netdev - Re: [PATCH net] udp: fix busy polling

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Message-ID: <65a9407bd77fc_1caa3f29452@willemb.c.googlers.com.notmuch>
Date: Thu, 18 Jan 2024 10:15:07 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: Willem de Bruijn <willemb@...gle.com>, 
 David Ahern <dsahern@...nel.org>, 
 netdev@...r.kernel.org, 
 eric.dumazet@...il.com, 
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] udp: fix busy polling

Eric Dumazet wrote:
> Generic sk_busy_loop_end() only looks at sk->sk_receive_queue
> for presence of packets.
> 
> Problem is that for UDP sockets after blamed commit, some packets
> could be present in another queue: udp_sk(sk)->reader_queue
> 
> In some cases, a busy poller could spin until timeout expiration,
> even if some packets are available in udp_sk(sk)->reader_queue.
> 
> Fixes: 2276f58ac589 ("udp: use a separate rx queue for packet reception")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/skmsg.h |  6 ------
>  include/net/sock.h    |  5 +++++
>  net/core/sock.c       | 10 +++++++++-
>  3 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 888a4b217829fd4d6baf52f784ce35e9ad6bd0ed..e65ec3fd27998a5b82fc2c4597c575125e653056 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -505,12 +505,6 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
>  	return !!psock->saved_data_ready;
>  }
>  
> -static inline bool sk_is_udp(const struct sock *sk)
> -{
> -	return sk->sk_type == SOCK_DGRAM &&
> -	       sk->sk_protocol == IPPROTO_UDP;
> -}
> -
>  #if IS_ENABLED(CONFIG_NET_SOCK_MSG)
>  
>  #define BPF_F_STRPARSER	(1UL << 1)
> diff --git a/include/net/sock.h b/include/net/sock.h
> index a7f815c7cfdfdf1296be2967fd100efdb10cdd63..b1ceba8e179aa5cc4c90e98d353551b3a3e1ab86 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -2770,6 +2770,11 @@ static inline bool sk_is_tcp(const struct sock *sk)
>  	return sk->sk_type == SOCK_STREAM && sk->sk_protocol == IPPROTO_TCP;
>  }
>  
> +static inline bool sk_is_udp(const struct sock *sk)
> +{
> +	return sk->sk_type == SOCK_DGRAM && sk->sk_protocol == IPPROTO_UDP;
> +}
> +

Since busy polling code is protocol (family) independent, is it safe
to assume sk->sk_family == PF_INET or PF_INET6 here?

>  static inline bool sk_is_stream_unix(const struct sock *sk)
>  {
>  	return sk->sk_family == AF_UNIX && sk->sk_type == SOCK_STREAM;
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 158dbdebce6a3693deb63e557e856d9cdd7500ae..e7e2435ed28681772bf3637b96ddd9334e6a639e 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -107,6 +107,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/poll.h>
>  #include <linux/tcp.h>
> +#include <linux/udp.h>
>  #include <linux/init.h>
>  #include <linux/highmem.h>
>  #include <linux/user_namespace.h>
> @@ -4143,8 +4144,15 @@ subsys_initcall(proto_init);
>  bool sk_busy_loop_end(void *p, unsigned long start_time)
>  {
>  	struct sock *sk = p;
> +	bool packet_ready;
>  
> -	return !skb_queue_empty_lockless(&sk->sk_receive_queue) ||
> +	packet_ready = !skb_queue_empty_lockless(&sk->sk_receive_queue);
> +	if (!packet_ready && sk_is_udp(sk)) {
> +		struct sk_buff_head *reader_queue = &udp_sk(sk)->reader_queue;
> +
> +		packet_ready = !skb_queue_empty_lockless(reader_queue);
> +	}
> +	return packet_ready ||
>  	       sk_busy_loop_timeout(sk, start_time);
>  }
>  EXPORT_SYMBOL(sk_busy_loop_end);
> -- 
> 2.43.0.381.gb435a96ce8-goog
>