[PATCH 18/32] drm/i915: Split execlist priority queue into rbtree + linked list

Chris Wilson chris at chris-wilson.co.uk
Sat Apr 29 14:46:02 UTC 2017


All the requests at the same priority are executed in FIFO order. They
do not need to be stored in the rbtree themselves, as they are a simple
list within a level. If we move the requests at one priority into a list,
we can then reduce the rbtree to the set of priorities. This should keep
the height of the rbtree small, as the number of active priorities can not
exceed the number of active requests and should be typically only a few.

Currently, we have ~2k possible different priority levels, that may
increase to allow even more fine grained selection. Allocating those in
advance seems a waste (and may be impossible), so we opt for allocating
upon first use, and freeing after its requests are depleted. To avoid
the possibility of an allocation failure causing us to lose a request,
we preallocate the default priority (0) and bump any request to that
priority if we fail to allocate it the appropriate plist. Having a
request (that is ready to run, so not leading to corruption) execute
out-of-order is better than leaking the request (and its dependency
tree) entirely.

There should be a benefit to reducing execlists_dequeue() to principally
using a simple list (and reducing the frequency of both rbtree iteration
and balancing on erase) but for typical workloads, request coalescing
should be small enough that we don't notice any change. The main gain is
from improving PI calls to schedule, and the explicit list within a
level should make request unwinding simpler (we just need to insert at
the head of the list rather than the tail and not have to make the
rbtree search more complicated).

v2: Avoid use-after-free when deleting a depleted priolist

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: MichaƂ Winiarski <michal.winiarski at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_debugfs.c        |  12 ++-
 drivers/gpu/drm/i915/i915_gem_request.c    |   4 +-
 drivers/gpu/drm/i915/i915_gem_request.h    |   2 +-
 drivers/gpu/drm/i915/i915_guc_submission.c |  53 ++++++----
 drivers/gpu/drm/i915/i915_utils.h          |   9 ++
 drivers/gpu/drm/i915/intel_lrc.c           | 159 +++++++++++++++++++----------
 drivers/gpu/drm/i915/intel_ringbuffer.h    |   7 ++
 7 files changed, 163 insertions(+), 83 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 0b5d7142d8d9..a8c7788d986e 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -3314,7 +3314,6 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 
 		if (i915.enable_execlists) {
 			u32 ptr, read, write;
-			struct rb_node *rb;
 			unsigned int idx;
 
 			seq_printf(m, "\tExeclist status: 0x%08x %08x\n",
@@ -3358,9 +3357,14 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 			rcu_read_unlock();
 
 			spin_lock_irq(&engine->timeline->lock);
-			for (rb = engine->execlist_first; rb; rb = rb_next(rb)) {
-				rq = rb_entry(rb, typeof(*rq), priotree.node);
-				print_request(m, rq, "\t\tQ ");
+			for (rb = engine->execlist_first; rb; rb = rb_next(rb)){
+				struct execlist_priolist *plist =
+					rb_entry(rb, typeof(*plist), node);
+
+				list_for_each_entry(rq,
+						    &plist->requests,
+						    priotree.link)
+					print_request(m, rq, "\t\tQ ");
 			}
 			spin_unlock_irq(&engine->timeline->lock);
 		} else if (INTEL_GEN(dev_priv) > 6) {
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 9074303c8888..1685bc712b6c 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -159,7 +159,7 @@ i915_priotree_fini(struct drm_i915_private *i915, struct i915_priotree *pt)
 {
 	struct i915_dependency *dep, *next;
 
-	GEM_BUG_ON(!RB_EMPTY_NODE(&pt->node));
+	GEM_BUG_ON(!list_empty(&pt->link));
 
