Am 18.08.23 um 05:08 schrieb Matthew Brost:
On Thu, Aug 17, 2023 at 01:13:31PM +0200, Danilo Krummrich wrote:
On 8/17/23 07:33, Christian König wrote:
Am 16.08.23 um 18:33 schrieb Danilo Krummrich:
On 8/16/23 16:59, Christian König wrote:
Am 16.08.23 um 14:30 schrieb Danilo Krummrich:
On 8/16/23 16:05, Christian König wrote:
Am 16.08.23 um 13:30 schrieb Danilo Krummrich:
Hi Matt,
On 8/11/23 04:31, Matthew Brost wrote:
In XE, the new Intel GPU driver, a choice has made to have a 1 to 1
mapping between a drm_gpu_scheduler and
drm_sched_entity. At first this
seems a bit odd but let us explain the reasoning below.
1. In XE the submission order from multiple drm_sched_entity is not
guaranteed to be the same completion even if
targeting the same hardware
engine. This is because in XE we have a firmware scheduler, the GuC,
which allowed to reorder, timeslice, and preempt
submissions. If a using
shared drm_gpu_scheduler across multiple
drm_sched_entity, the TDR falls
apart as the TDR expects submission order ==
completion order. Using a
dedicated drm_gpu_scheduler per drm_sched_entity solve this problem.
2. In XE submissions are done via programming a
ring buffer (circular
buffer), a drm_gpu_scheduler provides a limit on
number of jobs, if the
limit of number jobs is set to RING_SIZE /
MAX_SIZE_PER_JOB we get flow
control on the ring for free.
In XE, where does the limitation of MAX_SIZE_PER_JOB come from?
In Nouveau we currently do have such a limitation as
well, but it is derived from the RING_SIZE, hence
RING_SIZE / MAX_SIZE_PER_JOB would always be 1.
However, I think most jobs won't actually utilize
the whole ring.
Well that should probably rather be RING_SIZE /
MAX_SIZE_PER_JOB = hw_submission_limit (or even
hw_submission_limit - 1 when the hw can't distinct full
vs empty ring buffer).
Not sure if I get you right, let me try to clarify what I
was trying to say: I wanted to say that in Nouveau
MAX_SIZE_PER_JOB isn't really limited by anything other than
the RING_SIZE and hence we'd never allow more than 1 active
job.
But that lets the hw run dry between submissions. That is
usually a pretty horrible idea for performance.
Correct, that's the reason why I said it seems to be more efficient
to base ring flow control on the actual size of each incoming job
rather than the maximum size of a job.
However, it seems to be more efficient to base ring flow
control on the actual size of each incoming job rather than
the worst case, namely the maximum size of a job.
That doesn't sounds like a good idea to me. See we don't limit
the number of submitted jobs based on the ring size, but rather
we calculate the ring size based on the number of submitted
jobs.
My point isn't really about whether we derive the ring size from the
job limit or the other way around. It's more about the job size (or
its maximum size) being arbitrary.
As mentioned in my reply to Matt:
"In Nouveau, userspace can submit an arbitrary amount of addresses
of indirect bufferes containing the ring instructions. The ring on
the kernel side takes the addresses of the indirect buffers rather
than the instructions themself. Hence, technically there isn't
really a limit on the amount of IBs submitted by a job except for
the ring size."
So, my point is that I don't really want to limit the job size
artificially just to be able to fit multiple jobs into the ring even
if they're submitted at their "artificial" maximum size, but rather
track how much of the ring the submitted job actually occupies.
In other words the hw_submission_limit defines the ring size,
not the other way around. And you usually want the
hw_submission_limit as low as possible for good scheduler
granularity and to avoid extra overhead.
I don't think you really mean "as low as possible", do you?
No, I do mean as low as possible or in other words as few as possible.
Ideally the scheduler would submit only the minimum amount of work to
the hardware to keep the hardware busy. >
The hardware seems to work mostly the same for all vendors, but you
somehow seem to think that filling the ring is somehow beneficial which
is really not the case as far as I can see.
I think that's a misunderstanding. I'm not trying to say that it is *always*
beneficial to fill up the ring as much as possible. But I think it is under
certain circumstances, exactly those circumstances I described for Nouveau.
As mentioned, in Nouveau the size of a job is only really limited by the
ring size, which means that one job can (but does not necessarily) fill up
the whole ring. We both agree that this is inefficient, because it
potentially results into the HW run dry due to hw_submission_limit == 1.
I recognize you said that one should define hw_submission_limit and adjust
the other parts of the equation accordingly, the options I see are:
(1) Increase the ring size while keeping the maximum job size.
(2) Decrease the maximum job size while keeping the ring size.
(3) Let the scheduler track the actual job size rather than the maximum job
size.
(1) results into potentially wasted ring memory, because we're not always
reaching the maximum job size, but the scheduler assumes so.
(2) results into more IOCTLs from userspace for the same amount of IBs and
more jobs result into more memory allocations and more work being submitted
to the workqueue (with Matt's patches).
(3) doesn't seem to have any of those draw backs.
What would be your take on that?
Actually, if none of the other drivers is interested into a more precise way
of keeping track of the ring utilization, I'd be totally fine to do it in a
driver specific way. However, unfortunately I don't see how this would be
possible.
My proposal would be to just keep the hw_submission_limit (maybe rename it
to submission_unit_limit) and add a submission_units field to struct
drm_sched_job. By default a jobs submission_units field would be 0 and the
scheduler would behave the exact same way as it does now.
Accordingly, jobs with submission_units > 1 would contribute more than one
unit to the submission_unit_limit.
What do you think about that?
This seems reasonible to me and a very minimal change to the scheduler.
If you have a good use case for this then the approach sounds sane to me
as well.
My question is rather how exactly does Nouveau comes to have this use
case? Allowing the full ring size in the UAPI sounds a bit questionable.
Christian.
Matt
Besides all that, you said that filling up the ring just enough to not let
the HW run dry rather than filling it up entirely is desirable. Why do you
think so? I tend to think that in most cases it shouldn't make difference.
- Danilo
Regards,
Christian.
Because one really is the minimum if you want to do work at all, but
as you mentioned above a job limit of one can let the ring run dry.
In the end my proposal comes down to tracking the actual size of a
job rather than just assuming a pre-defined maximum job size, and
hence a dynamic job limit.
I don't think this would hurt the scheduler granularity. In fact, it
should even contribute to the desire of not letting the ring run dry
even better. Especially for sequences of small jobs, where the
current implementation might wrongly assume the ring is already full
although actually there would still be enough space left.
Christian.
Otherwise your scheduler might just overwrite the ring
buffer by pushing things to fast.
Christian.
Given that, it seems like it would be better to let
the scheduler keep track of empty ring "slots"
instead, such that the scheduler can deceide whether
a subsequent job will still fit on the ring and if
not re-evaluate once a previous job finished. Of
course each submitted job would be required to carry
the number of slots it requires on the ring.
What to you think of implementing this as
alternative flow control mechanism? Implementation
wise this could be a union with the existing
hw_submission_limit.
- Danilo
A problem with this design is currently a drm_gpu_scheduler uses a
kthread for submission / job cleanup. This doesn't scale if a large
number of drm_gpu_scheduler are used. To work
around the scaling issue,
use a worker rather than kthread for submission / job cleanup.
v2:
- (Rob Clark) Fix msm build
- Pass in run work queue
v3:
- (Boris) don't have loop in worker
v4:
- (Tvrtko) break out submit ready, stop,
start helpers into own patch
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
