Rob Clark wrote:
> > On Wed, Sep 23, 2020 at 8:25 AM Daniel Vetter 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 wrote:
> > > > >
> > > > > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
> > > > > >
> > > > > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter
> > > > > > wrote:
> > > > > > >
> > > > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > > > > > From: Rob Clark
> > > > > > > >
> > > > > > > > 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
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 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 wrote:
> > > >
> > > > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
> > > > >
> > > > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> > > > > >
> > > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > > > > From: Rob Clark
> > > > > > >
> > > > > > > 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
Re: [PATCH 0/3] drm: commit_work scheduling
On Wed, Sep 23, 2020 at 8:25 AM Daniel Vetter 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 wrote:
> > >
> > > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
> > > >
> > > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> > > > >
> > > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > > > From: Rob Clark
> > > > > >
> > > > > > 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
> I also thought kernel threads can be distinguished from
Re: [PATCH 0/3] drm: commit_work scheduling
On Tue, Sep 22, 2020 at 07:48:10AM -0700, Rob Clark wrote:
> On Mon, Sep 21, 2020 at 11:59 PM Daniel Vetter wrote:
> >
> > On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
> > >
> > > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> > > >
> > > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > > From: Rob Clark
> > > > >
> > > > > 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
Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 11:59 PM Daniel Vetter wrote:
>
> On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
> >
> > On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> > >
> > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > > From: Rob Clark
> > > >
> > > > 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()..
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):
> > > >
Re: [PATCH 0/3] drm: commit_work scheduling
On 09/19/20 12:37, Rob Clark wrote:
> From: Rob Clark
>
> 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.
Just a side note; this RT vs CFS inter-operatability is an issue that
creeps up every now and again.
https://lore.kernel.org/lkml/1567048502-6064-1-git-send-email-jing-ting...@mediatek.com/
Does the UI thread in Android ever run as RT by the way? I always suspected it
is one susceptible to such potential delays since it is part of the application
thread and thought it can't be trusted to become RT.
Those 120MHz displays will stress the pipeline :-)
>
> 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.
This is a function of num_cpus too. As long as nr_cpus > nr_running_rt_tasks
you should be fine.
Cheers
--
Qais Yousef
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Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 5:16 PM Rob Clark wrote:
>
> On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> >
> > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > From: Rob Clark
> > >
> > > 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?
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).
-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@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel
Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 9:10 AM Qais Yousef wrote:
>
> On 09/19/20 12:37, Rob Clark wrote:
> > From: Rob Clark
> >
> > 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.
>
> Just a side note; this RT vs CFS inter-operatability is an issue that
> creeps up every now and again.
>
> https://lore.kernel.org/lkml/1567048502-6064-1-git-send-email-jing-ting...@mediatek.com/
>
> Does the UI thread in Android ever run as RT by the way? I always suspected it
> is one susceptible to such potential delays since it is part of the
> application
> thread and thought it can't be trusted to become RT.
The application itself is not RT, since there is no good way to know
what other things the app may be doing. Although that is mentioned in
the lwn article (ie. the hypothetical SCHED_DEADLINE + token passing
part)
But at least once the app / ui thread is done, then it is all (or at
least in theory "all" if atomic-helper doesn't gum up the works ;-))
SCHED_FIFO from there to the display.
> Those 120MHz displays will stress the pipeline :-)
indeed
BR,
-R
> >
> > 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.
>
> This is a function of num_cpus too. As long as nr_cpus > nr_running_rt_tasks
> you should be fine.
>
> Cheers
>
> --
> Qais Yousef
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Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 8:16 AM Rob Clark wrote:
>
> On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> >
> > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > From: Rob Clark
> > >
> > > 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-devel@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
___
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Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 8:16 AM Rob Clark wrote:
>
> On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
> >
> > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > > From: Rob Clark
> > >
> > > 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?
Side-note, we have the same issue with work scheduled when GPU
completes a batch/submit.. I'm less sure what to do with that, so
figured I'd start with the commit_work because that was the "easy"
part ;-)
The retire_work tends to complete quickly, so maybe sched_set_fifo()
is sufficient. (That plus these days things that run android tend to
have 8 cores so you can kinda get away with multiple things at the
same RT priority level to some degree..)
> 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@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
___
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Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter wrote:
>
> On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> > From: Rob Clark
> >
> > 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?
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@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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Re: [PATCH 0/3] drm: commit_work scheduling
Hello, On Mon, Sep 21, 2020 at 11:21:54AM +0200, Daniel Vetter wrote: > 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. > > 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. Maybe not first but there haven't been many. The main benefit of workqueue is that the users get to pool the worker threads automatically. I don't think the existing workqueue design is something suitable for actual RT use cases. Furthermore, there are inherent conflicts between sharing resources and RT as this this patchset is already showing w/ needing per-crtc worker thread. Maybe we can further abstract it if there are more use cases but for now kthread_worker based implementation sounds about right to me. Thanks. -- tejun ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH 0/3] drm: commit_work scheduling
On Mon, Sep 21, 2020 at 11:21:54AM +0200, Daniel Vetter wrote: > 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? So regular workqueues do not support RT priorities, but you can set their nice value somewhere in /sys. The kthread_work stuff used in these patches result in a regular kthread and as such the user interface for changing its scheduling class or priority is that of any other 'random' task. ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH 0/3] drm: commit_work scheduling
On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote:
> From: Rob Clark
>
> 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.
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@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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[PATCH 0/3] drm: commit_work scheduling
From: Rob Clark
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.
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
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