kobject: factorize skb setup in kobject_uevent_net_broadcast()

We can build one skb and let it be cloned in netlink.

This is much faster, and use less memory (all clones will
share the same skb->head)

Tested:

time perf record (for f in `seq 1 3000` ; do ip netns add tast$f; done)
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 4.110 MB perf.data (~179584 samples) ]

real    0m24.227s # instead of 0m52.554s
user    0m0.329s
sys 0m23.753s # instead of 0m51.375s

    14.77%       ip  [kernel.kallsyms]  [k] __ip6addrlbl_add
    14.56%       ip  [kernel.kallsyms]  [k] netlink_broadcast_filtered
    11.65%       ip  [kernel.kallsyms]  [k] netlink_has_listeners
     6.19%       ip  [kernel.kallsyms]  [k] _raw_spin_lock_irqsave
     5.66%       ip  [kernel.kallsyms]  [k] kobject_uevent_env
     4.97%       ip  [kernel.kallsyms]  [k] memset_erms
     4.67%       ip  [kernel.kallsyms]  [k] refcount_sub_and_test
     4.41%       ip  [kernel.kallsyms]  [k] _raw_read_lock
     3.59%       ip  [kernel.kallsyms]  [k] refcount_inc_not_zero
     3.13%       ip  [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore
     1.55%       ip  [kernel.kallsyms]  [k] __wake_up
     1.20%       ip  [kernel.kallsyms]  [k] strlen
     1.03%       ip  [kernel.kallsyms]  [k] __wake_up_common
     0.93%       ip  [kernel.kallsyms]  [k] consume_skb
     0.92%       ip  [kernel.kallsyms]  [k] netlink_trim
     0.87%       ip  [kernel.kallsyms]  [k] insert_header
     0.63%       ip  [kernel.kallsyms]  [k] unmap_page_range
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/kobject_uevent.c

@@ -301,23 +301,26 @@ static int kobject_uevent_net_broadcast(struct kobject *kobj,
 {
 	int retval = 0;
 #if defined(CONFIG_NET)
+	struct sk_buff *skb = NULL;
 	struct uevent_sock *ue_sk;
 
 	/* send netlink message */
 	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
 		struct sock *uevent_sock = ue_sk->sk;
-		struct sk_buff *skb;
-		size_t len;
 
 		if (!netlink_has_listeners(uevent_sock, 1))
 			continue;
 
+		if (!skb) {
 			/* allocate message with the maximum possible size */
-		len = strlen(action_string) + strlen(devpath) + 2;
-		skb = alloc_skb(len + env->buflen, GFP_KERNEL);
-		if (skb) {
+			size_t len = strlen(action_string) + strlen(devpath) + 2;
 			char *scratch;
 
+			retval = -ENOMEM;
+			skb = alloc_skb(len + env->buflen, GFP_KERNEL);
+			if (!skb)
+				continue;
+
 			/* add header */
 			scratch = skb_put(skb, len);
 			sprintf(scratch, "%s@%s", action_string, devpath);
@@ -325,16 +328,17 @@ static int kobject_uevent_net_broadcast(struct kobject *kobj,
 			skb_put_data(skb, env->buf, env->buflen);
 
 			NETLINK_CB(skb).dst_group = 1;
-			retval = netlink_broadcast_filtered(uevent_sock, skb,
+		}
+
+		retval = netlink_broadcast_filtered(uevent_sock, skb_get(skb),
 						    0, 1, GFP_KERNEL,
 						    kobj_bcast_filter,
 						    kobj);
 		/* ENOBUFS should be handled in userspace */
 		if (retval == -ENOBUFS || retval == -ESRCH)
 			retval = 0;
-		} else
-			retval = -ENOMEM;
 	}
+	consume_skb(skb);
 #endif
 	return retval;
 }