Re: [Intel-gfx] [PATCH 06/54] drm/i915: Drop i915_request.lock requirement for intel_rps_boost()

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Since we use a flag within i915_request.flags to indicate when we have
> boosted the request (so that we only apply the boost) once, this can be
> used as the serialisation with i915_request_retire() to avoid having to
> explicitly take the i915_request.lock which is more heavily contended.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/gt/intel_rps.c | 15 ++++++---------
>  drivers/gpu/drm/i915/i915_request.c |  4 +---
>  2 files changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_rps.c b/drivers/gpu/drm/i915/gt/intel_rps.c
> index f74d5e09e176..e1397b8d3586 100644
> --- a/drivers/gpu/drm/i915/gt/intel_rps.c
> +++ b/drivers/gpu/drm/i915/gt/intel_rps.c
> @@ -917,17 +917,15 @@ void intel_rps_park(struct intel_rps *rps)
>  
>  void intel_rps_boost(struct i915_request *rq)
>  {
> -	struct intel_rps *rps = &READ_ONCE(rq->engine)->gt->rps;
> -	unsigned long flags;
> -
> -	if (i915_request_signaled(rq) || !intel_rps_is_active(rps))
> +	if (i915_request_signaled(rq) || i915_request_has_waitboost(rq))
>  		return;
>  
>  	/* Serializes with i915_request_retire() */
> -	spin_lock_irqsave(&rq->lock, flags);
> -	if (!i915_request_has_waitboost(rq) &&
> -	    !dma_fence_is_signaled_locked(&rq->fence)) {
> -		set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags);
> +	if (!test_and_set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags)) {
> +		struct intel_rps *rps = &READ_ONCE(rq->engine)->gt->rps;
> +
> +		if (!intel_rps_is_active(rps))
> +			return;
>  
>  		GT_TRACE(rps_to_gt(rps), "boost fence:%llx:%llx\n",
>  			 rq->fence.context, rq->fence.seqno);
> @@ -938,7 +936,6 @@ void intel_rps_boost(struct i915_request *rq)
>  
>  		atomic_inc(&rps->boosts);

Looks of it, this does not need to be atomic. But topic for another
patch.

>  	}
> -	spin_unlock_irqrestore(&rq->lock, flags);
>  }
>  
>  int intel_rps_set(struct intel_rps *rps, u8 val)
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 2d2882344e40..2a7bad88038b 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -307,10 +307,8 @@ bool i915_request_retire(struct i915_request *rq)
>  		spin_unlock_irq(&rq->lock);
>  	}
>  
> -	if (i915_request_has_waitboost(rq)) {
> -		GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
> +	if (test_and_set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags))
>  		atomic_dec(&rq->engine->gt->rps.num_waiters);

This should keep the num_waiters in sync.

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> -	}
>  
>  	/*
>  	 * We only loosely track inflight requests across preemption,
> -- 
> 2.20.1
>
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