Re: [PATCH] drm/nouveau: wait for the exclusive fence after the shared ones v2

[Stefan Fritsch <sf@sfritsch.de>]

On 09.12.21 11:23, Christian König wrote:
> Always waiting for the exclusive fence resulted on some performance
> regressions. So try to wait for the shared fences first, then the
> exclusive fence should always be signaled already.
> 
> v2: fix incorrectly placed "(", add some comment why we do this.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>

Tested-by: Stefan Fritsch <sf@sfritsch.de>

Please also add a cc for linux-stable, so that this is fixed in 5.15.x

Cheers,
Stefan

> ---
>   drivers/gpu/drm/nouveau/nouveau_fence.c | 28 +++++++++++++------------
>   1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> index 05d0b3eb3690..0ae416aa76dc 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> @@ -353,15 +353,22 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan, bool e
>   
>   		if (ret)
>   			return ret;
> -	}
>   
> -	fobj = dma_resv_shared_list(resv);
> -	fence = dma_resv_excl_fence(resv);
> +		fobj = NULL;
> +	} else {
> +		fobj = dma_resv_shared_list(resv);
> +	}
>   
> -	if (fence) {
> +	/* Waiting for the exclusive fence first causes performance regressions
> +	 * under some circumstances. So manually wait for the shared ones first.
> +	 */
> +	for (i = 0; i < (fobj ? fobj->shared_count : 0) && !ret; ++i) {
>   		struct nouveau_channel *prev = NULL;
>   		bool must_wait = true;
>   
> +		fence = rcu_dereference_protected(fobj->shared[i],
> +						dma_resv_held(resv));
> +
>   		f = nouveau_local_fence(fence, chan->drm);
>   		if (f) {
>   			rcu_read_lock();
> @@ -373,20 +380,13 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan, bool e
>   
>   		if (must_wait)
>   			ret = dma_fence_wait(fence, intr);
> -
> -		return ret;
>   	}
>   
> -	if (!exclusive || !fobj)
> -		return ret;
> -
> -	for (i = 0; i < fobj->shared_count && !ret; ++i) {
> +	fence = dma_resv_excl_fence(resv);
> +	if (fence) {
>   		struct nouveau_channel *prev = NULL;
>   		bool must_wait = true;
>   
> -		fence = rcu_dereference_protected(fobj->shared[i],
> -						dma_resv_held(resv));
> -
>   		f = nouveau_local_fence(fence, chan->drm);
>   		if (f) {
>   			rcu_read_lock();
> @@ -398,6 +398,8 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan, bool e
>   
>   		if (must_wait)
>   			ret = dma_fence_wait(fence, intr);
> +
> +		return ret;
>   	}
>   
>   	return ret;