[Intel-gfx] [PATCH v2] drm/i915: Only free the oldest stale object before a fresh allocation

[Chris Wilson]

Inspired by Tvrtko's critique of the reaping of the stale contexts
before allocating a new one, also limit the freed object reaping to the
oldest stale object before allocating a fresh object. Unlike contexts,
objects may have radically different sizes of backing storage, but
similar to contexts, whilst we want to prevent starvation due to
excessive freed lists, we also want do not want to delay fresh
allocations for too long. Only freeing the oldest on the freed object
list before each allocation is a reasonable compromise.

v2: Only a single consumer of llist_del_first() is allowed (although
multiple llist_add are still allowed in parallel). Unlike
i915_gem_context, i915_gem_flush_free_objects() is itself not serialized
and so we need to add our own spinlock. Otherwise KASAN eventually spots
a use-after-free for the race on *first->next.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com> #v1
---
 drivers/gpu/drm/i915/i915_drv.h |  1 +
 drivers/gpu/drm/i915/i915_gem.c | 14 ++++++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 667fb5c44483..e1c2bc66e0fd 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1447,6 +1447,7 @@ struct i915_gem_mm {
 	 */
 	struct llist_head free_list;
 	struct work_struct free_work;
+	spinlock_t free_lock;
 
 	/** Usable portion of the GTT for GEM */
 	dma_addr_t stolen_base; /* limited to low memory (32-bit) */
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 2c2907fef612..d57603f64e96 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4542,9 +4542,18 @@ static void i915_gem_flush_free_objects(struct drm_i915_private *i915)
 {
 	struct llist_node *freed;
 
-	freed = llist_del_all(&i915->mm.free_list);
-	if (unlikely(freed))
+	/* Free the oldest, most stale object to keep the free_list short */
+	freed = NULL;
+	if (!llist_empty(&i915->mm.free_list)) { /* quick test for hotpath */
+		/* Only one consumer of llist_del_first() allowed */
+		spin_lock(&i915->mm.free_lock);
+		freed = llist_del_first(&i915->mm.free_list);
+		spin_unlock(&i915->mm.free_lock);
+	}
+	if (unlikely(freed)) {
+		freed->next = NULL;
 		__i915_gem_free_objects(i915, freed);
+	}
 }
 
 static void __i915_gem_free_work(struct work_struct *work)
@@ -5026,6 +5035,7 @@ i915_gem_load_init(struct drm_i915_private *dev_priv)
 	INIT_WORK(&dev_priv->mm.free_work, __i915_gem_free_work);
 
 	spin_lock_init(&dev_priv->mm.obj_lock);
+	spin_lock_init(&dev_priv->mm.free_lock);
 	init_llist_head(&dev_priv->mm.free_list);
 	INIT_LIST_HEAD(&dev_priv->mm.unbound_list);
 	INIT_LIST_HEAD(&dev_priv->mm.bound_list);
-- 
2.13.3