Re: [RFC net-next] tcp: add support for SO_PEEK_OFF

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  1 Feb 2024 16:32:01 -0500
jmaloy@redhat.com wrote:

> From: Jon Maloy <jmaloy@redhat.com>
> 
> When reading received messages from a socket with MSG_PEEK, we may want
> to read the contents with an offset, like we can do with pread/preadv()
> when reading files. Currently, it is not possible to do that.
> 
> In this commit, we add support for the SO_PEEK_OFF socket option for TCP,
> in a similar way it is done for Unix Domain sockets.
> 
> In the iperf3 log examples shown below, we can observe a throughput
> improvement of 15-20 % in the direction host->namespace when using the
> protocol splicer 'pasta' (https://passt.top).
> This is a consistent result.
> 
> pasta(1) and passt(1) implement user-mode networking for network
> namespaces (containers) and virtual machines by means of a translation
> layer between Layer-2 network interface and native Layer-4 sockets
> (TCP, UDP, ICMP/ICMPv6 echo).
> 
> Received, pending TCP data to the container/guest is kept in kernel
> buffers until acknowledged, so the tool routinely needs to fetch new
> data from socket, skipping data that was already sent.
> 
> At the moment this is implemented using a dummy buffer passed to
> recvmsg(). With this change, we don't need a dummy buffer and the
> related buffer copy (copy_to_user()) anymore.
> 
> passt and pasta are supported in KubeVirt and libvirt/qemu.
> 
> jmaloy@freyr:~/passt$ perf record -g ./pasta --config-net -f
> SO_PEEK_OFF not supported by kernel.
> 
> jmaloy@freyr:~/passt# iperf3 -s
> -----------------------------------------------------------
> Server listening on 5201 (test #1)
> -----------------------------------------------------------
> Accepted connection from 192.168.122.1, port 44822
> [  5] local 192.168.122.180 port 5201 connected to 192.168.122.1 port 44832
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-1.00   sec  1.02 GBytes  8.78 Gbits/sec
> [  5]   1.00-2.00   sec  1.06 GBytes  9.08 Gbits/sec
> [  5]   2.00-3.00   sec  1.07 GBytes  9.15 Gbits/sec
> [  5]   3.00-4.00   sec  1.10 GBytes  9.46 Gbits/sec
> [  5]   4.00-5.00   sec  1.03 GBytes  8.85 Gbits/sec
> [  5]   5.00-6.00   sec  1.10 GBytes  9.44 Gbits/sec
> [  5]   6.00-7.00   sec  1.11 GBytes  9.56 Gbits/sec
> [  5]   7.00-8.00   sec  1.07 GBytes  9.20 Gbits/sec
> [  5]   8.00-9.00   sec   667 MBytes  5.59 Gbits/sec
> [  5]   9.00-10.00  sec  1.03 GBytes  8.83 Gbits/sec
> [  5]  10.00-10.04  sec  30.1 MBytes  6.36 Gbits/sec
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-10.04  sec  10.3 GBytes  8.78 Gbits/sec   receiver
> -----------------------------------------------------------
> Server listening on 5201 (test #2)
> -----------------------------------------------------------
> ^Ciperf3: interrupt - the server has terminated
> jmaloy@freyr:~/passt#
> logout
> [ perf record: Woken up 23 times to write data ]
> [ perf record: Captured and wrote 5.696 MB perf.data (35580 samples) ]
> jmaloy@freyr:~/passt$
> 
> jmaloy@freyr:~/passt$ perf record -g ./pasta --config-net -f
> SO_PEEK_OFF supported by kernel.
> 
> jmaloy@freyr:~/passt# iperf3 -s
> -----------------------------------------------------------
> Server listening on 5201 (test #1)
> -----------------------------------------------------------
> Accepted connection from 192.168.122.1, port 52084
> [  5] local 192.168.122.180 port 5201 connected to 192.168.122.1 port 52098
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-1.00   sec  1.32 GBytes  11.3 Gbits/sec
> [  5]   1.00-2.00   sec  1.19 GBytes  10.2 Gbits/sec
> [  5]   2.00-3.00   sec  1.26 GBytes  10.8 Gbits/sec
> [  5]   3.00-4.00   sec  1.36 GBytes  11.7 Gbits/sec
> [  5]   4.00-5.00   sec  1.33 GBytes  11.4 Gbits/sec
> [  5]   5.00-6.00   sec  1.21 GBytes  10.4 Gbits/sec
> [  5]   6.00-7.00   sec  1.31 GBytes  11.2 Gbits/sec
> [  5]   7.00-8.00   sec  1.25 GBytes  10.7 Gbits/sec
> [  5]   8.00-9.00   sec  1.33 GBytes  11.5 Gbits/sec
> [  5]   9.00-10.00  sec  1.24 GBytes  10.7 Gbits/sec
> [  5]  10.00-10.04  sec  56.0 MBytes  12.1 Gbits/sec
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-10.04  sec  12.9 GBytes  11.0 Gbits/sec                  receiver
> -----------------------------------------------------------
> Server listening on 5201 (test #2)
> -----------------------------------------------------------
> ^Ciperf3: interrupt - the server has terminated
> logout
> [ perf record: Woken up 20 times to write data ]
> [ perf record: Captured and wrote 5.040 MB perf.data (33411 samples) ]
> jmaloy@freyr:~/passt$
> 
> The perf record confirms this result. Below, we can observe that the
> CPU spends significantly less time in the function ____sys_recvmsg()
> when we have offset support.
> 
> Without offset support:
> ----------------------
> jmaloy@freyr:~/passt$ perf report -q --symbol-filter=do_syscall_64 -p ____sys_recvmsg -x --stdio -i  perf.data | head -1
>     46.32%     0.00%  passt.avx2  [kernel.vmlinux]  [k] do_syscall_64  ____sys_recvmsg
> 
> With offset support:
> ----------------------
> jmaloy@freyr:~/passt$ perf report -q --symbol-filter=do_syscall_64 -p ____sys_recvmsg -x --stdio -i  perf.data | head -1
>    28.12%     0.00%  passt.avx2  [kernel.vmlinux]  [k] do_syscall_64  ____sys_recvmsg
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

