Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

[Eric Dumazet <edumazet@google.com>]

On Tue, Feb 13, 2024 at 11:49 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> Oops,
>
> I just noticed Eric is missing from the recipients list, adding him
> now.
>

Hmmm thanks.

> On Fri, 2024-02-09 at 17:12 -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
> >
> > Suggested-by: Paolo Abeni <pabeni@redhat.com>
> > Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> >
> > ---
> > v3: - Applied changes suggested by Stefano Brivio and Paolo Abeni
> > ---
> >  net/ipv4/af_inet.c |  1 +
> >  net/ipv4/tcp.c     | 16 ++++++++++------
> >  2 files changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > index 4e635dd3d3c8..5f0e5d10c416 100644
> > --- a/net/ipv4/af_inet.c
> > +++ b/net/ipv4/af_inet.c
> > @@ -1071,6 +1071,7 @@ const struct proto_ops inet_stream_ops = {
> >  #endif
> >       .splice_eof        = inet_splice_eof,
> >       .splice_read       = tcp_splice_read,
> > +     .set_peek_off      = sk_set_peek_off,
> >       .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 7e2481b9eae1..1c8cab14a32c 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -1415,8 +1415,6 @@ static int tcp_peek_sndq(struct sock *sk, struct msghdr *msg, int len)
> >       struct sk_buff *skb;
> >       int copied = 0, err = 0;
> >
> > -     /* XXX -- need to support SO_PEEK_OFF */
> > -
> >       skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
> >               err = skb_copy_datagram_msg(skb, 0, msg, skb->len);
> >               if (err)
> > @@ -2327,6 +2325,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
> >       int target;             /* Read at least this many bytes */
> >       long timeo;
> >       struct sk_buff *skb, *last;
> > +     u32 peek_offset = 0;
> >       u32 urg_hole = 0;
> >
> >       err = -ENOTCONN;
> > @@ -2360,7 +2359,8 @@ 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;
> >       }
> >
> > @@ -2463,11 +2463,11 @@ 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;
> >
> > @@ -2508,7 +2508,10 @@ 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);

Yet another cache miss in TCP fast path...

We need to move sk_peek_off in a better location before we accept this patch.

I always thought MSK_PEEK was very inefficient, I am surprised we
allow arbitrary loops in recvmsg().