>
> > One transcode stream from my experience typically is 3-4 GPU
contexts
> > (buffer travels from vcs -> rcs -> vcs, maybe vecs) used from
a single
> > CPU thread. 4 contexts * 36 streams = 144 active contexts.
Multiply by
> > 60fps = 8640 jobs submitted and completed per second.
> >
> > 144 active contexts in the proposed scheme means possibly
means 144
> > kernel worker threads spawned (driven by 36 transcode CPU
threads). (I
> > don't think the pools would scale down given all are
constantly pinged
> > at 60fps.)
> >
> > And then each of 144 threads goes to grab the single GuC CT
mutex. First
> > threads are being made schedulable, then put to sleep as mutex
> > contention is hit, then woken again as mutexes are getting
released,
> > rinse, repeat.
> >
>
> Why is every submission grabbing the GuC CT mutex? I've not read
the GuC
> back-end yet but I was under the impression that most run_job()
would be
> just shoving another packet into a ring buffer. If we have to
send the GuC
> a message on the control ring every single time we submit a job,
that's
> pretty horrible.
>
Run job writes the ring buffer and moves the tail as the first
step (no
lock required). Next it needs to tell the GuC the xe_engine LRC
tail has
moved, this is done from a single Host to GuC channel which is
circular
buffer, the writing of the channel protected by the mutex. There are
little more nuances too but in practice there is always space in the
channel so the time mutex needs to held is really, really small
(check cached credits, write 3 dwords in payload, write 1 dword to
move
tail). I also believe mutexes in Linux are hybrid where they spin
for a
little bit before sleeping and certainly if there is space in the
channel we shouldn't sleep mutex contention.
Ok, that makes sense. It's maybe a bit clunky and it'd be nice if we
had some way to batch things up a bit so we only have to poke the GuC
channel once for every batch of things rather than once per job.
That's maybe something we can look into as a future improvement; not
fundamental.
Generally, though, it sounds like contention could be a real problem
if we end up ping-ponging that lock between cores. It's going to
depend on how much work it takes to get the next ready thing vs. the
cost of that atomic. But, also, anything we do is going to
potentially run into contention problems. *shrug* If we were going
to go for one-per-HW-engine, we may as well go one-per-device and then
we wouldn't need the lock. Off the top of my head, that doesn't sound
great either but IDK.
As far as this being horrible, well didn't design the GuC and this how
it is implemented for KMD based submission. We also have 256 doorbells
so we wouldn't need a lock but I think are other issues with that
design
too which need to be worked out in the Xe2 / Xe3 timeframe.
Yeah, not blaming you. Just surprised, that's all. How does it work
for userspace submission? What would it look like if the kernel
emulated userspace submission? Is that even possible?
What are these doorbell things? How do they play into it?
Also if you see my follow up response Xe is ~33k execs per second with
the current implementation on a 8 core (or maybe 8 thread) TGL which
seems to fine to me.
33k exec/sec is about 500/frame which should be fine. 500 is a lot for
a single frame. I typically tell game devs to shoot for dozens per
frame. The important thing is that it stays low even with hundreds of
memory objects bound. (Xe should be just fine there.)
--Jason
Matt
> --Jason
>
>
> (And yes this backend contention is there regardless of 1:1:1,
it would
> > require a different re-design to solve that. But it is just a
question
> > whether there are 144 contending threads, or just 6 with the
thread per
> > engine class scheme.)
> >
> > Then multiply all by 10 for a 4U server use case and you get
1440 worker
> > kthreads, yes 10 more CT locks, but contending on how many CPU
cores?
> > Just so they can grab a timeslice and maybe content on a mutex
as the
> > next step.
> >
> > This example is where it would hurt on large systems. Imagine
only an
> > even wider media transcode card...
> >
> > Second example is only a single engine class used (3d
desktop?) but with
> > a bunch of not-runnable jobs queued and waiting on a fence to
signal.
> > Implicit or explicit dependencies doesn't matter. Then the
fence signals
> > and call backs run. N work items get scheduled, but they all
submit to
> > the same HW engine. So we end up with:
> >
> > /-- wi1 --\
> > / .. .. \
> > cb --+--- wi.. ---+-- rq1 -- .. -- rqN
> > \ .. .. /
> > \-- wiN --/
> >
> >
> > All that we have achieved is waking up N CPUs to contend on
the same
> > lock and effectively insert the job into the same single HW
queue. I
> > don't see any positives there.
