client_stack(): Only stack windows once per mainloop

I was creating 2000 wiboxes in a loop (don't ask) and creating them took
forever. According to callgrind, there were about 2 million calls to
xcb_configure_window() and most (if not all) of them were from client_stack().
Awesome spent 70% of its cpu time in these client_stack() calls.

client_stack() is O(N^2) on the number of clients (it walks the list of clients
itself twice and each call to client_stack_above() walks the list too) and O(N)
on the number of wiboxes (it walks the wibox list twice). So obviously calls to
it should be rare.

This patch makes client_stack() only set a flag which is later checked. This
should reduce the number of restacks to the bare minimum. With this patch,
neither xcb_configure_window() nor anything else client_stack() related shows
up as having a lot of calls or using much cpu time.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>

client.c

@@ -344,8 +344,8 @@ client_layer_translator(client_t *c)
  * \todo It might be worth stopping to restack everyone and only stack `c'
  * relatively to the first matching in the list.
  */
-void
-client_stack()
+static void
+client_real_stack(void)
 {
     uint32_t config_win_vals[2];
     client_node_t *node, *last = *client_node_list_last(&globalconf.stack);
@@ -400,6 +400,15 @@ client_stack()
         }
 }
 
+void
+client_stack_refresh()
+{
+    if (!globalconf.client_need_stack_refresh)
+        return;
+    globalconf.client_need_stack_refresh = false;
+    client_real_stack();
+}
+
 /** Manage a new client.
  * \param w The window.
  * \param wgeom Window geometry.

client.h

@@ -55,7 +55,6 @@ DO_RCNT(client_t, client, client_delete)
 
 bool client_maybevisible(client_t *, int);
 client_t * client_getbywin(xcb_window_t);
-void client_stack(void);
 void client_ban(client_t *);
 void client_unban(client_t *);
 void client_manage(xcb_window_t, xcb_get_geometry_reply_t *, int, bool);
@@ -79,6 +78,7 @@ void client_focus(client_t *);
 void client_focus_update(client_t *);
 void client_unfocus(client_t *);
 void client_unfocus_update(client_t *);
+void client_stack_refresh(void);
 
 int luaA_client_newindex(lua_State *);
 
@@ -86,6 +86,12 @@ int luaA_client_userdata_new(lua_State *, client_t *);
 
 DO_SLIST(client_t, client, client_unref)
 
+static inline void
+client_stack(void)
+{
+    globalconf.client_need_stack_refresh = true;
+}
+
 /** Put client on top of the stack.
  * \param c The client to raise.
  */

event.h

@@ -26,12 +26,14 @@
 
 #include "wibox.h"
 #include "layout.h"
+#include "client.h"
 
 static inline int
 awesome_refresh(void)
 {
     layout_refresh();
     wibox_refresh();
+    client_stack_refresh();
     return xcb_flush(globalconf.connection);
 }
 

structs.h

@@ -381,6 +381,8 @@ struct awesome_t
     luaA_ref mousegrabber;
     /** Focused screen */
     screen_t *screen_focus;
+    /** Need to call client_stack_refresh() */
+    bool client_need_stack_refresh;
 };
 
 DO_ARRAY(const void *, void, DO_NOTHING)