drm/i915: Break up long runs of freeing objects

Before freeing the next batch of objects from the worker, check if the
worker's timeslice has expired and if so, defer the next batch to the
next invocation of the worker.

Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407102552.5781-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -4349,8 +4349,11 @@ static void __i915_gem_free_work(struct work_struct *work)
 	 * unbound now.
 	 */
 
-	while ((freed = llist_del_all(&i915->mm.free_list)))
+	while ((freed = llist_del_all(&i915->mm.free_list))) {
 		__i915_gem_free_objects(i915, freed);
+		if (need_resched())
+			break;
+	}
 }
 
 static void __i915_gem_free_object_rcu(struct rcu_head *head)