> >
> > This example I think can particularly hurt small / low power
devices
> > because of needless waking up of many cores for no benefit.
Granted, I
> > don't have a good feel on how common this pattern is in practice.
> >
> > >
> > > That
> > > is the number which drives the maximum number of
not-runnable jobs
> > that
> > > can become runnable at once, and hence spawn that many
work items,
> > and
> > > in turn unbound worker threads.
> > >
> > > Several problems there.
> > >
> > > It is fundamentally pointless to have potentially that
many more
> > > threads
> > > than the number of CPU cores - it simply creates a
scheduling storm.
> > >
> > > Unbound workers have no CPU / cache locality either and
no connection
> > > with the CPU scheduler to optimize scheduling patterns.
This may
> > matter
> > > either on large systems or on small ones. Whereas the
current design
> > > allows for scheduler to notice userspace CPU thread
keeps waking up
> > the
> > > same drm scheduler kernel thread, and so it can keep
them on the same
> > > CPU, the unbound workers lose that ability and so 2nd
CPU might be
> > > getting woken up from low sleep for every submission.
> > >
> > > Hence, apart from being a bit of a impedance mismatch,
the proposal
> > has
> > > the potential to change performance and power patterns
and both large
> > > and small machines.
> > >
> > >
> > > Ok, thanks for explaining the issue you're seeing in more
detail. Yes,
> > > deferred kwork does appear to mismatch somewhat with what
the scheduler
> > > needs or at least how it's worked in the past. How much
impact will
> > > that mismatch have? Unclear.
> > >
> > > > >>> Secondly, it probably demands separate
workers (not
> > > optional),
> > > > otherwise
> > > > >>> behaviour of shared workqueues has either
the potential
> > to
> > > > explode number
> > > > >>> kernel threads anyway, or add latency.
> > > > >>>
> > > > >>
> > > > >> Right now the system_unbound_wq is used which
does have a
> > > limit
> > > > on the
> > > > >> number of threads, right? I do have a FIXME
to allow a
> > > worker to be
> > > > >> passed in similar to TDR.
> > > > >>
> > > > >> WRT to latency, the 1:1 ratio could actually
have lower
> > > latency
> > > > as 2 GPU
> > > > >> schedulers can be pushing jobs into the backend /
> > cleaning up
> > > > jobs in
> > > > >> parallel.
> > > > >>
> > > > >
> > > > > Thought of one more point here where why in Xe we
> > > absolutely want
> > > > a 1 to
> > > > > 1 ratio between entity and scheduler - the way
we implement
> > > > timeslicing
> > > > > for preempt fences.
> > > > >
> > > > > Let me try to explain.
> > > > >
> > > > > Preempt fences are implemented via the generic
messaging
> > > > interface [1]
> > > > > with suspend / resume messages. If a suspend
messages is
> > > received to
> > > > > soon after calling resume (this is per entity)
we simply
> > > sleep in the
> > > > > suspend call thus giving the entity a
timeslice. This
> > > completely
> > > > falls
> > > > > apart with a many to 1 relationship as now a
entity
> > > waiting for a
> > > > > timeslice blocks the other entities. Could we
work aroudn
> > > this,
> > > > sure but
> > > > > just another bunch of code we'd have to add in
Xe. Being to
> > > > freely sleep
> > > > > in backend without affecting other entities is
really,
> > really
> > > > nice IMO
> > > > > and I bet Xe isn't the only driver that is
going to feel
> > > this way.
> > > > >
> > > > > Last thing I'll say regardless of how anyone
feels about
> > > Xe using
> > > > a 1 to
> > > > > 1 relationship this patch IMO makes sense as I
hope we can
> > all
> > > > agree a
> > > > > workqueue scales better than kthreads.
> > > >
> > > > I don't know for sure what will scale better and
for what use
> > > case,
> > > > combination of CPU cores vs number of GPU engines
to keep
> > > busy vs other
> > > > system activity. But I wager someone is bound to
ask for some
> > > > numbers to
> > > > make sure proposal is not negatively affecting
any other
> > drivers.
