Re: [PATCH] drm/i915: Prevent recursion by retiring requests when the ring is full

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Jan 27, 2014 at 10:43:07PM +0000, Chris Wilson wrote:
> As the VM do not track activity of objects and instead use a large
> hammer to forcibly idle and evict all of their associated objects when
> one is released, it is possible for that to cause a recursion when we
> need to wait for free space on a ring and call retire requests.
> (intel_ring_begin -> intel_ring_wait_request ->
> i915_gem_retire_requests_ring -> i915_gem_context_free ->
> i915_gem_evict_vm -> i915_gpu_idle -> intel_ring_begin etc)
> 
> In order to remove the requirement for calling retire-requests from
> intel_ring_wait_request, we have to inline a couple of steps from
> retiring requests, notably we have to record the position of the request
> we wait for and use that to update the available ring space.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Looks good to me.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

I do have a couple of questions about request->tail though.

We set it to -1 in intel_ring_wait_request(). Isn't that going to cause
problems for i915_request_guilty()?

When not -1, request->tail points to just before the commands that
.add_request() adds to the ring. So that means intel_ring_wait_request()
might have to wait for one extra request, and I guess more importantly
if the GPU hangs inside the .add_request() commands, we won't attribute
the hang to the request in question. Was it designe to be that way, or
is there a bug here?

> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 25 +++++--------------------
>  1 file changed, 5 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 10ff32d09c14..0da7c257159a 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1431,28 +1431,16 @@ void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring)
>  	cleanup_status_page(ring);
>  }
>  
> -static int intel_ring_wait_seqno(struct intel_ring_buffer *ring, u32 seqno)
> -{
> -	int ret;
> -
> -	ret = i915_wait_seqno(ring, seqno);
> -	if (!ret)
> -		i915_gem_retire_requests_ring(ring);
> -
> -	return ret;
> -}
> -
>  static int intel_ring_wait_request(struct intel_ring_buffer *ring, int n)
>  {
>  	struct drm_i915_gem_request *request;
> -	u32 seqno = 0;
> +	u32 seqno = 0, tail;
>  	int ret;
>  
> -	i915_gem_retire_requests_ring(ring);
> -
>  	if (ring->last_retired_head != -1) {
>  		ring->head = ring->last_retired_head;
>  		ring->last_retired_head = -1;
> +
>  		ring->space = ring_space(ring);
>  		if (ring->space >= n)
>  			return 0;
> @@ -1469,6 +1457,7 @@ static int intel_ring_wait_request(struct intel_ring_buffer *ring, int n)
>  			space += ring->size;
>  		if (space >= n) {
>  			seqno = request->seqno;
> +			tail = request->tail;
>  			break;
>  		}
>  
> @@ -1483,15 +1472,11 @@ static int intel_ring_wait_request(struct intel_ring_buffer *ring, int n)
>  	if (seqno == 0)
>  		return -ENOSPC;
>  
> -	ret = intel_ring_wait_seqno(ring, seqno);
> +	ret = i915_wait_seqno(ring, seqno);
>  	if (ret)
>  		return ret;
>  
> -	if (WARN_ON(ring->last_retired_head == -1))
> -		return -ENOSPC;
> -
> -	ring->head = ring->last_retired_head;
> -	ring->last_retired_head = -1;
> +	ring->head = tail;
>  	ring->space = ring_space(ring);
>  	if (WARN_ON(ring->space < n))
>  		return -ENOSPC;
> -- 
> 1.8.5.3
> 
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-- 
Ville Syrjälä
Intel OTC