| Message ID | 20240111230057.305672-1-jmaloy@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [RFC,net-next] tcp: add support for read with offset when using MSG_PEEK | expand |
On Thu, 2024-01-11 at 18:00 -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 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 <sbrivio@redhat.com> > Signed-off-by: Jon Maloy <jmaloy@redhat.com> > --- > 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
On 2024-01-16 05:49, Paolo Abeni wrote: > On Thu, 2024-01-11 at 18:00 -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. [...] >> + 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? I would expect doing the same thing with a different protocol to cause an EFAULT, as it should. But I haven't tried it. This is a change to TCP only, at least until somebody decides to implement it elsewhere (why not?) > > What about using/implementing SO_PEEK_OFF support instead? I looked at SO_PEEK_OFF, and it honestly looks both awkward and limited. We would have to make frequent calls to setsockopt(), something that would beat much of the purpose of this feature. I stand by my opinion here. This feature is simple, non-intrusive, totally backwards compatible and implies no changes to the API or BPI. I would love to hear other opinions on this, though. Regards /jon > > Cheers, > > Paolo >
On Thu, 18 Jan 2024 17:22:52 -0500 Jon Maloy <jmaloy@redhat.com> wrote: > On 2024-01-16 05:49, Paolo Abeni wrote: > > On Thu, 2024-01-11 at 18:00 -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. > [...] > >> + 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? > > I would expect doing the same thing with a different protocol to cause > an EFAULT, as it should. But I haven't tried it. So, out of curiosity, I actually tried: the current behaviour is recvmsg() failing with EFAULT, only as data is received (!), for TCP and UDP with AF_INET, and for AF_UNIX (both datagram and stream). EFAULT, however, is not in the list of "shall fail", nor "may fail" conditions described by POSIX.1-2008, so there isn't really anything that mandates it API-wise. Likewise, POSIX doesn't require any signal to be delivered (and no signals are delivered on Linux in any case: note that iov_base is not dereferenced). For TCP sockets only, passing a NULL buffer is already supported by recv() with MSG_TRUNC (same here, Linux extension). This change would finally make recvmsg() consistent with that TCP-specific bit. > This is a change to TCP only, at least until somebody decides to > implement it elsewhere (why not?) Side note, I can't really think of a reasonable use case for UDP -- it doesn't quite fit with the notion of message boundaries. Even letting alone the fact that passt(1) and pasta(1) don't need this for UDP (no acknowledgement means no need to keep unacknowledged data anywhere), if another application wants to do something conceptually similar, we should probably target recvmmsg(). > > What about using/implementing SO_PEEK_OFF support instead? > > I looked at SO_PEEK_OFF, and it honestly looks both awkward and limited. I think it's rather intended to skip headers with fixed size or suchlike. > We would have to make frequent calls to setsockopt(), something that > would beat much of the purpose of this feature. ...right, we would need to reset the SO_PEEK_OFF value at every recvmsg(), which is probably even worse than the current overhead. > I stand by my opinion here. > This feature is simple, non-intrusive, totally backwards compatible and > implies no changes to the API or BPI. My thoughts as well, plus the advantage for our user-mode networking case is quite remarkable given how simple the change is. > I would love to hear other opinions on this, though. > > Regards > /jon > > > > > Cheers, > > > > Paolo
On 2024-01-21 17:16, Stefano Brivio wrote: > On Thu, 18 Jan 2024 17:22:52 -0500 > Jon Maloy <jmaloy@redhat.com> wrote: > >> On 2024-01-16 05:49, Paolo Abeni wrote: >>> On Thu, 2024-01-11 at 18:00 -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. >> [...] >>>> + 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? >> I would expect doing the same thing with a different protocol to cause >> an EFAULT, as it should. But I haven't tried it. > So, out of curiosity, I actually tried: the current behaviour is > recvmsg() failing with EFAULT, only as data is received (!), for TCP > and UDP with AF_INET, and for AF_UNIX (both datagram and stream). > > EFAULT, however, is not in the list of "shall fail", nor "may fail" > conditions described by POSIX.1-2008, so there isn't really anything > that mandates it API-wise. > > Likewise, POSIX doesn't require any signal to be delivered (and no > signals are delivered on Linux in any case: note that iov_base is not > dereferenced). > > For TCP sockets only, passing a NULL buffer is already supported by > recv() with MSG_TRUNC (same here, Linux extension). This change would > finally make recvmsg() consistent with that TCP-specific bit. > >> This is a change to TCP only, at least until somebody decides to >> implement it elsewhere (why not?) > Side note, I can't really think of a reasonable use case for UDP -- it > doesn't quite fit with the notion of message boundaries. > > Even letting alone the fact that passt(1) and pasta(1) don't need this > for UDP (no acknowledgement means no need to keep unacknowledged data > anywhere), if another application wants to do something conceptually > similar, we should probably target recvmmsg(). > >>> What about using/implementing SO_PEEK_OFF support instead? >> I looked at SO_PEEK_OFF, and it honestly looks both awkward and limited. > I think it's rather intended to skip headers with fixed size or > suchlike. > >> We would have to make frequent calls to setsockopt(), something that >> would beat much of the purpose of this feature. > ...right, we would need to reset the SO_PEEK_OFF value at every > recvmsg(), which is probably even worse than the current overhead. > >> I stand by my opinion here. >> This feature is simple, non-intrusive, totally backwards compatible and >> implies no changes to the API or BPI. > My thoughts as well, plus the advantage for our user-mode networking > case is quite remarkable given how simple the change is. After pondering more upon this, and also some team internal discussions, I have decided to give it a try with SO_PEEK_OFF, just to see to see the outcome, both at kernel level and in user space. So please wait with any possible application of this , if that ever happens with RFCs. ///jon > >> I would love to hear other opinions on this, though. >> >> Regards >> /jon >> >>> Cheers, >>> >>> Paolo
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; + } } target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);