> > > >
> > > >
> > > > Then let them ask. Waving your hands vaguely in the
direction of
> > > the
> > > > rest of DRM and saying "Uh, someone (not me) might
object" is
> > > profoundly
> > > > unhelpful. Sure, someone might. That's why it's on
dri-devel.
> > > If you
> > > > think there's someone in particular who might have a
useful
> > > opinion on
> > > > this, throw them in the CC so they don't miss the
e-mail thread.
> > > >
> > > > Or are you asking for numbers? If so, what numbers
are you
> > > asking for?
> > >
> > > It was a heads up to the Xe team in case people weren't
appreciating
> > > how
> > > the proposed change has the potential influence power
and performance
> > > across the board. And nothing in the follow up
discussion made me
> > think
> > > it was considered so I don't think it was redundant to
raise it.
> > >
> > > In my experience it is typical that such core changes
come with some
> > > numbers. Which is in case of drm scheduler is tricky and
probably
> > > requires explicitly asking everyone to test (rather than
count on
> > > "don't
> > > miss the email thread"). Real products can fail to ship
due ten mW
> > here
> > > or there. Like suddenly an extra core prevented from
getting into
> > deep
> > > sleep.
> > >
> > > If that was "profoundly unhelpful" so be it.
> > >
> > >
> > > With your above explanation, it makes more sense what you're
asking.
> > > It's still not something Matt is likely to be able to
provide on his
> > > own. We need to tag some other folks and ask them to test
it out. We
> > > could play around a bit with it on Xe but it's not exactly
production
> > > grade yet and is going to hit this differently from most.
Likely
> > > candidates are probably AMD and Freedreno.
> >
> > Whoever is setup to check out power and performance would be
good to
> > give it a spin, yes.
> >
> > PS. I don't think I was asking Matt to test with other
devices. To start
> > with I think Xe is a team effort. I was asking for more
background on
> > the design decision since patch 4/20 does not say anything on that
> > angle, nor later in the thread it was IMO sufficiently addressed.
> >
> > > > Also, If we're talking about a design that might
paint us into an
> > > > Intel-HW-specific hole, that would be one thing. But
we're not.
> > > We're
> > > > talking about switching which kernel threading/task
mechanism to
> > > use for
> > > > what's really a very generic problem. The core Xe
design works
> > > without
> > > > this patch (just with more kthreads). If we land
this patch or
> > > > something like it and get it wrong and it causes a
performance
> > > problem
> > > > for someone down the line, we can revisit it.
> > >
> > > For some definition of "it works" - I really wouldn't
suggest
> > > shipping a
> > > kthread per user context at any point.
> > >
> > >
> > > You have yet to elaborate on why. What resources is it
consuming that's
> > > going to be a problem? Are you anticipating CPU affinity
problems? Or
> > > does it just seem wasteful?
> >
> > Well I don't know, commit message says the approach does not
scale. :)
> >
> > > I think I largely agree that it's probably
unnecessary/wasteful but
> > > reducing the number of kthreads seems like a tractable
problem to solve
> > > regardless of where we put the gpu_scheduler object. Is
this the right
> > > solution? Maybe not. It was also proposed at one point
that we could
> > > split the scheduler into two pieces: A scheduler which owns
the kthread,
> > > and a back-end which targets some HW ring thing where you
can have
> > > multiple back-ends per scheduler. That's certainly more
invasive from a
> > > DRM scheduler internal API PoV but would solve the kthread
problem in a
> > > way that's more similar to what we have now.
> > >
> > > > In any case that's a low level question caused by
the high
> > > level design
> > > > decision. So I'd think first focus on the high
level - which
> > > is the 1:1
> > > > mapping of entity to scheduler instance proposal.
> > > >
> > > > Fundamentally it will be up to the DRM
maintainers and the
> > > community to
> > > > bless your approach. And it is important to
stress 1:1 is
> > about
> > > > userspace contexts, so I believe unlike any other
current
> > > scheduler
> > > > user. And also important to stress this
effectively does not
> > > make Xe
> > > > _really_ use the scheduler that much.
