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 <[email protected]> Signed-off-by: David S. Miller <[email protected]>
kyagi · Dec 10, 2016 · 6b229cf · 6b229cf
1 parent c84d949
commit 6b229cf
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diff --git 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
@@ -1177,8 +1177,20 @@ int udp_sendpage(struct sock *sk, struct page *page, int offset,
 /* 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);