 	/* Everyone we depended upon (the fences we wait to be signaled)
 	 * should retire before us and remove themselves from our list.
@@ -185,7 +185,7 @@ i915_priotree_init(struct i915_priotree *pt)
 {
 	INIT_LIST_HEAD(&pt->signalers_list);
 	INIT_LIST_HEAD(&pt->waiters_list);
-	RB_CLEAR_NODE(&pt->node);
+	INIT_LIST_HEAD(&pt->link);
 	pt->priority = INT_MIN;
 }
 
diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
index feb81463abc9..6c58f7d87746 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.h
+++ b/drivers/gpu/drm/i915/i915_gem_request.h
@@ -67,7 +67,7 @@ struct i915_dependency {
 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 */
-	struct rb_node node;
+	struct list_head link;
 	int priority;
 #define I915_PRIORITY_MAX 1024
 #define I915_PRIORITY_DFL 0
diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
index 58f8fb3a7da7..9a8525db9659 100644
--- a/drivers/gpu/drm/i915/i915_guc_submission.c
+++ b/drivers/gpu/drm/i915/i915_guc_submission.c
@@ -674,32 +674,45 @@ static bool i915_guc_dequeue(struct intel_engine_cs *engine)
 
 	spin_lock_irq(&engine->timeline->lock);
 	rb = engine->execlist_first;
+	GEM_BUG_ON(rb_first(&engine->execlist_queue) != rb);
 	while (rb) {
-		struct drm_i915_gem_request *rq =
-			rb_entry(rb, typeof(*rq), priotree.node);
-
-		if (last && rq->ctx != last->ctx) {
-			if (port != engine->execlist_port)
-				break;
-
-			port_assign(port, last);
-			port++;
+		struct execlist_priolist *plist =
+			rb_entry(rb, typeof(*plist), node);
+		struct drm_i915_gem_request *rq, *rn;
+
+		list_for_each_entry_safe(rq, rn,
+					 &plist->requests, priotree.link) {
+			if (last && rq->ctx != last->ctx) {
+				if (port != engine->execlist_port) {
+					__list_del_many(&plist->requests,
+							&rq->priotree.link);
+					goto done;
+				}
+
+				port_assign(port, last);
+				port++;
+			}
+
+			INIT_LIST_HEAD(&rq->priotree.link);
+			rq->priotree.priority = INT_MAX;
+
+			i915_guc_submit(rq);
+			trace_i915_gem_request_in(rq, port - engine->execlist_port);
+			last = rq;
+			submit = true;
 		}
 
 		rb = rb_next(rb);
-		rb_erase(&rq->priotree.node, &engine->execlist_queue);
-		RB_CLEAR_NODE(&rq->priotree.node);
-		rq->priotree.priority = INT_MAX;
-
-		i915_guc_submit(rq);
-		trace_i915_gem_request_in(rq, port - engine->execlist_port);
-		last = rq;
-		submit = true;
-	}
-	if (submit) {
-		port_assign(port, last);
 		engine->execlist_first = rb;
+
+		rb_erase(&plist->node, &engine->execlist_queue);
+		INIT_LIST_HEAD(&plist->requests);
+		if (plist->priority != I915_PRIORITY_DFL)
+			kfree(plist);
 	}
+done:
+	if (submit)
+		port_assign(port, last);
 	spin_unlock_irq(&engine->timeline->lock);
 
 	return submit;
diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
index f0500c65726d..9c65f9ae2e97 100644
--- a/drivers/gpu/drm/i915/i915_utils.h
+++ b/drivers/gpu/drm/i915/i915_utils.h
@@ -99,4 +99,13 @@
 	__T;								\
 })
 
