[RFC PATCH 04/20] drm/sched: Convert drm scheduler to use a work queue rather than kthread

[Boris Brezillon]

On Fri, 30 Dec 2022 11:20:42 +0100
Boris Brezillon <boris.brezillon at collabora.com> wrote:

> Hello Matthew,
> 
> On Thu, 22 Dec 2022 14:21:11 -0800
> Matthew Brost <matthew.brost at intel.com> 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.  
> 
> Oh, that's interesting. I've been trying to solve the same sort of
> issues to support Arm's new Mali GPU which is relying on a FW-assisted
> scheduling scheme (you give the FW N streams to execute, and it does
> the scheduling between those N command streams, the kernel driver
> does timeslice scheduling to update the command streams passed to the
> FW). I must admit I gave up on using drm_sched at some point, mostly
> because the integration with drm_sched was painful, but also because I
> felt trying to bend drm_sched to make it interact with a
> timeslice-oriented scheduling model wasn't really future proof. Giving
> drm_sched_entity exlusive access to a drm_gpu_scheduler probably might
> help for a few things (didn't think it through yet), but I feel it's
> coming short on other aspects we have to deal with on Arm GPUs.

Ok, so I just had a quick look at the Xe driver and how it
instantiates the drm_sched_entity and drm_gpu_scheduler, and I think I
have a better understanding of how you get away with using drm_sched
while still controlling how scheduling is really done. Here
drm_gpu_scheduler is just a dummy abstract that let's you use the
drm_sched job queuing/dep/tracking mechanism. The whole run-queue
selection is dumb because there's only one entity ever bound to the
scheduler (the one that's part of the xe_guc_engine object which also
contains the drm_gpu_scheduler instance). I guess the main issue we'd
have on Arm is the fact that the stream doesn't necessarily get
scheduled when ->run_job() is called, it can be placed in the runnable
queue and be picked later by the kernel-side scheduler when a FW slot
gets released. That can probably be sorted out by manually disabling the
job timer and re-enabling it when the stream gets picked by the
scheduler. But my main concern remains, we're basically abusing
drm_sched here.

For the Arm driver, that means turning the following sequence

1. wait for job deps
2. queue job to ringbuf and push the stream to the runnable
   queue (if it wasn't queued already). Wakeup the timeslice scheduler
   to re-evaluate (if the stream is not on a FW slot already)
3. stream gets picked by the timeslice scheduler and sent to the FW for
   execution

into

1. queue job to entity which takes care of waiting for job deps for
   us
2. schedule a drm_sched_main iteration
3. the only available entity is picked, and the first job from this
   entity is dequeued. ->run_job() is called: the job is queued to the
   ringbuf and the stream is pushed to the runnable queue (if it wasn't
   queued already). Wakeup the timeslice scheduler to re-evaluate (if
   the stream is not on a FW slot already)
4. stream gets picked by the timeslice scheduler and sent to the FW for
   execution

That's one extra step we don't really need. To sum-up, yes, all the
job/entity tracking might be interesting to share/re-use, but I wonder
if we couldn't have that without pulling out the scheduling part of
drm_sched, or maybe I'm missing something, and there's something in
drm_gpu_scheduler you really need.