Re: [PATCH 1/3] drm/i915: Only spin whilst waiting on the current request

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Nov 18, 2015 at 09:56:06AM +0000, Chris Wilson wrote:
> Limit busywaiting only to the request currently being processed by the
> GPU. If the request is not currently being processed by the GPU, there
> is a very low likelihood of it being completed within the 2 microsecond
> spin timeout and so we will just be wasting CPU cycles.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/gpu/drm/i915/i915_drv.h | 2 +-
>  drivers/gpu/drm/i915/i915_gem.c | 8 +++++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 8afda459a26e..16095b95d2df 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2190,7 +2190,7 @@ struct drm_i915_gem_request {
>  	struct intel_engine_cs *ring;
>  
>  	/** GEM sequence number associated with this request. */
> -	uint32_t seqno;
> +	uint32_t seqno, spin_seqno;
>  
>  	/** Position in the ringbuffer of the start of the request */
>  	u32 head;
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 414150a0b8d5..af9ffa11ef44 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1193,9 +1193,14 @@ static int __i915_spin_request(struct drm_i915_gem_request *req, int state)
>  	 * takes to sleep on a request, on the order of a microsecond.
>  	 */
>  
> -	if (i915_gem_request_get_ring(req)->irq_refcount)
> +	if (req->ring->irq_refcount)
>  		return -EBUSY;
>  
> +	/* Only spin if we know the GPU is processing this request */
> +	if (i915_seqno_passed(req->ring->get_seqno(req->ring, false),
> +			      req->spin_seqno))
> +		return -EAGAIN;
> +
>  	timeout = local_clock_us(&cpu) + 2;
>  	while (!need_resched()) {
>  		if (i915_gem_request_completed(req, true))
> @@ -2592,6 +2597,7 @@ void __i915_add_request(struct drm_i915_gem_request *request,
>  	request->batch_obj = obj;
>  
>  	request->emitted_jiffies = jiffies;
> +	request->spin_seqno = ring->last_submitted_seqno;
>  	ring->last_submitted_seqno = request->seqno;
>  	list_add_tail(&request->list, &ring->request_list);
>  
> -- 
> 2.6.2
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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