[PATCH net-next v2 0/2] udp: avoid false sharing on receive

[PATCH net-next v2 0/2] udp: avoid false sharing on receive

[Paolo Abeni]

Under high UDP load, the BH processing and the user-space receiver can
run on different cores.

The UDP implementation does a lot of effort to avoid false sharing in
the receive path, but recent changes to the struct sock layout moved
the sk_forward_alloc and the sk_rcvbuf fields on the same cacheline:

        /* --- cacheline 4 boundary (256 bytes) --- */
                struct sk_buff *   tail;
        } sk_backlog;
        int                        sk_forward_alloc;
        unsigned int               sk_reserved_mem;
        unsigned int               sk_ll_usec;
        unsigned int               sk_napi_id;
        int                        sk_rcvbuf;

sk_forward_alloc is updated by the BH, while sk_rcvbuf is accessed by
udp_recvmsg(), causing false sharing.

A possible solution would be to re-order the struct sock fields to avoid
the false sharing. Such change is subject to being invalidated by future
changes and could have negative side effects on other workload.

Instead this series uses a different approach, touching only the UDP
socket layout. 

The first patch generalizes the custom setsockopt infrastructure, to
allow UDP tracking the buffer size, and the second patch addresses the
issue, copying the relevant buffer information into an already hot
cacheline.

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

v1 -> v2:
 - introduce and use a common helper to initialize the UDP v4/v6 sockets
   (Kuniyuki)

Paolo Abeni (2):
  net: introduce and use custom sockopt socket flag
  udp: track the forward memory release threshold in an hot cacheline

 include/linux/net.h  |  1 +
 include/linux/udp.h  |  3 +++
 include/net/udp.h    |  9 +++++++++
 net/ipv4/udp.c       | 18 +++++++++++++++---
 net/ipv6/udp.c       |  4 ++--
 net/mptcp/protocol.c |  4 ++++
 net/socket.c         |  8 +-------
 7 files changed, 35 insertions(+), 12 deletions(-)

-- 
2.37.3

[PATCH net-next v2 1/2] net: introduce and use custom sockopt socket flag

[Paolo Abeni]

We will soon introduce custom setsockopt for UDP sockets, too.
Instead of doing even more complex arbitrary checks inside
sock_use_custom_sol_socket(), add a new socket flag and set it
for the relevant socket types (currently only MPTCP).

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 include/linux/net.h  | 1 +
 net/mptcp/protocol.c | 4 ++++
 net/socket.c         | 8 +-------
 3 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/net.h b/include/linux/net.h
index 711c3593c3b8..59350fd85823 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -41,6 +41,7 @@ struct net;
 #define SOCK_NOSPACE		2
 #define SOCK_PASSCRED		3
 #define SOCK_PASSSEC		4
+#define SOCK_CUSTOM_SOCKOPT	5
 
 #ifndef ARCH_HAS_SOCKET_TYPES
 /**
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index f599ad44ed24..0448a5c3da3c 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2708,6 +2708,8 @@ static int mptcp_init_sock(struct sock *sk)
 	if (ret)
 		return ret;
 
+	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
+
 	/* fetch the ca name; do it outside __mptcp_init_sock(), so that clone will
 	 * propagate the correct value
 	 */
@@ -3684,6 +3686,8 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 		struct mptcp_subflow_context *subflow;
 		struct sock *newsk = newsock->sk;
 
+		set_bit(SOCK_CUSTOM_SOCKOPT, &newsock->flags);
+
 		lock_sock(newsk);
 
 		/* PM/worker can now acquire the first subflow socket
diff --git a/net/socket.c b/net/socket.c
index 00da9ce3dba0..55c5d536e5f6 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2199,13 +2199,7 @@ SYSCALL_DEFINE4(recv, int, fd, void __user *, ubuf, size_t, size,
 
 static bool sock_use_custom_sol_socket(const struct socket *sock)
 {
-	const struct sock *sk = sock->sk;
-
-	/* Use sock->ops->setsockopt() for MPTCP */
-	return IS_ENABLED(CONFIG_MPTCP) &&
-	       sk->sk_protocol == IPPROTO_MPTCP &&
-	       sk->sk_type == SOCK_STREAM &&
-	       (sk->sk_family == AF_INET || sk->sk_family == AF_INET6);
+	return test_bit(SOCK_CUSTOM_SOCKOPT, &sock->flags);
 }
 
 /*
-- 
2.37.3

Re: [PATCH net-next v2 1/2] net: introduce and use custom sockopt socket flag

[Eric Dumazet]

