Re: [PATCH v3 07/14] drm/i915/scheduler: Record all dependencies upon request construction

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

On 14/11/2016 08:56, Chris Wilson wrote:
> The scheduler needs to know the dependencies of each request for the
> lifetime of the request, as it may choose to reschedule the requests at
> any time and must ensure the dependency tree is not broken. This is in
> additional to using the fence to only allow execution after all
> dependencies have been completed.
>
> One option was to extend the fence to support the bidirectional
> dependency tracking required by the scheduler. However the mismatch in
> lifetimes between the submit fence and the request essentially meant
> that we had to build a completely separate struct (and we could not
> simply reuse the existing waitqueue in the fence for one half of the
> dependency tracking). The extra dependency tracking simply did not mesh
> well with the fence, and keeping it separate both keeps the fence
> implementation simpler and allows us to extend the dependency tracking
> into a priority tree (whilst maintaining support for reordering the
> tree).
>
> To avoid the additional allocations and list manipulations, the use of
> the priotree is disabled when there are no schedulers to use it.
>
> v2: Create a dedicated slab for i915_dependency.
>     Rename the lists.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_drv.h         |  1 +
>  drivers/gpu/drm/i915/i915_gem.c         | 11 +++-
>  drivers/gpu/drm/i915/i915_gem_request.c | 91 ++++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/i915/i915_gem_request.h | 33 ++++++++++++
>  4 files changed, 134 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index c0f1dfc7119e..ab4ad5522cf5 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1778,6 +1778,7 @@ struct drm_i915_private {
>  	struct kmem_cache *objects;
>  	struct kmem_cache *vmas;
>  	struct kmem_cache *requests;
> +	struct kmem_cache *dependencies;
>
>  	const struct intel_device_info info;
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index e1afa11609a0..b331e5966fe2 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4431,12 +4431,18 @@ i915_gem_load_init(struct drm_device *dev)
>  	if (!dev_priv->requests)
>  		goto err_vmas;
>
> +	dev_priv->dependencies = KMEM_CACHE(i915_dependency,
> +					    SLAB_HWCACHE_ALIGN |
> +					    SLAB_RECLAIM_ACCOUNT);
> +	if (!dev_priv->dependencies)
> +		goto err_requests;
> +
>  	mutex_lock(&dev_priv->drm.struct_mutex);
>  	INIT_LIST_HEAD(&dev_priv->gt.timelines);
>  	err = i915_gem_timeline_init__global(dev_priv);
>  	mutex_unlock(&dev_priv->drm.struct_mutex);
>  	if (err)
> -		goto err_requests;
> +		goto err_dependencies;
>
>  	INIT_LIST_HEAD(&dev_priv->context_list);
>  	INIT_WORK(&dev_priv->mm.free_work, __i915_gem_free_work);
> @@ -4464,6 +4470,8 @@ i915_gem_load_init(struct drm_device *dev)
>
>  	return 0;
>
> +err_dependencies:
> +	kmem_cache_destroy(dev_priv->dependencies);
>  err_requests:
>  	kmem_cache_destroy(dev_priv->requests);
>  err_vmas:
> @@ -4480,6 +4488,7 @@ void i915_gem_load_cleanup(struct drm_device *dev)
>
>  	WARN_ON(!llist_empty(&dev_priv->mm.free_list));
>
> +	kmem_cache_destroy(dev_priv->dependencies);
>  	kmem_cache_destroy(dev_priv->requests);
>  	kmem_cache_destroy(dev_priv->vmas);
>  	kmem_cache_destroy(dev_priv->objects);
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> index 1118cf48d6f0..78c87d94d205 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.c
> +++ b/drivers/gpu/drm/i915/i915_gem_request.c
> @@ -113,6 +113,77 @@ i915_gem_request_remove_from_client(struct drm_i915_gem_request *request)
>  	spin_unlock(&file_priv->mm.lock);
>  }
>
> +static struct i915_dependency *
> +i915_dependency_alloc(struct drm_i915_private *i915)
> +{
> +	return kmem_cache_alloc(i915->dependencies, GFP_KERNEL);
> +}
> +
> +static void
> +i915_dependency_free(struct drm_i915_private *i915,
> +		     struct i915_dependency *dep)
> +{
> +	kmem_cache_free(i915->dependencies, dep);
> +}
> +
> +static void
> +__i915_priotree_add_dependency(struct i915_priotree *pt,
> +			       struct i915_priotree *signal,
> +			       struct i915_dependency *dep,
> +			       unsigned long flags)
> +{
> +	list_add(&dep->wait_link, &signal->waiters_list);
> +	list_add(&dep->signal_link, &pt->signalers_list);
> +	dep->signaler = signal;
> +	dep->flags = flags;
> +}
> +
> +static int
> +i915_priotree_add_dependency(struct drm_i915_private *i915,
> +			     struct i915_priotree *pt,
> +			     struct i915_priotree *signal)
> +{
> +	struct i915_dependency *dep;
> +
> +	dep = i915_dependency_alloc(i915);
> +	if (!dep)
> +		return -ENOMEM;
> +
> +	__i915_priotree_add_dependency(pt, signal, dep, I915_DEPENDENCY_ALLOC);
> +	return 0;
> +}
> +
> +static void
