Re: [RFC v2] tcp: add support for read with offset when using MSG_PEEK

On Thu, 22 Jun 2023 22:12:27 -0400 Jon Maloy wrote:

When reading received messages with MSG_PEEK, we sometines have to read the leading bytes of the stream several times, only to reach the bytes we really want. This is clearly non-optimal.

What we would want is something similar to pread/preadv(), but working even for tcp sockets. At the same time, we obviously don't want to add any new arguments to the recv/recvmsg() calls.

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 making the iov_len field of that entry indicate the offset value. This way, there is no need to add any new arguments.

Ah-ha! I'm glad you found an acceptable way to pass a NULL pointer there. :)

This change is simple and non-intrusive, and should be safe addition to the socket API. We have measured it to give a throughput improvement of

...it would be nice to also do a bit of profiling with perf(1) -- that's where I originally noticed we were wasting cycles on filling up tcp_buf_discard. Plus, sure, there's also some value in dropping a useless 16 MiB buffer. If you need examples/inspiration: the pasta (automated) demo shows that, skip at 9:20 in: https://passt.top/passt/about/#pasta_2 (the one on the left) and that's simply done like this: https://passt.top/passt/tree/test/demo/pasta#n163

8-10 % for the protocol splicer 'passst', which is used in KubeVirt

passt

containers.

Those are virtual machines in containers -- but other than KubeVirt, passt is generally supported in libvirt (>= 9.2) with QEMU (>= 7.2)

Signed-off-by: Jon Maloy

works with original msghdr

Signed-off-by: Jon Maloy --- net/ipv4/tcp.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 33f559f491c8..1d89337e89b6 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -2428,6 +2428,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len, struct tcp_sock *tp = tcp_sk(sk); int copied = 0; u32 peek_seq; + u32 peek_offset;

Kernel networking code observes the reverse-Christmas tree notation (at least for new additions), this would need to go after *tp: struct tcp_sock *tp = tcp_sk(sk); u32 peek_offset;

u32 *seq; unsigned long used; int err; @@ -2435,7 +2436,6 @@ 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; -

Probably not intended.

err = -ENOTCONN; if (sk->sk_state == TCP_LISTEN) goto out; @@ -2469,6 +2469,14 @@ 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 == NULL) { + peek_offset = msg->msg_iter.iov[0].iov_len; + msg->msg_iter.iov = &msg->msg_iter.iov[1]; + msg->msg_iter.nr_segs -= 1;

Do you also need to make sure that nr_segs > 1 if iov[0].iov_base is NULL? I'm not sure if we need to check explicitly that msg_iter.iov[1] is valid here (but I haven't followed the whole path from the syscall handler).

+ msg->msg_iter.count -= peek_offset; + len -= peek_offset; + *seq += peek_offset; + } }

target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);

-- Stefano