On 04/04/2023 14:52, Matthew Brost wrote:
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.
Sounds like a fair enough requirement but if drivers will not be
comfortable with the wq conversion, it is probably possible to introduce
some vfuncs for the 1:1 case which would allow scheduler users override
the scheduler wakeup and select a special "pick one job" path. That
could allow 1:1 users do their thing, leaving rest as is. I mean you
already have the special single entity scheduler, you'd just need to add
some more specialization on the init, wake up, etc paths.
And I will mention once more that I find a wq item with a loop such as:
while (!READ_ONCE(sched->pause_run_wq)) {
...
A bit dodgy. If you piggy back on any system_wq it smells of system wide
starvation so for me any proposal with an option to use a system shared
wq is a no go.
Regards,
Tvrtko
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(-)
