drm/sched: Account entity GPU time

To implement fair scheduling we need a view into the GPU time consumed by
entities. Problem we have is that jobs and entities objects have decoupled
lifetimes, where at the point we have a view into accurate GPU time, we
cannot link back to the entity any longer.

Solve this by adding a light weight entity stats object which is reference
counted by both entity and the job and hence can safely be used from
either side.

With that, the only other thing we need is to add a helper for adding the
job's GPU time into the respective entity stats object, and call it once
the accurate GPU time has been calculated.

The most convenient place to do that is the free job worker for several
reasons. Doing the accounting from the job completion callback would mean
a few locks would need to become irq safe and we would also need to worry
about out of order completions (via dma_fence_is_signaled calls which we
cannot control). In-order completions are critical for GPU time accuracy
which is currently adjusted per fence in the free worker and requires
looking at the next job in the scheduler pending list. We would also need
to add a new lock to protect the scheduler average stats update.

In contrast to those complications, having the accounting done from the
free worker is serialized by definition and all the above complications
are avoided. Downside is there is potential for a time lag between job
completions and GPU time being accounted against the entity. Since that is
partly alleviated by batch processing the completed job queue, and the
scheduling algorithm does not attempt to be completely fair, which would
even be rather impossible to achieve in the GPU world with the current
DRM scheduler design and hardware with no or poor preemption support,
this downside is not considered critical. Plus, in practice the scheduler
is also affected by worker scheduling delays from other angles too. Not
least being able to promptly feed the GPU with new work.

We therefore choose the simple option and can later consider improving
upon it if the need arises.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Philipp Stanner <phasta@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Tested-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20260417103744.76020-9-tvrtko.ursulin@igalia.com

0 files changed, 0 insertions, 0 deletions