Re: [RFC net-next v4] tcp: add support for read with offset when using MSG_PEEK

[Jon Maloy <jmaloy@redhat.com>]

On 2024-01-11 08:19, Jon Maloy wrote:
> 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 allow the user to set iovec.iov_base in the first
> vector entry to NULL. This tells the socket to skip the first entry,
> hence letting the iov_len field of that entry indicate the offset value.
> This way, there is no need to add any new arguments or flags.
>
> In the iperf3 log examples shown below, we can observe a throughput
> improvement of ~15 % 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@lubu:~/passt$ perf record -g ./pasta --config-net -f
> MSG_PEEK with offset not supported by kernel.
>
> jmaloy@lubu:~/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@lubu:~/passt#
> logout
> [ perf record: Woken up 23 times to write data ]
> [ perf record: Captured and wrote 5.696 MB perf.data (35580 samples) ]
> jmaloy@lubu:~/passt$
>
> jmaloy@lubu:~/passt$ perf record -g ./pasta --config-net -f
> MSG_PEEK with offset supported by kernel.
>
> jmaloy@lubu:~/passt# iperf3 -s
> -----------------------------------------------------------
> Server listening on 5201 (test #1)
> -----------------------------------------------------------
> Accepted connection from 192.168.122.1, port 40854
> [  5] local 192.168.122.180 port 5201 connected to 192.168.122.1 port 40862
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-1.00   sec  1.22 GBytes  10.5 Gbits/sec
> [  5]   1.00-2.00   sec  1.19 GBytes  10.2 Gbits/sec
> [  5]   2.00-3.00   sec  1.22 GBytes  10.5 Gbits/sec
> [  5]   3.00-4.00   sec  1.11 GBytes  9.56 Gbits/sec
> [  5]   4.00-5.00   sec  1.20 GBytes  10.3 Gbits/sec
> [  5]   5.00-6.00   sec  1.14 GBytes  9.80 Gbits/sec
> [  5]   6.00-7.00   sec  1.17 GBytes  10.0 Gbits/sec
> [  5]   7.00-8.00   sec  1.12 GBytes  9.61 Gbits/sec
> [  5]   8.00-9.00   sec  1.13 GBytes  9.74 Gbits/sec
> [  5]   9.00-10.00  sec  1.26 GBytes  10.8 Gbits/sec
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID] Interval           Transfer     Bitrate
> [  5]   0.00-10.04  sec  11.8 GBytes  10.1 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@lubu:~/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@lubu:~/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@lubu:~/passt$ perf report -q --symbol-filter=do_syscall_64 -p ____sys_recvmsg -x --stdio -i  perf.data | head -1
>     27.24%     0.00%  passt.avx2  [kernel.vmlinux]  [k] do_syscall_64  ____sys_recvmsg
>
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
>
> ---
> v3:  Made changes suggested by Stefano Brivio:
>      - Added perf result to commit log
>      - Separated parameter sanity tests from code logics
>
> v4: - Simplified sanity test further.
>        'iov_iter.count' is caclulated as the sum of the segment sizes in
>         ___sys_recvmsg()->recvmsg_copy_msghdr()->copy_msghdr_from_user()->__import_iovec()->iov_iter_init()
>        'len' is the same as iov_iter.count, as returnrd in
>        sock_recvmsg_nosec()->msg_data_left()->iov_iter_count()
>        Hence, iov[0].iov_len cannot be larger than any of those, and no additional
>        testing is necessary.
>      - Improved description of passt/pasta in commit log
>      - Some cosmetic changes to the iperf3/perf output
Hi Stefano,
I think we are very close now. If I can get yourack on this today I will 
send it to net-next, and then we'll see...
///jon

> ---
>   net/ipv4/tcp.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index ff6838ca2e58..50dc997b82f9 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2353,6 +2353,20 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
>   	if (flags & MSG_PEEK) {
>   		peek_seq = tp->copied_seq;
>   		seq = &peek_seq;
> +		if (!msg->msg_iter.__iov[0].iov_base) {
> +			size_t peek_offset;
> +
> +			if (msg->msg_iter.nr_segs < 2) {
> +				err = -EINVAL;
> +				goto out;
> +			}
> +			peek_offset = msg->msg_iter.__iov[0].iov_len;
> +			msg->msg_iter.__iov = &msg->msg_iter.__iov[1];
> +			msg->msg_iter.nr_segs -= 1;
> +			msg->msg_iter.count -= peek_offset;
> +			len -= peek_offset;
> +			*seq += peek_offset;
> +		}
>   	}
>   
>   	target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);