Re: [PATCH v7 00/29] Fair(er) DRM scheduler

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

Gentle thread bump.

On 06/03/2026 16:34, Tvrtko Ursulin wrote:
> [disclaimer]
> Please note that as this series includes patches which touch a good number of
> drivers, I did not copy everyone on everything. Assumption is people are
> subscribed to dri-devel and for context can look at the whole series there.
> [/disclaimer]
> 
> As a summary, the new scheduling algorithm is insipired by the original Linux
> CFS and so far no scheduling regressions have been found relative to FIFO.
> There are improvements in fairness and scheduling of interactive clients when
> running in parallel with a heavy GPU load (for example Pierre-Eric has one
> viewperf medical test which shows a nice improvement with amdgpu).
> 
> On the high level main advantages of the series are:
> 
>   1. Scheduling quality - schedules better than FIFO, solves priority starvation.
>   2. Code simplification - no more multiple run queues and multiple algorithms.
>   3. Virtual GPU time based scheduling enables relatively simple addition
>      of a scheduling cgroup controller in the future.
> 
> There is a little bit more detailed write up on the motivation and results in
> the form of a blog post which may be easier to read:
> https://blogs.igalia.com/tursulin/fair-er-drm-gpu-scheduler/
> 
> 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 no more need
> for multiple run queues.
> 
> Series is structured in a way where we could apply the first 12 patches (up to
> and including "drm/sched: Switch default policy to fair") in one kernel release
> and then follow up with the rest of the cleanups after a release or two if
> things will be looking fine. Until the remainder of the series would be merged
> it would be easy to flip the default algorithm back.
> 
> Onto the performance evaluation. 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.)
> 
> Round-robin is the same as fair here.
> 
> 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.
> 
> Here round-robin flavours are the same as FIFO (same starvation issue).
> 
> 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
> 
> And again round-robin flavours are the same as FIFO.
> 
> 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/251008/4-heavy-vs-interactive.png
> 
> Here round-robin and round-robin rewritten on top of FIFO are best, with the
> fair algorithm being very close. FIFO is clearly the worst.
> 
> 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 the same
> 10% as above.
> 
> https://people.igalia.com/tursulin/drm-sched-fair/251008/4-very-heavy-vs-interactive.png
> 
> Here FIFO is even worse and fair is again almost as good as the two round-robin
> flavours.
> 
> 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/251008/4-low-hog-vs-interactive.png
> 
> All schedulers appear to handle this almost equally well but FIFO could still be
> the last while fair has a slight lead.
> 
> As before, I am looking for feedback, ideas for what other kinds of submission
> scenarios to test, testing on different GPUs and of course reviews.
> 
> 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".
> 
> v5:
>   * Rebase on top of latest upstream DRM scheduler changes.
>   * Kerneldoc fixup.
>   * Improve commit message justification for one patch. (Philipp)
>   * Add comment in drm_sched_alloc_wq. (Christian)
> 
> v6:
>   * Rebase for "drm/sched: De-clutter drm_sched_init" getting merged.
>   * Avoid NULL rq dereference from a bad rebase. (Maira)
>   * Added some kerneldoc throughout. (Maira)
>   * Removed some lockdep annotations not belonging to one patch. (Maira)
>   * Use dma_fence_is_signaled in "drm/sched: Avoid double re-lock on the job free path". (Maira, Philipp)
> 
> v7:
>   * Rebase for some prep patches getting merged.
>   * Dropped submit all ready jobs patch.
>   * Fixed 64-bit division in unit tests.
>   * Fixed some more rebase and patch re-ordering mistakes.
>   * Preserve entity RR order when re-entering the queue.
>   * Fine tuned the queue re-enter logic for better behaviour with interactive
>     clients.
>   * Removed some static inlines.
>   * Added more kerneldoc.
>   * Done some benchmarks in the round-robin scheduling modes.
> 
> v8:
>   * Rebased for upstream changes.
>   * Added assert for reverse numerical order of DRM_SCHED_PRIORITY enums.
>   * Fixed head of rq priority updates.
