[Intel-gfx] [PATCH] drm/i915/gt: Delay taking the spinlock for grabbing from the buffer pool

[Chris Wilson]

Some very low hanging fruit, but contention on the pool->lock is
noticeable between intel_gt_get_buffer_pool() and pool_retire(), with
the majority of the hold time due to the locked list iteration. If we
make the node itself RCU protected, we can perform the search for an
suitable node just under RCU, reserving taking the lock itself for
claiming the node and manipulating the list.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld at intel.com>
---
 .../gpu/drm/i915/gt/intel_gt_buffer_pool.c    | 65 +++++++++++++------
 .../drm/i915/gt/intel_gt_buffer_pool_types.h  |  6 +-
 2 files changed, 51 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
index 418ae184cecf..612aa2741e73 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
@@ -35,37 +35,54 @@ static void node_free(struct intel_gt_buffer_pool_node *node)
 {
 	i915_gem_object_put(node->obj);
 	i915_active_fini(&node->active);
-	kfree(node);
+	kfree_rcu(node, rcu);
 }
 
 static void pool_free_work(struct work_struct *wrk)
 {
 	struct intel_gt_buffer_pool *pool =
 		container_of(wrk, typeof(*pool), work.work);
-	struct intel_gt_buffer_pool_node *node, *next;
+	struct intel_gt_buffer_pool_node *node, *stale = NULL;
 	unsigned long old = jiffies - HZ;
 	bool active = false;
-	LIST_HEAD(stale);
 	int n;
 
 	/* Free buffers that have not been used in the past second */
-	spin_lock_irq(&pool->lock);
 	for (n = 0; n < ARRAY_SIZE(pool->cache_list); n++) {
 		struct list_head *list = &pool->cache_list[n];
 
-		/* Most recent at head; oldest at tail */
-		list_for_each_entry_safe_reverse(node, next, list, link) {
-			if (time_before(node->age, old))
-				break;
+		if (list_empty(list))
+			continue;
+
+		if (spin_trylock_irq(&pool->lock)) {
+			struct list_head *pos;
+
+			/* Most recent at head; oldest at tail */
+			list_for_each_prev(pos, list) {
+				node = list_entry(pos, typeof(*node), link);
+				if (time_before(node->age, old))
+					break;
+
+				/* Check we are the first to claim this node */
+				if (!xchg(&node->age, 0))
+					break;
 
-			list_move(&node->link, &stale);
+				node->free = stale;
+				stale = node;
+			}
+			if (!list_is_last(pos, list))
+				__list_del_many(pos, list);
+
+			spin_unlock_irq(&pool->lock);
 		}
+
 		active |= !list_empty(list);
 	}
-	spin_unlock_irq(&pool->lock);
 
-	list_for_each_entry_safe(node, next, &stale, link)
+	while ((node = stale)) {
+		stale = stale->free;
 		node_free(node);
+	}
 
 	if (active)
 		schedule_delayed_work(&pool->work,
@@ -108,9 +125,9 @@ static void pool_retire(struct i915_active *ref)
 	/* Return this object to the shrinker pool */
 	i915_gem_object_make_purgeable(node->obj);
 
+	WRITE_ONCE(node->age, jiffies ?: 1); /* 0 reserved for active nodes */
 	spin_lock_irqsave(&pool->lock, flags);
-	node->age = jiffies;
-	list_add(&node->link, list);
+	list_add_rcu(&node->link, list);
 	spin_unlock_irqrestore(&pool->lock, flags);
 
 	schedule_delayed_work(&pool->work,
@@ -151,20 +168,30 @@ intel_gt_get_buffer_pool(struct intel_gt *gt, size_t size)
 	struct intel_gt_buffer_pool *pool = &gt->buffer_pool;
 	struct intel_gt_buffer_pool_node *node;
 	struct list_head *list;
-	unsigned long flags;
 	int ret;
 
 	size = PAGE_ALIGN(size);
 	list = bucket_for_size(pool, size);
 
-	spin_lock_irqsave(&pool->lock, flags);
-	list_for_each_entry(node, list, link) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(node, list, link) {
+		unsigned long age;
+
 		if (node->obj->base.size < size)
 			continue;
-		list_del(&node->link);
-		break;
+
+		age = READ_ONCE(node->age);
+		if (!age)
+			continue;
+
+		if (cmpxchg(&node->age, age, 0) == age) {
+			spin_lock_irq(&pool->lock);
+			list_del_rcu(&node->link);
+			spin_unlock_irq(&pool->lock);
+			break;
+		}
 	}
-	spin_unlock_irqrestore(&pool->lock, flags);
+	rcu_read_unlock();
 
 	if (&node->link == list) {
 		node = node_create(pool, size);
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool_types.h b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool_types.h
index e28bdda771ed..bcf1658c9633 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool_types.h
@@ -25,7 +25,11 @@ struct intel_gt_buffer_pool_node {
 	struct i915_active active;
 	struct drm_i915_gem_object *obj;
 	struct list_head link;
-	struct intel_gt_buffer_pool *pool;
+	union {
+		struct intel_gt_buffer_pool *pool;
+		struct intel_gt_buffer_pool_node *free;
+		struct rcu_head rcu;
+	};
 	unsigned long age;
 };
 
-- 
2.20.1