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

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Apr 04, 2023 at 10:43:03AM +0100, Tvrtko Ursulin wrote:
> 
> 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?
> 

Addressed the other two on my reply to Christian...

For this one basically the concept of a single entity point IMO is a
very good concept which I'd like to keep. But most important reason
being the main execution thread (now worker) is kicked when a dependency
for a job is resolved, dependencies are dma-fences signaled via a
callback, and these call backs can be signaled in IRQ contexts. We
absolutely do not want to enter the backend in an IRQ context for a
variety of reasons.

Matt

> 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(-)
> >