[Intel-gfx] [PATCH] drm/i915/execlists: Drop setting sibling priority hint on virtual engines

[Tvrtko Ursulin]

On 25/03/2020 10:13, Chris Wilson wrote:
> We set the priority hint on execlists to avoid executing the tasklet for
> when we know that there will be no change in execution order. However,
> as we set it from the virtual engine for all siblings, but only one
> physical engine may respond, we leave the hint set on the others
> stopping direct submission that could take place.
> 
> If we do not set the hint, we may attempt direct submission even if we
> don't expect to submit. If we set the hint, we may not do any submission
> until the tasklet is run (and sometimes we may park the engine before
> that has had a chance). Ergo there's only a minor ill-effect on mixed
> virtual/physical engine workloads where we may try and fail to do direct
> submission more often than required. (Pure virtual / engine workloads
> will have redundant tasklet execution suppressed as normal.)
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/issues/1522
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_lrc.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 210f60e14ef4..f88d3b95c4e1 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -4985,10 +4985,8 @@ static void virtual_submission_tasklet(unsigned long data)
>   submit_engine:
>   		GEM_BUG_ON(RB_EMPTY_NODE(&node->rb));
>   		node->prio = prio;
> -		if (first && prio > sibling->execlists.queue_priority_hint) {
> -			sibling->execlists.queue_priority_hint = prio;
> +		if (first && prio > sibling->execlists.queue_priority_hint)
>   			tasklet_hi_schedule(&sibling->execlists.tasklet);
> -		}
>   
>   		spin_unlock(&sibling->active.lock);
>   	}
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

The queue_priority_hint scheme with virtual engine design is a bit 
problematic, since we have no way to unwind. And it's spreading it's 
tentacles all over the place. Oh well. Could we one day consider just 
peeking at the top of the tree(s)

Regards,

Tvrtko