[RFC PATCH 00/10] Xe DRM scheduler and long running workload plans

[Asahi Lina]

Hi, thanks for the Cc!

On 04/04/2023 09.22, 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.

Yup, we're in the exact same situation with drm/asahi, so this is very 
welcome! We've been using the existing scheduler as-is, but this should 
help remove some unneeded complexity in this use case.

Do you want me to pull in this series into our tree and make sure this 
all works out for us?

I also have a couple bugfixes for drm/sched I need to send out, but I 
think the rebase/merge with this series should be trivial. I'll send 
that out this week.

> - 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.

We haven't needed this so far (mostly by using fine-grained locking and 
refcounting all over the place) but I can see it being useful to 
simplify some of those constructs and maybe avoid potential deadlocks in 
some places. I'm not sure yet whether we can fully get rid of the main 
queue refcounting/locking (our completion/error signaling path doesn't 
map well to DMA fences directly so we still need something there to get 
from the global GPU completion signaling thread to individual queues) 
but it might be a step in the right direction at least!

~~ Lina