netdev - [PATCH net-next 2/2] udp: track the forward memory release threshold in an hot cacheline

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Message-Id: <dafe09ca2e14c4ab45f3d9db56b768e06750e382.1666173045.git.pabeni@redhat.com>
Date:   Wed, 19 Oct 2022 12:02:01 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, mptcp@...ts.linux.dev,
        David Ahern <dsahern@...nel.org>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Matthieu Baerts <matthieu.baerts@...sares.net>
Subject: [PATCH net-next 2/2] udp: track the forward memory release threshold in an hot cacheline

When the receiver process and the BH runs on different cores,
udp_rmem_release() experience a cache miss while accessing sk_rcvbuf,
as the latter shares the same cacheline with sk_forward_alloc, written
by the BH.

With this patch, UDP tracks the rcvbuf value and its update via custom
SOL_SOCKET socket options, and copies the forward memory threshold value
used by udp_rmem_release() in a different cacheline, already accessed by
the above function and uncontended.

Overall the above give a 10% peek throughput increase under UDP flood.

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/linux/udp.h |  3 +++
 net/ipv4/udp.c      | 22 +++++++++++++++++++---
 net/ipv6/udp.c      |  8 ++++++--
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/include/linux/udp.h b/include/linux/udp.h
index e96da4157d04..5cdba00a904a 100644
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -87,6 +87,9 @@ struct udp_sock {
 
 	/* This field is dirtied by udp_recvmsg() */
 	int		forward_deficit;
+
+	/* This fields follows rcvbuf value, and is touched by udp_recvmsg */
+	int		forward_threshold;
 };
 
 #define UDP_MAX_SEGMENTS	(1 << 6UL)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 8126f67d18b3..915f573587fa 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1448,7 +1448,7 @@ static void udp_rmem_release(struct sock *sk, int size, int partial,
 	if (likely(partial)) {
 		up->forward_deficit += size;
 		size = up->forward_deficit;
-		if (size < (sk->sk_rcvbuf >> 2) &&
+		if (size < READ_ONCE(up->forward_threshold) &&
 		    !skb_queue_empty(&up->reader_queue))
 			return;
 	} else {
@@ -1622,8 +1622,12 @@ static void udp_destruct_sock(struct sock *sk)
 
 int udp_init_sock(struct sock *sk)
 {
-	skb_queue_head_init(&udp_sk(sk)->reader_queue);
+	struct udp_sock *up = udp_sk(sk);
+
+	skb_queue_head_init(&up->reader_queue);
+	up->forward_threshold = sk->sk_rcvbuf >> 2;
 	sk->sk_destruct = udp_destruct_sock;
+	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
 	return 0;
 }
 
@@ -2671,6 +2675,18 @@ int udp_lib_setsockopt(struct sock *sk, int level, int optname,
 	int err = 0;
 	int is_udplite = IS_UDPLITE(sk);
 
+	if (level == SOL_SOCKET) {
+		err = sk_setsockopt(sk, level, optname, optval, optlen);
+
+		if (optname == SO_RCVBUF || optname == SO_RCVBUFFORCE) {
+			sockopt_lock_sock(sk);
+			/* paired with READ_ONCE in udp_rmem_release() */
+			WRITE_ONCE(up->forward_threshold, sk->sk_rcvbuf >> 2);
+			sockopt_release_sock(sk);
+		}
+		return err;
+	}
+
 	if (optlen < sizeof(int))
 		return -EINVAL;
 
@@ -2784,7 +2800,7 @@ EXPORT_SYMBOL(udp_lib_setsockopt);
 int udp_setsockopt(struct sock *sk, int level, int optname, sockptr_t optval,
 		   unsigned int optlen)
 {
-	if (level == SOL_UDP  ||  level == SOL_UDPLITE)
+	if (level == SOL_UDP  ||  level == SOL_UDPLITE || level == SOL_SOCKET)
 		return udp_lib_setsockopt(sk, level, optname,
 					  optval, optlen,
 					  udp_push_pending_frames);
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 8d09f0ea5b8c..1ed20bcfd7a0 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -64,8 +64,12 @@ static void udpv6_destruct_sock(struct sock *sk)
 
 int udpv6_init_sock(struct sock *sk)
 {
-	skb_queue_head_init(&udp_sk(sk)->reader_queue);
+	struct udp_sock *up = udp_sk(sk);
+
+	skb_queue_head_init(&up->reader_queue);
+	up->forward_threshold = sk->sk_rcvbuf >> 2;
 	sk->sk_destruct = udpv6_destruct_sock;
+	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
 	return 0;
 }
 
@@ -1671,7 +1675,7 @@ void udpv6_destroy_sock(struct sock *sk)
 int udpv6_setsockopt(struct sock *sk, int level, int optname, sockptr_t optval,
 		     unsigned int optlen)
 {
-	if (level == SOL_UDP  ||  level == SOL_UDPLITE)
+	if (level == SOL_UDP  ||  level == SOL_UDPLITE || level == SOL_SOCKET)
 		return udp_lib_setsockopt(sk, level, optname,
 					  optval, optlen,
 					  udp_v6_push_pending_frames);
-- 
2.37.3