> 
> *** RFC -> PATCH change log version reset. ***
> 
> v9 / v1:
>   * RFC -> PATCH for the series as agreed during the XDC.
>   * Updated interactive benchmark graphs.
>   * Improved handling of interactive clients by replacing the random noise on tie
>     approach with the average job duration statistics.
>   * Document in code why we track entity GPU stats in a reference counted structures.
>   * Document the new structure fields added by the fair policy.
>   * Undo some tab vs spaces damage.
>   * More accurate wording in the fair policy commit message.
>   * Default to fair policy in a separate patch.
>   * Renamed drm_sched_rq_select_entity to drm_sched_select_entity and make it only take sched.
>   * Fixed kerneldoc after removing scheduling policies and renaming the rq.
>   * Reversed arguments of drm_sched_rq_init and cleanup callers. (New patch)
>   * Removed unused num_rqs from struct drm_sched_args. (New patches)
>   * Unit tests:
>     * Added wait duration comments.
>     * Data structure comments.
>     * Better name for a local variable.
>     * Added comment to the short job duration assert.
>     * Added comment for cond_resched().
>     * Tweaked some comments
>     * Added client_done() helper and documented the READ_ONCE.
>     * Clarified cycles per second calculation.
> 
> v2:
>   * Dropped the sched_rq_init() argument re-order patch.
>   * Pulled run queue management consolidation patches to start of the series.
>   * Drm_sched_rq_get_rr_ts() renamed to drm_sched_rq_next_rr_ts().
>   * Added DRM_SCHED_PRIORITY_INVALID local define.
>   * Added locking around the ktime_t vruntime read.
>   * Expanded comment relating to the unlocked entity->priority read.
>   * Expanded comment relating to the unlocked scheduler average job duration read.
>   * Various kerneldoc tweaks.
>   * Undo some tab vs spaces damage harder.
>   * Undo one empty line rebase damage.
>   * Reworded commit message for the FIFO and RR removal patch.
>   * Various commit message spelling, grammar and typo fixes.
>   * Unit tests:
>     * Various style changes.
>     * More descriptive test names.
>     * Print legend for measured metrics.
>     * Check for memory allocation failures.
> 
> v3:
>   * Marked fair policy as default in the correct patch.
>   * Further improved comment in drm_sched_rq_update_prio().
>   * Moved DRM_SCHED_PRIORITY_INVALID to gpu_scheduler.h.
>   * Added description of the GPU time to vruntime scaling factors.
>   * Recorded more collected acks.
> 
> v4:
>   * Rebased, collect ackes and r-b-s receives so far.
>   * New patch in the series for handling the new ethosu accel driver.
>   * Bump lower priorities lower down in the run queue when they re-join, in order
>     to penalize them a bit more than simply making them second in the line to
>     run as it was in v3.
> 
> v5:
>   * Rebased, collect ackes and r-b-s received so far.
>   * Fix lost return statement introduced in v2 refactoring.
>   * Use virtual runtime based on average job durations when placing the re-
>     joining lower priority entity in the run-queue.
> 
> v6:
>   * Renamed drm_sched_entity_stats_alloc to drm_sched_entity_stats_new.
>   * Moved drm_sched_entity_stats_job_add_gpu_time to sched_entity.c.
>   * Improved kerneldoc for struct drm_sched_entity_stats.
>   * Explained GPU accounting placement in the respective commit text.
>   * Removed duplicated sched list assignment int the embedding the run-queue
>     patch.
> 
> v7:
>   * Rebased to catchup with upstream changes.
>   * Dropped the two amdgpu dependencies from the series - they have been merged
>     and have propagated to drm-next and drm-tip.
>   * Applied acks/reviews received since v6.