> +i915_priotree_fini(struct drm_i915_private *i915, struct i915_priotree *pt)
> +{
> +	struct i915_dependency *dep, *next;
> +
> +	/* Everyone we depended upon (the fences we wait to be signaled)
> +	 * should retire before us and remove themselves from our list.
> +	 * However, retirement is run independently on each timeline and
> +	 * so we may be called out-of-order.
> +	 */
> +	list_for_each_entry_safe(dep, next, &pt->signalers_list, signal_link) {
> +		list_del(&dep->wait_link);
> +		if (dep->flags & I915_DEPENDENCY_ALLOC)
> +			i915_dependency_free(i915, dep);
> +	}
> +
> +	/* Remove ourselves from everyone who depends upon us */
> +	list_for_each_entry_safe(dep, next, &pt->waiters_list, wait_link) {
> +		list_del(&dep->signal_link);
> +		if (dep->flags & I915_DEPENDENCY_ALLOC)
> +			i915_dependency_free(i915, dep);
> +	}
> +}
> +
> +static void
> +i915_priotree_init(struct i915_priotree *pt)
> +{
> +	INIT_LIST_HEAD(&pt->signalers_list);
> +	INIT_LIST_HEAD(&pt->waiters_list);
> +}
> +
>  void i915_gem_retire_noop(struct i915_gem_active *active,
>  			  struct drm_i915_gem_request *request)
>  {
> @@ -182,6 +253,8 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
>  	i915_gem_context_put(request->ctx);
>
>  	dma_fence_signal(&request->fence);
> +
> +	i915_priotree_fini(request->i915, &request->priotree);
>  	i915_gem_request_put(request);
>  }
>
> @@ -467,6 +540,8 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
>  	 */
>  	i915_sw_fence_await_sw_fence(&req->execute, &req->submit, &req->execq);
>
> +	i915_priotree_init(&req->priotree);
> +
>  	INIT_LIST_HEAD(&req->active_list);
>  	req->i915 = dev_priv;
>  	req->engine = engine;
> @@ -520,6 +595,14 @@ i915_gem_request_await_request(struct drm_i915_gem_request *to,
>
>  	GEM_BUG_ON(to == from);
>
> +	if (to->engine->schedule) {
> +		ret = i915_priotree_add_dependency(to->i915,
> +						   &to->priotree,
> +						   &from->priotree);
> +		if (ret < 0)
> +			return ret;
> +	}
> +
>  	if (to->timeline == from->timeline)
>  		return 0;
>
> @@ -743,9 +826,15 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
>
>  	prev = i915_gem_active_raw(&timeline->last_request,
>  				   &request->i915->drm.struct_mutex);
> -	if (prev)
> +	if (prev) {
>  		i915_sw_fence_await_sw_fence(&request->submit, &prev->submit,
>  					     &request->submitq);
> +		if (engine->schedule)
> +			__i915_priotree_add_dependency(&request->priotree,
> +						       &prev->priotree,
> +						       &request->dep,
> +						       0);
> +	}
>
>  	spin_lock_irq(&timeline->lock);
>  	list_add_tail(&request->link, &timeline->requests);
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
> index 4d2784633d9f..943c39d2a62a 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.h
> +++ b/drivers/gpu/drm/i915/i915_gem_request.h
> @@ -44,6 +44,28 @@ struct intel_signal_node {
>  	struct intel_wait wait;
>  };
>
> +struct i915_dependency {
> +	struct i915_priotree *signaler;
> +	struct list_head signal_link;
> +	struct list_head wait_link;
> +	unsigned long flags;
> +#define I915_DEPENDENCY_ALLOC BIT(0)
> +};
> +
> +/* Requests exist in a complex web of interdependencies. Each request
> + * has to wait for some other request to complete before it is ready to be run
> + * (e.g. we have to wait until the pixels have been rendering into a texture
> + * before we can copy from it). We track the readiness of a request in terms
> + * of fences, but we also need to keep the dependency tree for the lifetime
> + * of the request (beyond the life of an individual fence). We use the tree
> + * at various points to reorder the requests whilst keeping the requests
> + * in order with respect to their various dependencies.
> + */
> +struct i915_priotree {
> +	struct list_head signalers_list; /* those before us, we depend upon */
> +	struct list_head waiters_list; /* those after us, they depend upon us */
> +};
> +
>  /**
>   * Request queue structure.
>   *
> @@ -105,6 +127,17 @@ struct drm_i915_gem_request {
>  	wait_queue_t submitq;
>  	wait_queue_t execq;
>
> +	/* A list of everyone we wait upon, and everyone who waits upon us.
> +	 * Even though we will not be submitted to the hardware before the
> +	 * submit fence is signaled (it waits for all external events as well
> +	 * as our own requests), the scheduler still needs to know the
> +	 * dependency tree for the lifetime of the request (from execbuf
> +	 * to retirement), i.e. bidirectional dependency information for the
> +	 * request not tied to individual fences.
> +	 */
> +	struct i915_priotree priotree;
> +	struct i915_dependency dep;
> +
>  	u32 global_seqno;
>
>  	/** GEM sequence number associated with the previous request,
>

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

Regards,

Tvrtko

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