> > > >
> > > >
> > > > I don't think this makes Xe nearly as much of a
one-off as you
> > > think it
> > > > does. I've already told the Asahi team working on
Apple M1/2
> > > hardware
> > > > to do it this way and it seems to be a pretty good
mapping for
> > > them. I
> > > > believe this is roughly the plan for nouveau as
well. It's not
> > > the way
> > > > it currently works for anyone because most other
groups aren't
> > > doing FW
> > > > scheduling yet. In the world of FW scheduling and
hardware
> > > designed to
> > > > support userspace direct-to-FW submit, I think the
design makes
> > > perfect
> > > > sense (see below) and I expect we'll see more drivers
move in this
> > > > direction as those drivers evolve. (AMD is doing some
customish
> > > thing
> > > > for how with gpu_scheduler on the front-end somehow.
I've not dug
> > > into
> > > > those details.)
> > > >
> > > > I can only offer my opinion, which is that the
two options
> > > mentioned in
> > > > this thread (either improve drm scheduler to cope
with what is
> > > > required,
> > > > or split up the code so you can use just the parts of
> > > drm_sched which
> > > > you want - which is frontend dependency tracking)
shouldn't
> > be so
> > > > readily dismissed, given how I think the idea was
for the new
> > > driver to
> > > > work less in a silo and more in the community
(not do kludges
> > to
> > > > workaround stuff because it is thought to be too
hard to
> > > improve common
> > > > code), but fundamentally, "goto previous
paragraph" for what
> > I am
> > > > concerned.
> > > >
> > > >
> > > > Meta comment: It appears as if you're falling into
the standard
> > > i915
> > > > team trap of having an internal discussion about what the
> > community
> > > > discussion might look like instead of actually having the
> > community
> > > > discussion. If you are seriously concerned about
interactions
> > with
> > > > other drivers or whether or setting common direction,
the right
> > > way to
> > > > do that is to break a patch or two out into a
separate RFC series
> > > and
> > > > tag a handful of driver maintainers. Trying to
predict the
> > > questions
> > > > other people might ask is pointless. Cc them and
asking for their
> > > input
> > > > instead.
> > >
> > > I don't follow you here. It's not an internal discussion
- I am
> > raising
> > > my concerns on the design publicly. I am supposed to
write a patch to
> > > show something, but am allowed to comment on a RFC series?
> > >
> > >
> > > I may have misread your tone a bit. It felt a bit like too many
> > > discussions I've had in the past where people are trying to
predict what
> > > others will say instead of just asking them. Reading it
again, I was
> > > probably jumping to conclusions a bit. Sorry about that.
> >
> > Okay no problem, thanks. In any case we don't have to keep
discussing
> > it, since I wrote one or two emails ago it is fundamentally on the
> > maintainers and community to ack the approach. I only felt
like RFC did
> > not explain the potential downsides sufficiently so I wanted
to probe
> > that area a bit.
> >
> > > It is "drm/sched: Convert drm scheduler to use a work
queue rather
> > than
> > > kthread" which should have Cc-ed _everyone_ who use drm
scheduler.
> > >
> > >
> > > Yeah, it probably should have. I think that's mostly what
I've been
> > > trying to say.
> > >
> > > >
> > > > Regards,
> > > >
> > > > Tvrtko
> > > >
> > > > P.S. And as a related side note, there are more
areas where
> > > drm_sched
> > > > could be improved, like for instance priority
handling.
> > > > Take a look at msm_submitqueue_create /
> > > msm_gpu_convert_priority /
> > > > get_sched_entity to see how msm works around the
drm_sched
> > > hardcoded
> > > > limit of available priority levels, in order to
avoid having
> > > to leave a
> > > > hw capability unused. I suspect msm would be
happier if they
> > > could have
> > > > all priority levels equal in terms of whether
they apply only
> > > at the
> > > > frontend level or completely throughout the pipeline.
> > > >
> > > > > [1]
> > > >
> > >
https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1
<https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1>
> > >
<https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1
<https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1>
> > >
> > > >
> > > <
> >
https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1
<https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1>
<
> >
https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1
<https://patchwork.freedesktop.org/patch/515857/?series=112189&rev=1>>>
> > > > >
> > > > >>> What would be interesting to learn is
whether the option
> > of
> > > > refactoring
> > > > >>> drm_sched to deal with out of order
completion was
> > > considered
> > > > and what were
> > > > >>> the conclusions.
