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

[Jon Maloy <jmaloy@redhat.com>]

On 2023-06-23 07:02, Stefano Brivio wrote:
> On Thu, 22 Jun 2023 22:12:27 -0400
> Jon Maloy <jmaloy@redhat.com> 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
This looks like a script language, but I don't recognize it. How do I 
run it?

>
>> 8-10 % for the protocol splicer 'passst', which is used in KubeVirt
Now at 20+ %
> 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 <jmaloy@redhat.com>
>>
>> works with original msghdr
>>
>> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
>> ---
>>   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).
Checked. This is taken care of by the generic code.

///jon

>
>> +			msg->msg_iter.count -= peek_offset;
>> +			len -= peek_offset;
>> +			*seq += peek_offset;
>> +		}
>>   	}
>>   
>>   	target = sock_rcvlowat(sk, flags & MSG_WAITALL, len);