drm/amdkfd: Asynchronously free events

The synchronize_rcu call in destroy_events can take several ms, which
noticeably slows down applications destroying many events. Use kfree_rcu
to free the event structure asynchronously and eliminate the
synchronize_rcu call in the user thread.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_events.c

@@ -262,8 +262,7 @@ static void destroy_event(struct kfd_process *p, struct kfd_event *ev)
 		p->signal_event_count--;
 
 	idr_remove(&p->event_idr, ev->event_id);
-	synchronize_rcu();
-	kfree(ev);
+	kfree_rcu(ev, rcu);
 }
 
 static void destroy_events(struct kfd_process *p)

drivers/gpu/drm/amd/amdkfd/kfd_events.h

@@ -70,6 +70,8 @@ struct kfd_event {
 		struct kfd_hsa_memory_exception_data memory_exception_data;
 		struct kfd_hsa_hw_exception_data hw_exception_data;
 	};
+
+	struct rcu_head rcu; /* for asynchronous kfree_rcu */
 };
 
 #define KFD_EVENT_TIMEOUT_IMMEDIATE 0