[Intel-gfx] [PATCH] drm/i915: Do not add an interrupt for a context switch

[Daniel Vetter]

On Wed, Aug 07, 2013 at 10:29:05PM +0100, Chris Wilson wrote:
> We use the request to ensure we hold a reference to the context for the
> duration that it remains in use by the ring. Each request only holds a
> reference to the current context, hence we emit a request after
> switching contexts with the final reference to the old context. However,
> the extra interrupt caused by that request is not useful (no timing
> critical function will wait for the context object), instead the overhead
> of servicing the IRQ shows up in some (lightweight) benchmarks. In order
> to keep the useful property of using the request to manage the context
> lifetime, we want to add a dummy request that is associated with the
> interrupt from the subsequent real request following the batch.
> 
> The extra interrupt was added as a side-effect of using
> i915_add_request() in
> 
> commit 112522f6789581824903f6f72082b5b841a7f0f9
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Thu May 2 16:48:07 2013 +0300
> 
>     drm/i915: put context upon switching
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Ben Widawsky <ben at bwidawsk.net>
> Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h         |  1 +
>  drivers/gpu/drm/i915/i915_gem.c         | 32 +++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/i915/i915_gem_context.c |  2 +-
>  3 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 9a8fac3..e822390 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1873,6 +1873,7 @@ int __i915_add_request(struct intel_ring_buffer *ring,
>  		       struct drm_file *file,
>  		       struct drm_i915_gem_object *batch_obj,
>  		       u32 *seqno);
> +int __i915_add_request_marker(struct intel_ring_buffer *ring);
>  #define i915_add_request(ring, seqno) \
>  	__i915_add_request(ring, NULL, NULL, seqno)
>  int __must_check i915_wait_seqno(struct intel_ring_buffer *ring,
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 0fa45e0..7568bf0 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2110,6 +2110,37 @@ i915_gem_get_seqno(struct drm_device *dev, u32 *seqno)
>  	return 0;
>  }
>  
> +int
> +__i915_add_request_marker(struct intel_ring_buffer *ring)
> +{
> +	struct drm_i915_gem_request *request;
> +
> +	request = kmalloc(sizeof(*request), GFP_KERNEL);
> +	if (request == NULL)
> +		return -ENOMEM;
> +
> +	request->ring = ring;
> +	request->seqno = intel_ring_get_seqno(ring);
> +	request->tail = request->head = intel_ring_get_tail(ring);
> +	request->batch_obj = NULL;
> +
> +	/* Whilst this request exists, batch_obj will be on the
> +	 * active_list, and so will hold the active reference. Only when this
> +	 * request is retired will the the batch_obj be moved onto the
> +	 * inactive_list and lose its active reference. Hence we do not need
> +	 * to explicitly hold another reference here.
> +	 */
> +	request->ctx = ring->last_context;
> +	if (request->ctx)
> +		i915_gem_context_reference(request->ctx);
> +
> +	request->emitted_jiffies = jiffies;
> +	list_add_tail(&request->list, &ring->request_list);
> +	request->file_priv = NULL;
> +
> +	return 0;
> +}
> +
>  int __i915_add_request(struct intel_ring_buffer *ring,
>  		       struct drm_file *file,
>  		       struct drm_i915_gem_object *obj,
> @@ -2137,7 +2168,6 @@ int __i915_add_request(struct intel_ring_buffer *ring,
>  	if (request == NULL)
>  		return -ENOMEM;
>  
> -
>  	/* Record the position of the start of the request so that
>  	 * should we detect the updated seqno part-way through the
>  	 * GPU processing the request, we never over-estimate the
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index db94aca..88a9425 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -466,7 +466,7 @@ static int do_switch(struct i915_hw_context *to)
>  		from->obj->dirty = 1;
>  		BUG_ON(from->obj->ring != ring);
>  
> -		ret = i915_add_request(ring, NULL);
> +		ret = __i915_add_request_marker(ring);

Can't we just ditch the add_request here? The actual backing storage of
the old context won't get reaped too early (due to the oustanding lazy
request logic) and as long as we don't schedule a batch we don't care one
bit that there's no request with a context linked to it. Only once we emit
batches we care, since otherwise the hangcheck code can't assign the
blame.

Ripping out the add_request would also allow us to ditch the scary WARN +
mi_set_context stuff below.

Or do I massively miss something here?
-Daniel

>  		if (ret) {
>  			/* Too late, we've already scheduled a context switch.
>  			 * Try to undo the change so that the hw state is
> -- 
> 1.8.4.rc1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch