Re: [PATCH 4/4] drm/i915: Remove the spin-request during execbuf await_request

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 05/06/2017 11:26, Chris Wilson wrote:
> Originally we would enable and disable the breadcrumb interrupt
> immediately on demand. This was slow enough to have a large impact
> (>30%) on tasks that hopped between engines. However, by using a shadow
> to keep the irq alive for an extra interrupt (see commit 67b807a89230
> ("drm/i915: Delay disabling the user interrupt for breadcrumbs")) and
> by recently reducing the cost in adding ourselves to the signal tree, we
> no longer need to spin-request during await_request to avoid delays in
> throughput tests. Without the earlier patches to stop the wakeup when
> signaling if the irq was already active, we saw no improvement in
> execbuf overhead (and corresponding contention in other clients) despite
> the removal of the spinner in a simple test like glxgears. This means
> that will be scenarios where now we spend longer enabling the interrupt

"There will be" I guess?

> than we would have spent spinning, but these are not likely to have as
> noticeable an impact as the high frequency test cases (where there
> should not be any regression).
> 
> Ulterior motive: generalising the engine->sync_to to handle different
> types of semaphores and non-semaphores.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Oscar Mateo <oscar.mateo@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem_request.c | 18 ++++++------------
>   1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> index 46d869e26b4d..8c59c79cbd8b 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.c
> +++ b/drivers/gpu/drm/i915/i915_gem_request.c
> @@ -683,7 +683,6 @@ static int
>   i915_gem_request_await_request(struct drm_i915_gem_request *to,
>   			       struct drm_i915_gem_request *from)
>   {
> -	u32 seqno;
>   	int ret;
>   
>   	GEM_BUG_ON(to == from);
> @@ -707,18 +706,14 @@ i915_gem_request_await_request(struct drm_i915_gem_request *to,
>   		return ret < 0 ? ret : 0;
>   	}
>   
> -	seqno = i915_gem_request_global_seqno(from);
> -	if (!seqno)
> -		goto await_dma_fence;
> +	if (to->engine->semaphore.sync_to) {
> +		u32 seqno;
>   
> -	if (!to->engine->semaphore.sync_to) {
> -		if (!__i915_gem_request_started(from, seqno))
> -			goto await_dma_fence;
> +		GEM_BUG_ON(!from->engine->semaphore.signal);
>   
> -		if (!__i915_spin_request(from, seqno, TASK_INTERRUPTIBLE, 2))
> +		seqno = i915_gem_request_global_seqno(from);
> +		if (!seqno)
>   			goto await_dma_fence;
> -	} else {
> -		GEM_BUG_ON(!from->engine->semaphore.signal);
>   
>   		if (seqno <= to->timeline->global_sync[from->engine->id])
>   			return 0;
> @@ -729,10 +724,9 @@ i915_gem_request_await_request(struct drm_i915_gem_request *to,
>   			return ret;
>   
>   		to->timeline->global_sync[from->engine->id] = seqno;
> +		return 0;
>   	}
>   
> -	return 0;
> -
>   await_dma_fence:
>   	ret = i915_sw_fence_await_dma_fence(&to->submit,
>   					    &from->fence, 0,
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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