Re: [PATCH 3/3] drm/i915: Limit request busywaiting

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Nov 18, 2015 at 09:56:08AM +0000, Chris Wilson wrote:
> If we suppose that on average it takes 1ms to do a fullscreen paint
> (which is fairly typical across generations, as the GPUs get faster the
> displays get larger), then the likelihood of a synchronous wait on the last
> request completing within 2 microseconds is miniscule. On the other
> hand, not everything attempts a fullscreen repaint (or is ratelimited by
> such) and for those a short busywait is much faster than setting up the
> interrupt driven waiter. The challenge is identifying such tasks. Here
> we opt to never spin for an unlocked wait (such as from a set-domain or
> wait ioctl) and instead only spin in circumstances where we are holding
> the lock and have either already completed one unlocked wait or we are
> in a situation where we are being forced to drain old requests (which we
> know must be close to completion).
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Do you have some perf datat for this?
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_drv.h         |  1 +
>  drivers/gpu/drm/i915/i915_gem.c         | 12 +++++++-----
>  drivers/gpu/drm/i915/intel_ringbuffer.c |  2 +-
>  3 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index f47d701f2ddb..dc8f2a87dac0 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2979,6 +2979,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>  			s64 *timeout,
>  			struct intel_rps_client *rps);
>  #define I915_WAIT_INTERRUPTIBLE 0x1
> +#define I915_WAIT_MAY_SPIN 0x2
>  int __must_check i915_wait_request(struct drm_i915_gem_request *req);
>  int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf);
>  int __must_check
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index d4d5c6e8c02f..cfa101fad479 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1273,9 +1273,11 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>  	before = ktime_get_raw_ns();
>  
>  	/* Optimistic spin for the next jiffie before touching IRQs */
> -	ret = __i915_spin_request(req, state);
> -	if (ret == 0)
> -		goto out;
> +	if (flags & I915_WAIT_MAY_SPIN) {
> +		ret = __i915_spin_request(req, state);
> +		if (ret == 0)
> +			goto out;
> +	}
>  
>  	if (!irq_test_in_progress && WARN_ON(!ring->irq_get(ring))) {
>  		ret = -ENODEV;
> @@ -1467,7 +1469,7 @@ i915_wait_request(struct drm_i915_gem_request *req)
>  	if (ret)
>  		return ret;
>  
> -	flags = 0;
> +	flags = I915_WAIT_MAY_SPIN;
>  	if (dev_priv->mm.interruptible)
>  		flags |= I915_WAIT_INTERRUPTIBLE;
>  
> @@ -4107,7 +4109,7 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
>  
>  	ret = __i915_wait_request(target,
>  				  reset_counter,
> -				  I915_WAIT_INTERRUPTIBLE,
> +				  I915_WAIT_INTERRUPTIBLE | I915_WAIT_MAY_SPIN,
>  				  NULL, NULL);
>  	if (ret == 0)
>  		queue_delayed_work(dev_priv->wq, &dev_priv->mm.retire_work, 0);
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 664ce0b20b23..d3142af0f347 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -2241,7 +2241,7 @@ int intel_ring_idle(struct intel_engine_cs *ring)
>  			struct drm_i915_gem_request,
>  			list);
>  
> -	flags = 0;
> +	flags = I915_WAIT_MAY_SPIN;
>  	if (to_i915(ring->dev)->mm.interruptible)
>  		flags |= I915_WAIT_INTERRUPTIBLE;
>  
> -- 
> 2.6.2
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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