[Intel-gfx] [PATCH 3/3] drm/i915/execlists: Suppress redundant preemption

[Chris Wilson]

Quoting Chris Wilson (2019-01-23 12:36:02)
> On unwinding the active request we give it a small (limited to internal
> priority levels) boost to prevent it from being gazumped a second time.
> However, this means that it can be promoted to above the request that
> triggered the preemption request, causing a preempt-to-idle cycle for no
> change. We can avoid this if we take the boost into account when
> checking if the preemption request is valid.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index d9d744f6ab2c..74726f647e47 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -163,6 +163,8 @@
>  #define WA_TAIL_DWORDS 2
>  #define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)
>  
> +#define ACTIVE_PRIORITY (I915_PRIORITY_NEWCLIENT)
> +
>  static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
>                                             struct intel_engine_cs *engine,
>                                             struct intel_context *ce);
> @@ -181,13 +183,31 @@ static inline int rq_prio(const struct i915_request *rq)
>         return rq->sched.attr.priority;
>  }
>  
> +static inline int active_prio(const struct i915_request *rq)
> +{
> +       int prio = rq_prio(rq);
> +
> +       /*
> +        * On unwinding the active request, we give it a priority bump
> +        * equivalent to a freshly submitted request. This protects it from
> +        * being gazumped again, but it would be preferrable if we didn't
> +        * let it be gazumped in the first place!
> +        *
> +        * See __unwind_incomplete_requests()
> +        */
> +       if (i915_request_started(rq))
> +               prio |= ACTIVE_PRIORITY;

Hmm, actually we are put to the tail of that priolist so we don't get
rerun ahead of the preemptee if we end up at the same priority.
ACTIVE_PRIORITY - 1 would seem to be the right compromise.
-Chris