udp: add batching to udp_rmem_release()

If udp_recvmsg() constantly releases sk_rmem_alloc
for every read packet, it gives opportunity for
producers to immediately grab spinlocks and desperatly
try adding another packet, causing false sharing.

We can add a simple heuristic to give the signal
by batches of ~25 % of the queue capacity.

This patch considerably increases performance under
flood by about 50 %, since the thread draining the queue
is no longer slowed by false sharing.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/udp.h b/include/linux/udp.h
index d1fd8cd..c0f5308 100644 (file)
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -79,6 +79,9 @@ struct udp_sock {
        int                     (*gro_complete)(struct sock *sk,
                                                struct sk_buff *skb,
                                                int nhoff);
+
+       /* This field is dirtied by udp_recvmsg() */
+       int             forward_deficit;
 };
 
 static inline struct udp_sock *udp_sk(const struct sock *sk)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index c608334..5a38faa 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1177,8 +1177,20 @@ out:
 /* fully reclaim rmem/fwd memory allocated for skb */
 static void udp_rmem_release(struct sock *sk, int size, int partial)
 {
+       struct udp_sock *up = udp_sk(sk);
        int amt;
 
+       if (likely(partial)) {
+               up->forward_deficit += size;
+               size = up->forward_deficit;
+               if (size < (sk->sk_rcvbuf >> 2) &&
+                   !skb_queue_empty(&sk->sk_receive_queue))
+                       return;
+       } else {
+               size += up->forward_deficit;
+       }
+       up->forward_deficit = 0;
+
        atomic_sub(size, &sk->sk_rmem_alloc);
        sk->sk_forward_alloc += size;
        amt = (sk->sk_forward_alloc - partial) & ~(SK_MEM_QUANTUM - 1);