[Intel-gfx] [RFC v2 0/5] Waitboost drm syncobj waits

[Rob Clark]

On Fri, Feb 10, 2023 at 5:07 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>
> In i915 we have this concept of "wait boosting" where we give a priority boost
> for instance to fences which are actively waited upon from userspace. This has
> it's pros and cons and can certainly be discussed at lenght. However fact is
> some workloads really like it.
>
> Problem is that with the arrival of drm syncobj and a new userspace waiting
> entry point it added, the waitboost mechanism was bypassed. Hence I cooked up
> this mini series really (really) quickly to see if some discussion can be had.
>
> It adds a concept of "wait count" to dma fence, which is incremented for every
> explicit dma_fence_enable_sw_signaling and dma_fence_add_wait_callback (like
> dma_fence_add_callback but from explicit/userspace wait paths).

I was thinking about a similar thing, but in the context of dma_fence
(or rather sync_file) fd poll()ing.  How does the kernel differentiate
between "housekeeping" poll()ers that don't want to trigger boost but
simply know when to do cleanup, and waiters who are waiting with some
urgency.  I think we could use EPOLLPRI for this purpose.

Not sure how that translates to waits via the syncobj.  But I think we
want to let userspace give some hint about urgent vs housekeeping
waits.

Also, on a related topic: https://lwn.net/Articles/868468/

BR,
-R

> Individual drivers can then inspect this via dma_fence_wait_count() and decide
> to wait boost the waits on such fences.
>
> Again, quickly put together and smoke tested only - no guarantees whatsoever and
> I will rely on interested parties to test and report if it even works or how
> well.
>
> v2:
>  * Small fixups based on CI feedback:
>     * Handle decrement correctly for already signalled case while adding callback.
>     * Remove i915 assert which was making sure struct i915_request does not grow.
>  * Split out the i915 patch into three separate functional changes.
>
> Tvrtko Ursulin (5):
>   dma-fence: Track explicit waiters
>   drm/syncobj: Mark syncobj waits as external waiters
>   drm/i915: Waitboost external waits
>   drm/i915: Mark waits as explicit
>   drm/i915: Wait boost requests waited upon by others
>
>  drivers/dma-buf/dma-fence.c               | 102 ++++++++++++++++------
>  drivers/gpu/drm/drm_syncobj.c             |   6 +-
>  drivers/gpu/drm/i915/gt/intel_engine_pm.c |   1 -
>  drivers/gpu/drm/i915/i915_request.c       |  13 ++-
>  include/linux/dma-fence.h                 |  14 +++
>  5 files changed, 101 insertions(+), 35 deletions(-)
>
> --
> 2.34.1
>