On Thu, Oct 20, 2022 at 10:49 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> We will soon introduce custom setsockopt for UDP sockets, too.
> Instead of doing even more complex arbitrary checks inside
> sock_use_custom_sol_socket(), add a new socket flag and set it
> for the relevant socket types (currently only MPTCP).
>
> Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net-next v2 1/2] net: introduce and use custom sockopt socket flag

[Kuniyuki Iwashima]

From:   Paolo Abeni <pabeni@redhat.com>
Date:   Thu, 20 Oct 2022 19:48:51 +0200
> We will soon introduce custom setsockopt for UDP sockets, too.
> Instead of doing even more complex arbitrary checks inside
> sock_use_custom_sol_socket(), add a new socket flag and set it
> for the relevant socket types (currently only MPTCP).
> 
> Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Acked-by: Kuniyuki Iwashima <kuniyu@amazon.com>


> ---
>  include/linux/net.h  | 1 +
>  net/mptcp/protocol.c | 4 ++++
>  net/socket.c         | 8 +-------
>  3 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 711c3593c3b8..59350fd85823 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -41,6 +41,7 @@ struct net;
>  #define SOCK_NOSPACE		2
>  #define SOCK_PASSCRED		3
>  #define SOCK_PASSSEC		4
> +#define SOCK_CUSTOM_SOCKOPT	5
>  
>  #ifndef ARCH_HAS_SOCKET_TYPES
>  /**
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index f599ad44ed24..0448a5c3da3c 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -2708,6 +2708,8 @@ static int mptcp_init_sock(struct sock *sk)
>  	if (ret)
>  		return ret;
>  
> +	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
> +
>  	/* fetch the ca name; do it outside __mptcp_init_sock(), so that clone will
>  	 * propagate the correct value
>  	 */
> @@ -3684,6 +3686,8 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
>  		struct mptcp_subflow_context *subflow;
>  		struct sock *newsk = newsock->sk;
>  
> +		set_bit(SOCK_CUSTOM_SOCKOPT, &newsock->flags);
> +
>  		lock_sock(newsk);
>  
>  		/* PM/worker can now acquire the first subflow socket
> diff --git a/net/socket.c b/net/socket.c
> index 00da9ce3dba0..55c5d536e5f6 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2199,13 +2199,7 @@ SYSCALL_DEFINE4(recv, int, fd, void __user *, ubuf, size_t, size,
>  
>  static bool sock_use_custom_sol_socket(const struct socket *sock)
>  {
> -	const struct sock *sk = sock->sk;
> -
> -	/* Use sock->ops->setsockopt() for MPTCP */
> -	return IS_ENABLED(CONFIG_MPTCP) &&
> -	       sk->sk_protocol == IPPROTO_MPTCP &&
> -	       sk->sk_type == SOCK_STREAM &&
> -	       (sk->sk_family == AF_INET || sk->sk_family == AF_INET6);
> +	return test_bit(SOCK_CUSTOM_SOCKOPT, &sock->flags);
>  }
>  
>  /*
> -- 
> 2.37.3

[PATCH net-next v2 2/2] udp: track the forward memory release threshold in an hot cacheline

[Paolo Abeni]

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.

Since the UDP socket init operation grown a bit, factor out the common
code between v4 and v6 in a shared helper.

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

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v1 -> v2:
 - factor out common init helper for udp && udpv6 sock (Kuniyuki)
---
 include/linux/udp.h |  3 +++
 include/net/udp.h   |  9 +++++++++
 net/ipv4/udp.c      | 18 +++++++++++++++---
 net/ipv6/udp.c      |  4 ++--
 4 files changed, 29 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/include/net/udp.h b/include/net/udp.h
index fee053bcd17c..de4b528522bb 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -174,6 +174,15 @@ INDIRECT_CALLABLE_DECLARE(int udpv6_rcv(struct sk_buff *));
 struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 				  netdev_features_t features, bool is_ipv6);
 
+static inline void udp_lib_init_sock(struct sock *sk)
+{
+	struct udp_sock *up = udp_sk(sk);
+
+	skb_queue_head_init(&up->reader_queue);
+	up->forward_threshold = sk->sk_rcvbuf >> 2;
+	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
+}
+
 /* hash routines shared between UDPv4/6 and UDP-Litev4/6 */
 static inline int udp_lib_hash(struct sock *sk)
 {
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 8126f67d18b3..e361ad93999e 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 {
@@ -1622,7 +1622,7 @@ static void udp_destruct_sock(struct sock *sk)
 
 int udp_init_sock(struct sock *sk)
 {
-	skb_queue_head_init(&udp_sk(sk)->reader_queue);
+	udp_lib_init_sock(sk);
 	sk->sk_destruct = udp_destruct_sock;
 	return 0;
 }
@@ -2671,6 +2671,18 @@ 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 +2796,7 @@ 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..b0bc4e27ec2f 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -64,7 +64,7 @@ static void udpv6_destruct_sock(struct sock *sk)
 
 int udpv6_init_sock(struct sock *sk)
 {
-	skb_queue_head_init(&udp_sk(sk)->reader_queue);
+	udp_lib_init_sock(sk);
 	sk->sk_destruct = udpv6_destruct_sock;
 	return 0;
 }
@@ -1671,7 +1671,7 @@ 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

