[PATCH 9/9] drm/i915: Trim struct_mutex hold duration for i915_gem_free_objects

[Chris Wilson]

We free objects in bulk after they wait for their RCU grace period.
Currently, we take struct_mutex and unbind all the objects. This can lead
to a long lock duration during which time those objects have their pages
unfreeable (i.e. the shrinker is prevented from reaping those pages). If
we only process a single object under the struct_mutex and then free the
pages, the number of objects locked away from the shrinker is minimal
and we allow regular clients better access to struct_mutex if they need
it.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 4c1534480ea2..a4cc66e0b0f4 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4491,13 +4491,14 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 {
 	struct drm_i915_gem_object *obj, *on;
 
-	mutex_lock(&i915->drm.struct_mutex);
 	intel_runtime_pm_get(i915);
-	llist_for_each_entry(obj, freed, freed) {
+	llist_for_each_entry_safe(obj, on, freed, freed) {
 		struct i915_vma *vma, *vn;
 
 		trace_i915_gem_object_destroy(obj);
 
+		mutex_lock(&i915->drm.struct_mutex);
+
 		GEM_BUG_ON(i915_gem_object_is_active(obj));
 		list_for_each_entry_safe(vma, vn,
 					 &obj->vma_list, obj_link) {
@@ -4520,13 +4521,8 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 			spin_unlock(&i915->mm.obj_lock);
 		}
 
-	}
-	intel_runtime_pm_put(i915);
-	mutex_unlock(&i915->drm.struct_mutex);
-
-	cond_resched();
+		mutex_unlock(&i915->drm.struct_mutex);
 
-	llist_for_each_entry_safe(obj, on, freed, freed) {
 		GEM_BUG_ON(obj->bind_count);
 		GEM_BUG_ON(obj->userfault_count);
 		GEM_BUG_ON(atomic_read(&obj->frontbuffer_bits));
@@ -4549,7 +4545,11 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 
 		kfree(obj->bit_17);
 		i915_gem_object_free(obj);
+
+		if (on)
+			cond_resched();
 	}
+	intel_runtime_pm_put(i915);
 }
 
 static void i915_gem_flush_free_objects(struct drm_i915_private *i915)
-- 
2.15.0.rc0