[RFC v4 00/16] Fair DRM scheduler

Fri Apr 25 10:20:18 UTC 2025

V4 is quite different from v3 in that I have replaced the deadline + queue-depth
approach with a fair GPU time based approach. This is because Pierre-Eric found
a viewperf workload which showed queue-depth based approach regressing and
without it there was a regression on one of my synthetic workloads I was not
happy with.

In my experiments the fair scheduler looks solid so lets see how it fares after
wider testing.

On the high level main advantages of the series are:

 1. Scheduling quality - schedules better than FIFO.
 2. Code simplification - no more multiple run queues.

First patches add some unit tests which allow for easy evaluation of scheduling
behaviour against different client submission patterns. From there onwards it is
hopefully a natural progression of cleanups, enablers, adding the fair policy,
and finally removing FIFO and RR and simplifying the code base due not more need
for multiple run queues.

As a headline result I have tested three simultaneous clients on the Steam Deck:

One instance of a deferredmultisampling Vulkan demo running with low priority,
one normal priority instance of the same demo, and the Unigine Heaven benchmark.

With the FIFO scheduler we can see that the low priority client is completely
starved and the GPU time distribution between the other two clients is uneven:

https://people.igalia.com/tursulin/drm-sched-fair/fifo-starvation.png

Switching to the fair scheduler, GPU time distribution is almost equal and the
low priority client does get a small share of the GPU:

https://people.igalia.com/tursulin/drm-sched-fair/fair-no-starvation.png

Moving onto the synthetic submission patterns, they are about two simultaneous
clients which broadly cover the following categories:

 * Deep queue clients
 * Hogs versus interactive
 * Priority handling

Lets look at the results:

1. Two normal priority deep queue clients.

These ones submit one second worth of 8ms jobs. As fast as they can, no
dependencies etc. There is no difference in runtime between FIFO and fair but
the latter allows both clients to progress with work more evenly:

https://people.igalia.com/tursulin/drm-sched-fair/normal-normal.png

(X axis is time, Y is submitted queue-depth, hence lowering of qd corresponds
  with work progress for both clients, tested with both schedulers separately.)

2. Same two clients but one is now low priority.

https://people.igalia.com/tursulin/drm-sched-fair/normal-low.png

Normal priority client is a solid line, low priority dotted. We can see how FIFO
completely starves the low priority client until the normal priority is fully
done. Only then the low priority client gets any GPU time.

In constrast, fair scheduler allows some GPU time to the low priority client.

3. Same clients but now high versus normal priority.

Similar behaviour as in the previous one with normal a bit less de-prioritised
relative to high, than low was against normal.

https://people.igalia.com/tursulin/drm-sched-fair/high-normal.png

4. Heavy load vs interactive client.

Heavy client emits a 75% GPU load in the format of 3x 2.5ms jobs followed by a
2.5ms wait. Interactive client emits a 10% GPU load in the format of 1x 1ms job
followed by a 9ms wait.

This simulates an interactive graphical client used on top of a relatively heavy
background load but no GPU oversubscription.

Graphs show the interactive client only and from now on, instead of looking at
the client's queue depth, we look at its "fps".

https://people.igalia.com/tursulin/drm-sched-fair/heavy-interactive.png

We can see that fair scheduler allows a higher fps for the interactive client
which is good.

5. An even heavier load vs interactive client.

This one is oversubscribing the GPU by submitting 4x 50ms jobs and waiting for
only one microsecond before repeating the cycle. Interactive client is thje same
10% as above.

https://people.igalia.com/tursulin/drm-sched-fair/veryheavy-interactive.png

Here the difference is even more dramatic with fair scheduler enabling ~3x the
framerate for the interactive client.

6. Low priority GPU hog versus heavy-interactive.

Low priority client: 3x 2.5ms jobs client followed by a 0.5ms wait.
Interactive client: 1x 0.5ms job followed by a 10ms wait.

https://people.igalia.com/tursulin/drm-sched-fair/lowhog-interactive.png

Slight win for the fair scheduler but could be just noise.

7. Last set of test scenarios will have three subgroups.

In all cases we have two interactive (synchronous, single job at a time) clients
with a 50% "duty cycle" GPU time usage.