I guess this was Suggested-by: Paolo Abeni <pabeni@redhat.com> ? :)

> ---
>  include/net/tcp.h  |  1 +
>  net/ipv4/af_inet.c |  1 +
>  net/ipv4/tcp.c     | 25 +++++++++++++++++++------
>  3 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 87f0e6c2e1f2..7eca7f2ac102 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -357,6 +357,7 @@ void tcp_twsk_purge(struct list_head *net_exit_list, int family);
>  ssize_t tcp_splice_read(struct socket *sk, loff_t *ppos,
>  			struct pipe_inode_info *pipe, size_t len,
>  			unsigned int flags);
> +int tcp_set_peek_offset(struct sock *sk, int val);
>  struct sk_buff *tcp_stream_alloc_skb(struct sock *sk, gfp_t gfp,
>  				     bool force_schedule);
>  
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index fb81de10d332..7a8b3a91257f 100644
> --- a/net/ipv4/af_inet.c
> +++ b/net/ipv4/af_inet.c
> @@ -1068,6 +1068,7 @@ const struct proto_ops inet_stream_ops = {
>  #endif
>  	.splice_eof	   = inet_splice_eof,
>  	.splice_read	   = tcp_splice_read,
> +	.set_peek_off      = tcp_set_peek_offset,
>  	.read_sock	   = tcp_read_sock,
>  	.read_skb	   = tcp_read_skb,
>  	.sendmsg_locked    = tcp_sendmsg_locked,
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index fce5668a6a3d..33ade88633de 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -863,6 +863,14 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>  }
>  EXPORT_SYMBOL(tcp_splice_read);
>  
> +int tcp_set_peek_offset(struct sock *sk, int val)
> +{
> +	WRITE_ONCE(sk->sk_peek_off, val);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(tcp_set_peek_offset);
> +
>  struct sk_buff *tcp_stream_alloc_skb(struct sock *sk, gfp_t gfp,
>  				     bool force_schedule)
>  {
> @@ -2302,7 +2310,6 @@ static int tcp_inq_hint(struct sock *sk)
>   *	tricks with *seq access order and skb->users are not required.
>   *	Probably, code can be easily improved even more.
>   */
> -

Stray change.

>  static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  			      int flags, struct scm_timestamping_internal *tss,
>  			      int *cmsg_flags)
> @@ -2317,6 +2324,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  	long timeo;
>  	struct sk_buff *skb, *last;
>  	u32 urg_hole = 0;
> +	u32 peek_offset = 0;
>  
>  	err = -ENOTCONN;
>  	if (sk->sk_state == TCP_LISTEN)
> @@ -2349,7 +2357,8 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  
>  	seq = &tp->copied_seq;
>  	if (flags & MSG_PEEK) {
> -		peek_seq = tp->copied_seq;
> +		peek_offset = max(sk_peek_offset(sk, flags), 0);
> +		peek_seq = tp->copied_seq + peek_offset;
>  		seq = &peek_seq;
>  	}
>  

And with this, explicit support in tcp_peek_sndq() is not actually
needed, but this comment in that function:

        /* XXX -- need to support SO_PEEK_OFF */

should be removed now I guess.

> @@ -2452,11 +2461,11 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  		}
>  
>  		if ((flags & MSG_PEEK) &&
> -		    (peek_seq - copied - urg_hole != tp->copied_seq)) {
> +		    (peek_seq - peek_offset - copied - urg_hole != tp->copied_seq)) {
>  			net_dbg_ratelimited("TCP(%s:%d): Application bug, race in MSG_PEEK\n",
>  					    current->comm,
>  					    task_pid_nr(current));
> -			peek_seq = tp->copied_seq;
> +			peek_seq = tp->copied_seq + peek_offset;
>  		}
>  		continue;
>  
> @@ -2497,7 +2506,10 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>  		WRITE_ONCE(*seq, *seq + used);
>  		copied += used;
>  		len -= used;
> -
> +		if (flags & MSG_PEEK)
> +			sk_peek_offset_fwd(sk, used);
> +		else
> +			sk_peek_offset_bwd(sk, used);
>  		tcp_rcv_space_adjust(sk);
>  
>  skip_copy:
> @@ -2774,6 +2786,7 @@ void __tcp_close(struct sock *sk, long timeout)
>  		data_was_unread += len;
>  		__kfree_skb(skb);
>  	}
> +	sk_set_peek_off(sk, -1);
>  
>  	/* If socket has been already reset (e.g. in tcp_reset()) - kill it. */
>  	if (sk->sk_state == TCP_CLOSE)
> @@ -4492,7 +4505,7 @@ void tcp_done(struct sock *sk)
>  		reqsk_fastopen_remove(sk, req, false);
>  
>  	WRITE_ONCE(sk->sk_shutdown, SHUTDOWN_MASK);
> -
> +	sk_set_peek_off(sk, -1);
>  	if (!sock_flag(sk, SOCK_DEAD))
>  		sk->sk_state_change(sk);
>  	else

-- 
Stefano