On Thu, 2024-01-11 at 18:00 -0500, jmaloy@redhat.com wrote:

From: Jon Maloy

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@freyr:~/passt$ perf record -g ./pasta --config-net -f MSG_PEEK with offset 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 MSG_PEEK with offset supported by kernel.

jmaloy@freyr:~/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@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 27.24% 0.00% passt.avx2 [kernel.vmlinux] [k] do_syscall_64 ____sys_recvmsg

Reviewed-by: Stefano Brivio Signed-off-by: Jon Maloy --- net/ipv4/tcp.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 1baa484d2190..82e1da3f0f98 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -2351,6 +2351,20 @@ 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; + }

IMHO this does not look like the correct interface to expose such functionality. Doing the same with a different protocol should cause a SIGSEG or the like, right? What about using/implementing SO_PEEK_OFF support instead? Cheers, Paolo