> > > > >>>
> > > > >>
> > > > >> I coded this up a while back when trying to
convert the
> > > i915 to
> > > > the DRM
> > > > >> scheduler it isn't all that hard either. The
free flow
> > > control
> > > > on the
> > > > >> ring (e.g. set job limit == SIZE OF RING /
MAX JOB SIZE)
> > is
> > > > really what
> > > > >> sold me on the this design.
> > > >
> > > >
> > > > You're not the only one to suggest supporting
out-of-order
> > > completion.
> > > > However, it's tricky and breaks a lot of internal
assumptions of
> > the
> > > > scheduler. It also reduces functionality a bit
because it can no
> > > longer
> > > > automatically rate-limit HW/FW queues which are often
> > > fixed-size. (Ok,
> > > > yes, it probably could but it becomes a substantially
harder
> > > problem.)
> > > >
> > > > It also seems like a worse mapping to me. The goal
here is to
> > turn
> > > > submissions on a userspace-facing engine/queue into
submissions
> > > to a FW
> > > > queue submissions, sorting out any dma_fence
dependencies. Matt's
> > > > description of saying this is a 1:1 mapping between
sched/entity
> > > doesn't
> > > > tell the whole story. It's a 1:1:1 mapping between
xe_engine,
> > > > gpu_scheduler, and GuC FW engine. Why make it a
1:something:1
> > > mapping?
> > > > Why is that better?
> > >
> > > As I have stated before, what I think what would fit
well for Xe is
> > one
> > > drm_scheduler per engine class. In specific terms on our
current
> > > hardware, one drm scheduler instance for render,
compute, blitter,
> > > video
> > > and video enhance. Userspace contexts remain scheduler
entities.
> > >
> > >
> > > And this is where we fairly strongly disagree. More in a bit.
> > >
> > > That way you avoid the whole kthread/kworker story and
you have it
> > > actually use the entity picking code in the scheduler,
which may be
> > > useful when the backend is congested.
> > >
> > >
> > > What back-end congestion are you referring to here? Running
out of FW
> > > queue IDs? Something else?
> >
> > CT channel, number of context ids.
> >
> > >
> > > Yes you have to solve the out of order problem so in my
mind that is
> > > something to discuss. What the problem actually is (just
TDR?), how
> > > tricky and why etc.
> > >
> > > And yes you lose the handy LRCA ring buffer size
management so you'd
> > > have to make those entities not runnable in some other way.
> > >
> > > Regarding the argument you raise below - would any of
that make the
> > > frontend / backend separation worse and why? Do you
think it is less
> > > natural? If neither is true then all remains is that it
appears extra
> > > work to support out of order completion of entities has been
> > discounted
> > > in favour of an easy but IMO inelegant option.
> > >
> > >
> > > Broadly speaking, the kernel needs to stop thinking about
GPU scheduling
> > > in terms of scheduling jobs and start thinking in terms of
scheduling
> > > contexts/engines. There is still some need for scheduling
individual
> > > jobs but that is only for the purpose of delaying them as
needed to
> > > resolve dma_fence dependencies. Once dependencies are
resolved, they
> > > get shoved onto the context/engine queue and from there the
kernel only
> > > really manages whole contexts/engines. This is a major
architectural
> > > shift, entirely different from the way i915 scheduling
works. It's also
> > > different from the historical usage of DRM scheduler which I
think is
> > > why this all looks a bit funny.
> > >
> > > To justify this architectural shift, let's look at where
we're headed.
> > > In the glorious future...
> > >
> > > 1. Userspace submits directly to firmware queues. The
kernel has no
> > > visibility whatsoever into individual jobs. At most it can
pause/resume
> > > FW contexts as needed to handle eviction and memory management.
> > >
> > > 2. Because of 1, apart from handing out the FW queue IDs
at the
> > > beginning, the kernel can't really juggle them that much.
Depending on
> > > FW design, it may be able to pause a client, give its IDs to
another,
> > > and then resume it later when IDs free up. What it's not
doing is
> > > juggling IDs on a job-by-job basis like i915 currently is.
