Re: [PATCH 10/11] drm/i915: Use a preemption timeout to enforce interactivity

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 10/11] drm/i915: Use a preemption timeout to enforce interactivity
Date: Tue, 27 Mar 2018 09:35:17 +0100	[thread overview]
Message-ID: <3df73d2a-c55e-3e0f-0670-6a263a134199@linux.intel.com> (raw)
In-Reply-To: <20180326115044.2505-11-chris@chris-wilson.co.uk>

On 26/03/2018 12:50, Chris Wilson wrote:
> Use a liberal timeout of 20ms to ensure that the rendering for an
> interactive pageflip is started in a timely fashion, and that
> user interaction is not blocked by GPU, or CPU, hogs. This is at the cost
> of resetting whoever was blocking the preemption, likely leading to that
> context/process being banned from submitting future requests.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_drv.h      |  3 ++-
>   drivers/gpu/drm/i915/i915_gem.c      | 18 ++++++++++--------
>   drivers/gpu/drm/i915/intel_display.c | 18 +++++++++++++++++-
>   3 files changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 800230ba1c3b..87388feb973d 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3150,8 +3150,9 @@ int i915_gem_object_wait(struct drm_i915_gem_object *obj,
>   			 struct intel_rps_client *rps);
>   int i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
>   				  unsigned int flags,
> -				  int priority);
> +				  int priority, unsigned int timeout);
>   #define I915_PRIORITY_DISPLAY I915_PRIORITY_MAX
> +#define I915_PREEMPTION_TIMEOUT_DISPLAY (20 * 1000 * 1000) /* 20 ms */
>   
>   int __must_check
>   i915_gem_object_set_to_wc_domain(struct drm_i915_gem_object *obj, bool write);
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 675d6bb59337..252c6b58e739 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -469,7 +469,8 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
>   	return timeout;
>   }
>   
> -static void __fence_set_priority(struct dma_fence *fence, int prio)
> +static void __fence_set_priority(struct dma_fence *fence,
> +				 int prio, unsigned int timeout)
>   {
>   	struct i915_request *rq;
>   	struct intel_engine_cs *engine;
> @@ -482,11 +483,12 @@ static void __fence_set_priority(struct dma_fence *fence, int prio)
>   
>   	rcu_read_lock();
>   	if (engine->schedule)
> -		engine->schedule(rq, prio, 0);
> +		engine->schedule(rq, prio, timeout);
>   	rcu_read_unlock();
>   }
>   
> -static void fence_set_priority(struct dma_fence *fence, int prio)
> +static void fence_set_priority(struct dma_fence *fence,
> +			       int prio, unsigned int timeout)
>   {
>   	/* Recurse once into a fence-array */
>   	if (dma_fence_is_array(fence)) {
> @@ -494,16 +496,16 @@ static void fence_set_priority(struct dma_fence *fence, int prio)
>   		int i;
>   
>   		for (i = 0; i < array->num_fences; i++)
> -			__fence_set_priority(array->fences[i], prio);
> +			__fence_set_priority(array->fences[i], prio, timeout);
>   	} else {
> -		__fence_set_priority(fence, prio);
> +		__fence_set_priority(fence, prio, timeout);
>   	}
>   }
>   
>   int
>   i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
>   			      unsigned int flags,
> -			      int prio)
> +			      int prio, unsigned int timeout)
>   {
>   	struct dma_fence *excl;
>   
> @@ -518,7 +520,7 @@ i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
>   			return ret;
>   
>   		for (i = 0; i < count; i++) {
> -			fence_set_priority(shared[i], prio);
> +			fence_set_priority(shared[i], prio, timeout);
>   			dma_fence_put(shared[i]);
>   		}
>   
> @@ -528,7 +530,7 @@ i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
>   	}
>   
>   	if (excl) {
> -		fence_set_priority(excl, prio);
> +		fence_set_priority(excl, prio, timeout);
>   		dma_fence_put(excl);
>   	}
>   	return 0;
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 4c30c7c04f9c..830f2d4e540f 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -12786,7 +12786,23 @@ intel_prepare_plane_fb(struct drm_plane *plane,
>   
>   	ret = intel_plane_pin_fb(to_intel_plane_state(new_state));
>   
> -	i915_gem_object_wait_priority(obj, 0, I915_PRIORITY_DISPLAY);
> +	/*
> +	 * Reschedule our dependencies, and ensure we run within a timeout.
> +	 *
> +	 * Note that if the timeout is exceeded, then whoever was running that
> +	 * prevented us from acquiring the GPU is declared rogue and reset. An
> +	 * unresponsive process will then be banned in order to preserve
> +	 * interactivity. Since this can be seen as a bit heavy-handed, we
> +	 * select a timeout for when the dropped frames start to become a
> +	 * noticeable nuisance for the user (20 ms, i.e. preemption was blocked
> +	 * for more than a frame). Note, this is only a timeout for a delay in
> +	 * preempting the current request in order to run our dependency chain,
> +	 * our dependency chain may itself take a long time to run to completion
> +	 * before we can present the framebuffer.
> +	 */
> +	i915_gem_object_wait_priority(obj, 0,
> +				      I915_PRIORITY_DISPLAY,
> +				      I915_PREEMPTION_TIMEOUT_DISPLAY);

API signature changes to allow timeouts are fine, but I think this hunk 
should be separate patch at the end of the series. (Since it has the 
potential to make things which used to work, albeit stutteringly (?), 
start crashing.

Regards,

Tvrtko

>   
>   	mutex_unlock(&dev_priv->drm.struct_mutex);
>   	i915_gem_object_unpin_pages(obj);
> 
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