Re: [PATCH net-next v2 2/2] udp: track the forward memory release threshold in an hot cacheline

[Eric Dumazet]

On Thu, Oct 20, 2022 at 10:49 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> 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.
>
> Since the UDP socket init operation grown a bit, factor out the common
> code between v4 and v6 in a shared helper.
>
> Overall the above give a 10% peek throughput increase under UDP flood.
>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>


Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net-next v2 2/2] udp: track the forward memory release threshold in an hot cacheline

[Kuniyuki Iwashima]

From:   Paolo Abeni <pabeni@redhat.com>
Date:   Thu, 20 Oct 2022 19:48:52 +0200
> 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.
> 
> Since the UDP socket init operation grown a bit, factor out the common
> code between v4 and v6 in a shared helper.
> 
> Overall the above give a 10% peek throughput increase under UDP flood.
> 
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Acked-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Thank you!


> ---
> v1 -> v2:
>  - factor out common init helper for udp && udpv6 sock (Kuniyuki)
> ---
>  include/linux/udp.h |  3 +++
>  include/net/udp.h   |  9 +++++++++
>  net/ipv4/udp.c      | 18 +++++++++++++++---
>  net/ipv6/udp.c      |  4 ++--
>  4 files changed, 29 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/include/net/udp.h b/include/net/udp.h
> index fee053bcd17c..de4b528522bb 100644
> --- a/include/net/udp.h
> +++ b/include/net/udp.h
> @@ -174,6 +174,15 @@ INDIRECT_CALLABLE_DECLARE(int udpv6_rcv(struct sk_buff *));
>  struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>  				  netdev_features_t features, bool is_ipv6);
>  
> +static inline void udp_lib_init_sock(struct sock *sk)
> +{
> +	struct udp_sock *up = udp_sk(sk);
> +
> +	skb_queue_head_init(&up->reader_queue);
> +	up->forward_threshold = sk->sk_rcvbuf >> 2;
> +	set_bit(SOCK_CUSTOM_SOCKOPT, &sk->sk_socket->flags);
> +}
> +
>  /* hash routines shared between UDPv4/6 and UDP-Litev4/6 */
>  static inline int udp_lib_hash(struct sock *sk)
>  {
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 8126f67d18b3..e361ad93999e 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 {
> @@ -1622,7 +1622,7 @@ static void udp_destruct_sock(struct sock *sk)
>  
>  int udp_init_sock(struct sock *sk)
>  {
> -	skb_queue_head_init(&udp_sk(sk)->reader_queue);
> +	udp_lib_init_sock(sk);
>  	sk->sk_destruct = udp_destruct_sock;
>  	return 0;
>  }
> @@ -2671,6 +2671,18 @@ 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 +2796,7 @@ 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..b0bc4e27ec2f 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -64,7 +64,7 @@ static void udpv6_destruct_sock(struct sock *sk)
>  
>  int udpv6_init_sock(struct sock *sk)
>  {
> -	skb_queue_head_init(&udp_sk(sk)->reader_queue);
> +	udp_lib_init_sock(sk);
>  	sk->sk_destruct = udpv6_destruct_sock;
>  	return 0;
>  }
> @@ -1671,7 +1671,7 @@ 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

Re: [PATCH net-next v2 0/2] udp: avoid false sharing on receive

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (master)
by David S. Miller <davem@davemloft.net>:

On Thu, 20 Oct 2022 19:48:50 +0200 you wrote:
> Under high UDP load, the BH processing and the user-space receiver can
> run on different cores.
> 
> The UDP implementation does a lot of effort to avoid false sharing in
> the receive path, but recent changes to the struct sock layout moved
> the sk_forward_alloc and the sk_rcvbuf fields on the same cacheline:
> 
> [...]

Here is the summary with links:
  - [net-next,v2,1/2] net: introduce and use custom sockopt socket flag
    https://git.kernel.org/netdev/net-next/c/a5ef058dc4d9
  - [net-next,v2,2/2] udp: track the forward memory release threshold in an hot cacheline
    https://git.kernel.org/netdev/net-next/c/8a3854c7b8e4

You are awesome, thank you!
-- 
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