+#include <linux/list.h>
+
+static inline void __list_del_many(struct list_head *head,
+				   struct list_head *first)
+{
+	head->next = first;
+	first->prev = head;
+}
+
 #endif /* !__I915_UTILS_H */
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 05496a111b88..062f309a5c61 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -437,57 +437,79 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 
 	spin_lock_irq(&engine->timeline->lock);
 	rb = engine->execlist_first;
+	GEM_BUG_ON(rb_first(&engine->execlist_queue) != rb);
 	while (rb) {
-		struct drm_i915_gem_request *cursor =
-			rb_entry(rb, typeof(*cursor), priotree.node);
-
-		/* Can we combine this request with the current port? It has to
-		 * be the same context/ringbuffer and not have any exceptions
-		 * (e.g. GVT saying never to combine contexts).
-		 *
-		 * If we can combine the requests, we can execute both by
-		 * updating the RING_TAIL to point to the end of the second
-		 * request, and so we never need to tell the hardware about
-		 * the first.
-		 */
-		if (last && !can_merge_ctx(cursor->ctx, last->ctx)) {
-			/* If we are on the second port and cannot combine
-			 * this request with the last, then we are done.
-			 */
-			if (port != engine->execlist_port)
-				break;
-
-			/* If GVT overrides us we only ever submit port[0],
-			 * leaving port[1] empty. Note that we also have
-			 * to be careful that we don't queue the same
-			 * context (even though a different request) to
-			 * the second port.
+		struct execlist_priolist *plist =
+			rb_entry(rb, typeof(*plist), node);
+		struct drm_i915_gem_request *rq, *rn;
+
+		list_for_each_entry_safe(rq, rn,
+					 &plist->requests, priotree.link) {
+			/*
+			 * Can we combine this request with the current port?
+			 * It has to be the same context/ringbuffer and not
+			 * have any exceptions (e.g. GVT saying never to
+			 * combine contexts).
+			 *
+			 * If we can combine the requests, we can execute both
+			 * by updating the RING_TAIL to point to the end of the
+			 * second request, and so we never need to tell the
+			 * hardware about the first.
 			 */
-			if (ctx_single_port_submission(last->ctx) ||
-			    ctx_single_port_submission(cursor->ctx))
-				break;
+			if (last && !can_merge_ctx(rq->ctx, last->ctx)) {
+				/*
+				 * If we are on the second port and cannot
+				 * combine this request with the last, then we
+				 * are done.
+				 */
+				if (port != engine->execlist_port) {
+					__list_del_many(&plist->requests,
+							&rq->priotree.link);
+					goto done;
+				}
+
+				/*
+				 * If GVT overrides us we only ever submit
+				 * port[0], leaving port[1] empty. Note that we
+				 * also have to be careful that we don't queue
+				 * the same context (even though a different
+				 * request) to the second port.
+				 */
+				if (ctx_single_port_submission(last->ctx) ||
+				    ctx_single_port_submission(rq->ctx)) {
+					__list_del_many(&plist->requests,
+							&rq->priotree.link);
+					goto done;
+				}
+
+				GEM_BUG_ON(last->ctx == rq->ctx);
+
+				if (submit)
+					port_assign(port, last);
+				port++;
+			}
 
-			GEM_BUG_ON(last->ctx == cursor->ctx);
+			INIT_LIST_HEAD(&rq->priotree.link);
+			rq->priotree.priority = INT_MAX;
 
-			if (submit)
-				port_assign(port, last);
-			port++;
+			__i915_gem_request_submit(rq);
+			trace_i915_gem_request_in(rq,
+						  port - engine->execlist_port);
+			last = rq;
+			submit = true;
 		}
 
 		rb = rb_next(rb);
-		rb_erase(&cursor->priotree.node, &engine->execlist_queue);
-		RB_CLEAR_NODE(&cursor->priotree.node);
-		cursor->priotree.priority = INT_MAX;
+		engine->execlist_first = rb;
 
-		__i915_gem_request_submit(cursor);
-		trace_i915_gem_request_in(cursor, port - engine->execlist_port);
-		last = cursor;
-		submit = true;
+		rb_erase(&plist->node, &engine->execlist_queue);
+		INIT_LIST_HEAD(&plist->requests);
+		if (plist->priority != I915_PRIORITY_DFL)
+			kfree(plist);
 	}
-	if (submit) {
+done:
+	if (submit)
 		port_assign(port, last);
-		engine->execlist_first = rb;
-	}
 	spin_unlock_irq(&engine->timeline->lock);
 
 	if (submit)
@@ -611,28 +633,53 @@ static void intel_lrc_irq_handler(unsigned long data)
 	intel_uncore_forcewake_put(dev_priv, engine->fw_domains);
 }
 
