Re: [Intel-xe] [RFC PATCH 00/10] Xe DRM scheduler and long running workload plans

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 04/04/2023 01:22, Matthew Brost wrote:
> Hello,
> 
> As a prerequisite to merging the new Intel Xe DRM driver [1] [2], we
> have been asked to merge our common DRM scheduler patches first as well
> as develop a common solution for long running workloads with the DRM
> scheduler. This RFC series is our first attempt at doing this. We
> welcome any and all feedback.
> 
> This can we thought of as 4 parts detailed below.
> 
> - DRM scheduler changes for 1 to 1 relationship between scheduler and
> entity (patches 1-3)
> 
> In Xe all of the scheduling of jobs is done by a firmware scheduler (the
> GuC) which is a new paradigm WRT to the DRM scheduler and presents
> severals problems as the DRM was originally designed to schedule jobs on
> hardware queues. The main problem being that DRM scheduler expects the
> submission order of jobs to be the completion order of jobs even across
> multiple entities. This assumption falls apart with a firmware scheduler
> as a firmware scheduler has no concept of jobs and jobs can complete out
> of order. A novel solution for was originally thought of by Faith during
> the initial prototype of Xe, create a 1 to 1 relationship between scheduler
> and entity. I believe the AGX driver [3] is using this approach and
> Boris may use approach as well for the Mali driver [4].
> 
> To support a 1 to 1 relationship we move the main execution function
> from a kthread to a work queue and add a new scheduling mode which
> bypasses code in the DRM which isn't needed in a 1 to 1 relationship.
> The new scheduling mode should unify all drivers usage with a 1 to 1
> relationship and can be thought of as using scheduler as a dependency /
> infligt job tracker rather than a true scheduler.

Once you add capability for a more proper 1:1 via 
DRM_SCHED_POLICY_SINGLE_ENTITY, do you still have further need to 
replace kthreads with a wq?

Or in other words, what purpose does the offloading of a job picking 
code to a separate execution context serve? Could it be done directly in 
the 1:1 mode and leave kthread setup for N:M?

Apart from those design level questions, low level open IMO still is 
that default fallback of using the system_wq has the potential to 
affect latency for other drivers. But that's for those driver owners to 
approve.

Regards,

Tvrtko

> - Generic messaging interface for DRM scheduler
> 
> Idea is to be able to communicate to the submission backend with in band
> (relative to main execution function) messages. Messages are backend
> defined and flexable enough for any use case. In Xe we use these
> messages to clean up entites, set properties for entites, and suspend /
> resume execution of an entity [5]. I suspect other driver can leverage
> this messaging concept too as it a convenient way to avoid races in the
> backend.
> 
> - Support for using TDR for all error paths of a scheduler / entity
> 
> Fix a few races / bugs, add function to dynamically set the TDR timeout.
> 
> - Annotate dma-fences for long running workloads.
> 
> The idea here is to use dma-fences only as sync points within the
> scheduler and never export them for long running workloads. By
> annotating these fences as long running we ensure that these dma-fences
> are never used in a way that breaks the dma-fence rules. A benefit of
> thus approach is the scheduler can still safely flow control the
> execution ring buffer via the job limit without breaking the dma-fence
> rules.
> 
> Again this a first draft and looking forward to feedback.
> 
> Enjoy - Matt
> 
> [1] https://gitlab.freedesktop.org/drm/xe/kernel
> [2] https://patchwork.freedesktop.org/series/112188/
> [3] https://patchwork.freedesktop.org/series/114772/
> [4] https://patchwork.freedesktop.org/patch/515854/?series=112188&rev=1
> [5] https://gitlab.freedesktop.org/drm/xe/kernel/-/blob/drm-xe-next/drivers/gpu/drm/xe/xe_guc_submit.c#L1031
> 
> Matthew Brost (8):
>    drm/sched: Convert drm scheduler to use a work queue rather than
>      kthread
>    drm/sched: Move schedule policy to scheduler / entity
>    drm/sched: Add DRM_SCHED_POLICY_SINGLE_ENTITY scheduling policy
>    drm/sched: Add generic scheduler message interface
>    drm/sched: Start run wq before TDR in drm_sched_start
>    drm/sched: Submit job before starting TDR
>    drm/sched: Add helper to set TDR timeout
>    drm/syncobj: Warn on long running dma-fences
> 
> Thomas Hellström (2):
>    dma-buf/dma-fence: Introduce long-running completion fences
>    drm/sched: Support long-running sched entities
> 
>   drivers/dma-buf/dma-fence.c                 | 142 +++++++---
>   drivers/dma-buf/dma-resv.c                  |   5 +
>   drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c |  14 +-
>   drivers/gpu/drm/amd/amdgpu/amdgpu_device.c  |  15 +-
>   drivers/gpu/drm/drm_syncobj.c               |   5 +-
>   drivers/gpu/drm/etnaviv/etnaviv_sched.c     |   5 +-
>   drivers/gpu/drm/lima/lima_sched.c           |   5 +-
>   drivers/gpu/drm/msm/adreno/adreno_device.c  |   6 +-
>   drivers/gpu/drm/msm/msm_ringbuffer.c        |   5 +-
>   drivers/gpu/drm/panfrost/panfrost_job.c     |   5 +-
>   drivers/gpu/drm/scheduler/sched_entity.c    | 127 +++++++--
>   drivers/gpu/drm/scheduler/sched_fence.c     |   6 +-
>   drivers/gpu/drm/scheduler/sched_main.c      | 278 +++++++++++++++-----
>   drivers/gpu/drm/v3d/v3d_sched.c             |  25 +-
>   include/drm/gpu_scheduler.h                 | 130 +++++++--
>   include/linux/dma-fence.h                   |  60 ++++-
>   16 files changed, 649 insertions(+), 184 deletions(-)
>