> > >
> > > 3. Long-running compute jobs may not complete for days.
This means
> > > that memory management needs to happen in terms of
pause/resume of
> > > entire contexts/engines using the memory rather than based
on waiting
> > > for individual jobs to complete or pausing individual jobs
until the
> > > memory is available.
> > >
> > > 4. Synchronization happens via userspace memory fences
(UMF) and the
> > > kernel is mostly unaware of most dependencies and when a
context/engine
> > > is or is not runnable. Instead, it keeps as many of them
minimally
> > > active (memory is available, even if it's in system RAM) as
possible and
> > > lets the FW sort out dependencies. (There may need to be
some facility
> > > for sleeping a context until a memory change similar to
futex() or
> > > poll() for userspace threads. There are some details TBD.)
> > >
> > > Are there potential problems that will need to be solved
here? Yes. Is
> > > it a good design? Well, Microsoft has been living in this
future for
> > > half a decade or better and it's working quite well for
them. It's also
> > > the way all modern game consoles work. It really is just
Linux that's
> > > stuck with the same old job model we've had since the
monumental shift
> > > to DRI2.
> > >
> > > To that end, one of the core goals of the Xe project was to
make the
> > > driver internally behave as close to the above model as
possible while
> > > keeping the old-school job model as a very thin layer on
top. As the
> > > broader ecosystem problems (window-system support for UMF,
for instance)
> > > are solved, that layer can be peeled back. The core driver
will already
> > > be ready for it.
> > >
> > > To that end, the point of the DRM scheduler in Xe isn't to
schedule
> > > jobs. It's to resolve syncobj and dma-buf implicit sync
dependencies
> > > and stuff jobs into their respective context/engine queue
once they're
> > > ready. All the actual scheduling happens in firmware and
any scheduling
> > > the kernel does to deal with contention, oversubscriptions,
too many
> > > contexts, etc. is between contexts/engines, not individual
jobs. Sure,
> > > the individual job visibility is nice, but if we design
around it, we'll
> > > never get to the glorious future.
> > >
> > > I really need to turn the above (with a bit more detail)
into a blog
> > > post.... Maybe I'll do that this week.
> > >
> > > In any case, I hope that provides more insight into why Xe
is designed
> > > the way it is and why I'm pushing back so hard on trying to
make it more
> > > of a "classic" driver as far as scheduling is concerned.
Are there
> > > potential problems here? Yes, that's why Xe has been labeled a
> > > prototype. Are such radical changes necessary to get to
said glorious
> > > future? Yes, I think they are. Will it be worth it? I
believe so.
> >
> > Right, that's all solid I think. My takeaway is that frontend
priority
> > sorting and that stuff isn't needed and that is okay. And that
there are
> > multiple options to maybe improve drm scheduler, like the fore
mentioned
> > making it deal with out of order, or split into functional
components,
> > or split frontend/backend what you suggested. For most of them
cost vs
> > benefit is more or less not completely clear, neither how much
effort
> > was invested to look into them.
> >
> > One thing I missed from this explanation is how drm_scheduler
per engine
> > class interferes with the high level concepts. And I did not
manage to
> > pick up on what exactly is the TDR problem in that case. Maybe
the two
> > are one and the same.
> >
> > Bottom line is I still have the concern that conversion to
kworkers has
> > an opportunity to regress. Possibly more opportunity for some
Xe use
> > cases than to affect other vendors, since they would still be
using per
> > physical engine / queue scheduler instances.
> >
> > And to put my money where my mouth is I will try to put testing Xe
> > inside the full blown ChromeOS environment in my team plans.
It would
> > probably also be beneficial if Xe team could take a look at
real world
> > behaviour of the extreme transcode use cases too. If the stack
is ready
> > for that and all. It would be better to know earlier rather
than later
> > if there is a fundamental issue.
> >
> > For the patch at hand, and the cover letter, it certainly
feels it would
> > benefit to record the past design discussion had with AMD
folks, to
> > explicitly copy other drivers, and to record the theoretical
pros and
> > cons of threads vs unbound workers as I have tried to
highlight them.
> >
> > Regards,
> >
> > Tvrtko
> >