-static bool insert_request(struct i915_priotree *pt, struct rb_root *root)
+static bool
+insert_request(struct intel_engine_cs *engine,
+	       struct i915_priotree *pt,
+	       int prio)
 {
+	struct execlist_priolist *plist;
 	struct rb_node **p, *rb;
 	bool first = true;
 
+find_plist:
 	/* most positive priority is scheduled first, equal priorities fifo */
 	rb = NULL;
-	p = &root->rb_node;
+	p = &engine->execlist_queue.rb_node;
 	while (*p) {
-		struct i915_priotree *pos;
-
 		rb = *p;
-		pos = rb_entry(rb, typeof(*pos), node);
-		if (pt->priority > pos->priority) {
+		plist = rb_entry(rb, typeof(*plist), node);
+		if (prio > plist->priority) {
 			p = &rb->rb_left;
-		} else {
+		} else if (prio < plist->priority) {
 			p = &rb->rb_right;
 			first = false;
+		} else {
+			list_add_tail(&pt->link, &plist->requests);
+			return false;
 		}
 	}
-	rb_link_node(&pt->node, rb, p);
-	rb_insert_color(&pt->node, root);
+
+	if (prio == I915_PRIORITY_DFL) {
+		plist = &engine->default_priolist;
+	} else {
+		plist = kmalloc(sizeof(*plist), GFP_ATOMIC);
+		/* Convert an allocation failure to a priority bump */
+		if (unlikely(!plist)) {
+			prio = I915_PRIORITY_DFL; /* recurses just once */
+			goto find_plist;
+		}
+	}
+
+	plist->priority = prio;
+	rb_link_node(&plist->node, rb, p);
+	rb_insert_color(&plist->node, &engine->execlist_queue);
+
+	INIT_LIST_HEAD(&plist->requests);
+	list_add_tail(&pt->link, &plist->requests);
+
+	if (first)
+		engine->execlist_first = &plist->node;
 
 	return first;
 }
@@ -645,8 +692,9 @@ static void execlists_submit_request(struct drm_i915_gem_request *request)
 	/* Will be called from irq-context when using foreign fences. */
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 
-	if (insert_request(&request->priotree, &engine->execlist_queue)) {
-		engine->execlist_first = &request->priotree.node;
+	if (insert_request(engine,
+			   &request->priotree,
+			   request->priotree.priority)) {
 		if (execlists_elsp_ready(engine))
 			tasklet_hi_schedule(&engine->irq_tasklet);
 	}
@@ -735,10 +783,9 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 			continue;
 
 		pt->priority = prio;
-		if (!RB_EMPTY_NODE(&pt->node)) {
-			rb_erase(&pt->node, &engine->execlist_queue);
-			if (insert_request(pt, &engine->execlist_queue))
-				engine->execlist_first = &pt->node;
+		if (!list_empty(&pt->link)) {
+			__list_del_entry(&pt->link);
+			insert_request(engine, pt, prio);
 		}
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 696b45024cb2..914d171f61d8 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -177,6 +177,12 @@ enum intel_engine_id {
 	VECS
 };
 
+struct execlist_priolist {
+	struct rb_node node;
+	struct list_head requests;
+	int priority;
+};
+
 #define INTEL_ENGINE_CS_MAX_NAME 8
 
 struct intel_engine_cs {
@@ -367,6 +373,7 @@ struct intel_engine_cs {
 
 	/* Execlists */
 	struct tasklet_struct irq_tasklet;
+	struct execlist_priolist default_priolist;
 	struct execlist_port {
 		struct drm_i915_gem_request *request_count;
 #define EXECLIST_COUNT_BITS 2
-- 
2.11.0



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