net: speedup sk_wake_async()

An incoming datagram must bring into cpu cache *lot* of cache lines,
in particular : (other parts omitted (hash chains, ip route cache...))

On 32bit arches :

offsetof(struct sock, sk_rcvbuf)       =0x30    (read)
offsetof(struct sock, sk_lock)         =0x34   (rw)

offsetof(struct sock, sk_sleep)        =0x50 (read)
offsetof(struct sock, sk_rmem_alloc)   =0x64   (rw)
offsetof(struct sock, sk_receive_queue)=0x74   (rw)

offsetof(struct sock, sk_forward_alloc)=0x98   (rw)

offsetof(struct sock, sk_callback_lock)=0xcc    (rw)
offsetof(struct sock, sk_drops)        =0xd8 (read if we add dropcount support, rw if frame dropped)
offsetof(struct sock, sk_filter)       =0xf8    (read)

offsetof(struct sock, sk_socket)       =0x138 (read)

offsetof(struct sock, sk_data_ready)   =0x15c   (read)


We can avoid sk->sk_socket and socket->fasync_list referencing on sockets
with no fasync() structures. (socket->fasync_list ptr is probably already in cache
because it shares a cache line with socket->wait, ie location pointed by sk->sk_sleep)

This avoids one cache line load per incoming packet for common cases (no fasync())

We can leave (or even move in a future patch) sk->sk_socket in a cold location

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/net/socket.c b/net/socket.c
index 7565536..d53ad11 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1100,11 +1100,14 @@ static int sock_fasync(int fd, struct file *filp, int on)
 		fna->fa_next = sock->fasync_list;
 		write_lock_bh(&sk->sk_callback_lock);
 		sock->fasync_list = fna;
+		sock_set_flag(sk, SOCK_FASYNC);
 		write_unlock_bh(&sk->sk_callback_lock);
 	} else {
 		if (fa != NULL) {
 			write_lock_bh(&sk->sk_callback_lock);
 			*prev = fa->fa_next;
+			if (!sock->fasync_list)
+				sock_reset_flag(sk, SOCK_FASYNC);
 			write_unlock_bh(&sk->sk_callback_lock);
 			kfree(fa);
 		}