Client 1: 1.5ms job + 1.5ms wait (aka short bursty)
Client 2: 2.5ms job + 2.5ms wait (aka long bursty)

a) Both normal priority.

https://people.igalia.com/tursulin/drm-sched-fair/5050-short.png
https://people.igalia.com/tursulin/drm-sched-fair/5050-long.png

Both schedulers favour the higher frequency duty cycle with fair giving it a
little bit more which should be good for interactivity.

b) Normal vs low priority.

https://people.igalia.com/tursulin/drm-sched-fair/5050-normal-low-normal.png
https://people.igalia.com/tursulin/drm-sched-fair/5050-normal-low-low.png

Fair scheduler gives a bit more GPU time to the normal priority client which is
again good.

c) High vs normal priority.

https://people.igalia.com/tursulin/drm-sched-fair/5050-high-normal-high.png
https://people.igalia.com/tursulin/drm-sched-fair/5050-high-normal-normal.png

Again, fair scheduler gives a bit more share to the higher priority client.

On the overall fair looks like a potential improvement in terms of fairness,
especially avoiding priority starvation. There do not appear to be any
regressions with the tested workloads.

As before, I am looking for feedback, ideas for what kind of submission
scenarios to test. Testers on different GPUs would be very welcome too.

And I should probably test round-robin at some point, to see if we are maybe
okay to drop unconditionally, it or further work improving fair would be needed
if some use cases rely on round-robin.

v2:
 * Fixed many rebase errors.
 * Added some new patches.
 * Dropped single shot dependecy handling.

v3:
 * Added scheduling quality unit tests.
 * Refined a tiny bit by adding some fairness.
 * Dropped a few patches for now.

v4:
 * Replaced deadline with fair!
 * Refined scheduling quality unit tests.
 * Pulled one cleanup patch earlier.
 * Fixed "drm/sched: Avoid double re-lock on the job free path".

Cc: Christian König <christian.koenig at amd.com>
Cc: Danilo Krummrich <dakr at kernel.org>
CC: Leo Liu <Leo.Liu at amd.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Cc: Philipp Stanner <phasta at kernel.org>
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com>
Cc: Michel Dänzer <michel.daenzer at mailbox.org>

Tvrtko Ursulin (16):
  drm/sched: Add some scheduling quality unit tests
  drm/sched: Add some more scheduling quality unit tests
  drm/sched: De-clutter drm_sched_init
  drm/sched: Avoid double re-lock on the job free path
  drm/sched: Consolidate drm_sched_job_timedout
  drm/sched: Consolidate drm_sched_rq_select_entity_rr
  drm/sched: Implement RR via FIFO
  drm/sched: Consolidate entity run queue management
  drm/sched: Move run queue related code into a separate file
  drm/sched: Free all finished jobs at once
  drm/sched: Account entity GPU time
  drm/sched: Remove idle entity from tree
  drm/sched: Add fair scheduling policy
  drm/sched: Remove FIFO and RR and simplify to a single run queue
  drm/sched: Queue all free credits in one worker invocation
  drm/sched: Embed run queue singleton into the scheduler

 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c        |   6 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_job.c       |  27 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_job.h       |   5 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_trace.h     |   8 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_sdma.c   |   8 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_xcp.c       |   8 +-
 drivers/gpu/drm/scheduler/Makefile            |   2 +-
 drivers/gpu/drm/scheduler/sched_entity.c      | 121 +--
 drivers/gpu/drm/scheduler/sched_fence.c       |   2 +-
 drivers/gpu/drm/scheduler/sched_internal.h    | 114 ++-
 drivers/gpu/drm/scheduler/sched_main.c        | 570 +++---------
 drivers/gpu/drm/scheduler/sched_rq.c          | 214 +++++
 drivers/gpu/drm/scheduler/tests/Makefile      |   3 +-
 .../gpu/drm/scheduler/tests/tests_scheduler.c | 815 ++++++++++++++++++
 include/drm/gpu_scheduler.h                   |  23 +-
 15 files changed, 1348 insertions(+), 578 deletions(-)
 create mode 100644 drivers/gpu/drm/scheduler/sched_rq.c
 create mode 100644 drivers/gpu/drm/scheduler/tests/tests_scheduler.c

-- 
2.48.0