> 
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> CC: Leo Liu <Leo.Liu@amd.com>
> Cc: Lucas Stach <l.stach@pengutronix.de>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Philipp Stanner <phasta@kernel.org>
> Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
> Cc: Michel Dänzer <michel.daenzer@mailbox.org>
> 
> Tvrtko Ursulin (29):
>    drm/sched: Disallow initializing entities with no schedulers
>    drm/sched: Consolidate entity run queue management
>    drm/sched: Move run queue related code into a separate file
>    drm/sched: Add some scheduling quality unit tests
>    drm/sched: Add some more scheduling quality unit tests
>    drm/sched: Implement RR via FIFO
>    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: Favour interactive clients slightly
>    drm/sched: Switch default policy to fair
>    drm/sched: Remove FIFO and RR and simplify to a single run queue
>    drm/sched: Embed run queue singleton into the scheduler
>    accel/amdxdna: Remove drm_sched_init_args->num_rqs usage
>    accel/rocket: Remove drm_sched_init_args->num_rqs usage
>    accel/ethosu: Remove drm_sched_init_args->num_rqs usage
>    drm/amdgpu: Remove drm_sched_init_args->num_rqs usage
>    drm/etnaviv: Remove drm_sched_init_args->num_rqs usage
>    drm/imagination: Remove drm_sched_init_args->num_rqs usage
>    drm/lima: Remove drm_sched_init_args->num_rqs usage
>    drm/msm: Remove drm_sched_init_args->num_rqs usage
>    drm/nouveau: Remove drm_sched_init_args->num_rqs usage
>    drm/panfrost: Remove drm_sched_init_args->num_rqs usage
>    drm/panthor: Remove drm_sched_init_args->num_rqs usage
>    drm/sched: Remove drm_sched_init_args->num_rqs usage
>    drm/v3d: Remove drm_sched_init_args->num_rqs usage
>    drm/xe: Remove drm_sched_init_args->num_rqs usage
>    drm/sched: Remove drm_sched_init_args->num_rqs
> 
>   drivers/accel/amdxdna/aie2_ctx.c              |   1 -
>   drivers/accel/ethosu/ethosu_job.c             |   1 -
>   drivers/accel/rocket/rocket_job.c             |   1 -
>   drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c        |   6 +-
>   drivers/gpu/drm/amd/amdgpu/amdgpu_device.c    |   1 -
>   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/etnaviv/etnaviv_sched.c       |   1 -
>   drivers/gpu/drm/imagination/pvr_queue.c       |   1 -
>   drivers/gpu/drm/lima/lima_sched.c             |   1 -
>   drivers/gpu/drm/msm/msm_gem_vma.c             |   1 -
>   drivers/gpu/drm/msm/msm_ringbuffer.c          |   1 -
>   drivers/gpu/drm/nouveau/nouveau_sched.c       |   1 -
>   drivers/gpu/drm/panfrost/panfrost_job.c       |   1 -
>   drivers/gpu/drm/panthor/panthor_mmu.c         |   1 -
>   drivers/gpu/drm/panthor/panthor_sched.c       |   1 -
>   drivers/gpu/drm/scheduler/Makefile            |   2 +-
>   drivers/gpu/drm/scheduler/sched_entity.c      | 163 ++--
>   drivers/gpu/drm/scheduler/sched_fence.c       |   2 +-
>   drivers/gpu/drm/scheduler/sched_internal.h    |  70 +-
>   drivers/gpu/drm/scheduler/sched_main.c        | 351 +------
>   drivers/gpu/drm/scheduler/sched_rq.c          | 379 ++++++++
>   drivers/gpu/drm/scheduler/tests/Makefile      |   3 +-
>   .../gpu/drm/scheduler/tests/mock_scheduler.c  |   1 -
>   .../gpu/drm/scheduler/tests/tests_scheduler.c | 884 ++++++++++++++++++
>   drivers/gpu/drm/v3d/v3d_sched.c               |   1 -
>   drivers/gpu/drm/xe/xe_dep_scheduler.c         |   1 -
>   drivers/gpu/drm/xe/xe_execlist.c              |   1 -
>   drivers/gpu/drm/xe/xe_gpu_scheduler.c         |   1 -
>   include/drm/gpu_scheduler.h                   |  44 +-
>   33 files changed, 1497 insertions(+), 481 deletions(-)
>   create mode 100644 drivers/gpu/drm/scheduler/sched_rq.c
>   create mode 100644 drivers/gpu/drm/scheduler/tests/tests_scheduler.c
>