[Intel-gfx] [PATCH 3/6] drm/i915/scheduler: Record all dependencies upon request construction

[Chris Wilson]

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.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_request.c | 71 ++++++++++++++++++++++++++++++++-
 drivers/gpu/drm/i915/i915_gem_request.h | 23 +++++++++++
 2 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 864ab686af33..cf06b036b47a 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -113,6 +113,59 @@ i915_gem_request_remove_from_client(struct drm_i915_gem_request *request)
 	spin_unlock(&file_priv->mm.lock);
 }
 
+static int
+i915_priotree_add_dependency(struct i915_priotree *pt,
+			     struct i915_priotree *signal,
+			     struct i915_dependency *dep)
+{
+	unsigned long flags = 0;
+
+	if (!dep) {
+		dep = kmalloc(sizeof(*dep), GFP_KERNEL);
+		if (!dep)
+			return -ENOMEM;
+
+		flags |= I915_DEPENDENCY_ALLOC;
+	}
+
+	dep->signal = signal;
+	dep->flags = flags;
+
+	list_add(&dep->post_link, &signal->post_list);
+	list_add(&dep->pre_link, &pt->pre_list);
+	return 0;
+}
+
+static void
+i915_priotree_fini(struct i915_priotree *pt)
+{
+	struct i915_dependency *dep, *next;
+
+	/* Everyone we depended upon 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->pre_list, pre_link) {
+		list_del(&dep->post_link);
+		if (dep->flags & I915_DEPENDENCY_ALLOC)
+			kfree(dep);
+	}
+
+	/* Remove ourselves from everyone who depends upon us */
+	list_for_each_entry_safe(dep, next, &pt->post_list, post_link) {
+		list_del(&dep->pre_link);
+		if (dep->flags & I915_DEPENDENCY_ALLOC)
+			kfree(dep);
+	}
+}
+
+static void
+i915_priotree_init(struct i915_priotree *pt)
+{
+	INIT_LIST_HEAD(&pt->pre_list);
+	INIT_LIST_HEAD(&pt->post_list);
+}
+
 void i915_gem_retire_noop(struct i915_gem_active *active,
 			  struct drm_i915_gem_request *request)
 {
@@ -182,6 +235,8 @@ 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->priotree);
 	i915_gem_request_put(request);
 }
 
@@ -441,6 +496,7 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
 		       __timeline_get_seqno(req->timeline->common));
 
 	i915_sw_fence_init(&req->submit, submit_notify);
+	i915_priotree_init(&req->priotree);
 
 	INIT_LIST_HEAD(&req->active_list);
 	req->i915 = dev_priv;
@@ -495,6 +551,14 @@ 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->priotree,
+						   &from->priotree,
+						   NULL);
+		if (ret < 0)
+			return ret;
+	}
+
 	if (to->timeline == from->timeline)
 		return 0;
 
@@ -718,9 +782,14 @@ 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->depq);
+	}
 
 	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 75f8360b3421..5841bbfa39ad 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.h
+++ b/drivers/gpu/drm/i915/i915_gem_request.h
@@ -41,6 +41,18 @@ struct intel_signal_node {
 	struct intel_wait wait;
 };
 
+struct i915_dependency {
+	struct i915_priotree *signal;
+	struct list_head pre_link, post_link;
+	unsigned long flags;
+#define I915_DEPENDENCY_ALLOC BIT(0)
+};
+
+struct i915_priotree {
+	struct list_head pre_list; /* who is before us, we depend upon */
+	struct list_head post_list; /* who is after us, they depend upon us */
+};
+
 /**
  * Request queue structure.
  *
@@ -87,6 +99,17 @@ struct drm_i915_gem_request {
 	struct i915_sw_fence submit;
 	wait_queue_t submitq;
 
+	/* 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 depq;
+
 	u32 global_seqno;
 
 	/** GEM sequence number associated with the previous request,
-- 
2.10.1