Re: [PATCH v2 12/22] tcp, tcp_splice: Helpers for getting sockets from the pools

[Stefano Brivio <sbrivio@redhat.com>]

On Tue,  6 Feb 2024 12:17:24 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> We maintain pools of ready-to-connect sockets in both the original and
> (for pasta) guest namespace to reduce latency when starting new TCP
> connections.  If we exhaust those pools we have to take a higher
> latency path to get a new socket.
> 
> Currently we open-code that fallback in the places we need it.  To
> improve clarity encapsulate that into helper functions.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tcp.c        | 29 ++++++++++++++++++++++++-----
>  tcp_conn.h   |  2 +-
>  tcp_splice.c | 46 +++++++++++++++++++++++++---------------------
>  3 files changed, 50 insertions(+), 27 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index e15b932f..c06d1cc4 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1792,7 +1792,7 @@ int tcp_conn_pool_sock(int pool[])
>   *
>   * Return: socket number on success, negative code if socket creation failed
>   */
> -int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
> +static int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
>  {
>  	int s;
>  
> @@ -1811,6 +1811,27 @@ int tcp_conn_new_sock(const struct ctx *c, sa_family_t af)
>  	return s;
>  }
>  
> +/**
> + * tcp_conn_sock() - Obtain a connectable socket in the host/init namespace
> + * @c:		Execution context
> + * @af:		Address family (AF_INET or AF_INET6)
> + *
> + * Return: Socket fd on success, -errno on failure
> + */
> +int tcp_conn_sock(const struct ctx *c, sa_family_t af)
> +{
> +	int *pool = af == AF_INET6 ? init_sock_pool6 : init_sock_pool4;
> +	int s;
> +
> +	if ((s = tcp_conn_pool_sock(pool)) >= 0)
> +		return s;
> +
> +	/* If the pool is empty we just open a new one without refilling the
> +	 * pool to keep latency down.
> +	 */
> +	return tcp_conn_new_sock(c, af);
> +}
> +
>  /**
>   * tcp_conn_tap_mss() - Get MSS value advertised by tap/guest
>   * @conn:	Connection pointer
> @@ -1909,7 +1930,6 @@ static void tcp_conn_from_tap(struct ctx *c, sa_family_t af,
>  			      const struct tcphdr *th, const char *opts,
>  			      size_t optlen, const struct timespec *now)
>  {
> -	int *pool = af == AF_INET6 ? init_sock_pool6 : init_sock_pool4;
>  	struct sockaddr_in addr4 = {
>  		.sin_family = AF_INET,
>  		.sin_port = th->dest,
> @@ -1931,9 +1951,8 @@ static void tcp_conn_from_tap(struct ctx *c, sa_family_t af,
>  	if (!(flow = flow_alloc()))
>  		return;
>  
> -	if ((s = tcp_conn_pool_sock(pool)) < 0)
> -		if ((s = tcp_conn_new_sock(c, af)) < 0)
> -			goto cancel;
> +	if ((s = tcp_conn_sock(c, af)) < 0)
> +		goto cancel;
>  
>  	if (!c->no_map_gw) {
>  		if (af == AF_INET && IN4_ARE_ADDR_EQUAL(daddr, &c->ip4.gw))
> diff --git a/tcp_conn.h b/tcp_conn.h
> index 20c7cb8b..e55edafe 100644
> --- a/tcp_conn.h
> +++ b/tcp_conn.h
> @@ -159,7 +159,7 @@ bool tcp_flow_defer(union flow *flow);
>  bool tcp_splice_flow_defer(union flow *flow);
>  void tcp_splice_timer(const struct ctx *c, union flow *flow);
>  int tcp_conn_pool_sock(int pool[]);
> -int tcp_conn_new_sock(const struct ctx *c, sa_family_t af);
> +int tcp_conn_sock(const struct ctx *c, sa_family_t af);
>  void tcp_sock_refill_pool(const struct ctx *c, int pool[], sa_family_t af);
>  void tcp_splice_refill(const struct ctx *c);
>  
> diff --git a/tcp_splice.c b/tcp_splice.c
> index 576fe9be..609f3242 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -90,7 +90,7 @@ static const char *tcp_splice_flag_str[] __attribute((__unused__)) = {
>  };
>  
>  /* Forward declaration */
> -static int tcp_sock_refill_ns(void *arg);
> +static int tcp_conn_sock_ns(const struct ctx *c, sa_family_t af);
>  
>  /**
>   * tcp_splice_conn_epoll_events() - epoll events masks for given state
> @@ -380,36 +380,19 @@ static int tcp_splice_connect(const struct ctx *c, struct tcp_splice_conn *conn,
>  static int tcp_splice_new(const struct ctx *c, struct tcp_splice_conn *conn,
>  			  in_port_t port, uint8_t pif)
>  {
> +	sa_family_t af = CONN_V6(conn) ? AF_INET6 : AF_INET;
>  	int s = -1;
>  
> -	/* If the pool is empty we take slightly different approaches
> -	 * for init or ns sockets.  For init sockets we just open a
> -	 * new one without refilling the pool to keep latency down.
> -	 * For ns sockets, we're going to incur the latency of
> -	 * entering the ns anyway, so we might as well refill the
> -	 * pool.
> -	 */
>  	if (pif == PIF_SPLICE) {
> -		int *p = CONN_V6(conn) ? init_sock_pool6 : init_sock_pool4;
> -		sa_family_t af = CONN_V6(conn) ? AF_INET6 : AF_INET;
> -
>  		port += c->tcp.fwd_out.delta[port];
>  
> -		s = tcp_conn_pool_sock(p);
> -		if (s < 0)
> -			s = tcp_conn_new_sock(c, af);
> +		s = tcp_conn_sock(c, af);
>  	} else {
> -		int *p = CONN_V6(conn) ? ns_sock_pool6 : ns_sock_pool4;
> -
>  		ASSERT(pif == PIF_HOST);
>  
>  		port += c->tcp.fwd_in.delta[port];
>  
> -		/* If pool is empty, refill it first */
> -		if (p[TCP_SOCK_POOL_SIZE-1] < 0)
> -			NS_CALL(tcp_sock_refill_ns, c);
> -
> -		s = tcp_conn_pool_sock(p);
> +		s = tcp_conn_sock_ns(c, af);
>  	}
>  
>  	if (s < 0) {
> @@ -709,6 +692,27 @@ static int tcp_sock_refill_ns(void *arg)
>  	return 0;
>  }
>  
> +/**
> + * tcp_conn_sock_ns() - Obtain a connectable socket in the namespace
> + * @c:		Execution context
> + * @af:		Address family (AF_INET or AF_INET6)
> + *
> + * Return: Socket fd in the namespace on success, -errno on failure
> + */
> +static int tcp_conn_sock_ns(const struct ctx *c, sa_family_t af)
> +{
> +	int *p = af == AF_INET6 ? ns_sock_pool6 : ns_sock_pool4;
> +
> +	/* If the pool is empty we have to incur the latency of entering the ns.
> +	 * Therefore, we might as well refill the whole pool while we're at it,
> +	 * which differs from tcp_conn_sock().
> +	 */
> +	if (p[TCP_SOCK_POOL_SIZE-1] < 0)

Nit, for consistency (but yes, it was already like this):

	if (p[TCP_SOCK_POOL_SIZE - 1] < 0)

-- 
Stefano