[PATCH] per-task delay accounting: avoid send without listeners

Don't send taskstats (per-pid or per-tgid) on thread exit when no one is
listening for such data.

Currently the taskstats interface allocates a structure, fills it in and
calls netlink to send out per-pid and per-tgid stats regardless of whether
a userspace listener for the data exists (netlink layer would check for
that and avoid the multicast).

As a result of this patch, the check for the no-listener case is performed
early, avoiding the redundant allocation and filling up of the taskstats
structures.
Signed-off-by: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Jay Lan <jlan@engr.sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/taskstats_kern.h

@@ -9,6 +9,7 @@
 
 #include <linux/taskstats.h>
 #include <linux/sched.h>
+#include <net/genetlink.h>
 
 enum {
 	TASKSTATS_MSG_UNICAST,		/* send data only to requester */
@@ -19,8 +20,18 @@ enum {
 extern kmem_cache_t *taskstats_cache;
 extern struct mutex taskstats_exit_mutex;
 
+static inline int taskstats_has_listeners(void)
+{
+	if (!genl_sock)
+		return 0;
+	return netlink_has_listeners(genl_sock, TASKSTATS_LISTEN_GROUP);
+}
+
+
 static inline void taskstats_exit_alloc(struct taskstats **ptidstats)
 {
+	*ptidstats = NULL;
+	if (taskstats_has_listeners())
 		*ptidstats = kmem_cache_zalloc(taskstats_cache, SLAB_KERNEL);
 }