netdev - Re: [PATCH net] datagram: When peeking datagrams with offset

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Date:   Mon, 14 Aug 2017 15:39:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Thiago Macieira <thiago.macieira@...el.com>
Cc:     Matthew Dawson <matthew@...systems.ca>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] datagram: When peeking datagrams with offset < 0
 don't skip empty skbs

On Mon, Aug 14, 2017 at 3:15 PM, Thiago Macieira
<thiago.macieira@...el.com> wrote:
> On Monday, 14 August 2017 12:03:16 PDT Willem de Bruijn wrote:
>> On Mon, Aug 14, 2017 at 2:58 PM, Thiago Macieira
>>
>> <thiago.macieira@...el.com> wrote:
>> > On Monday, 14 August 2017 11:46:42 PDT Willem de Bruijn wrote:
>> >> > By the way, what were the usecases for the peek offset feature?
>> >>
>> >> The idea was to be able to peek at application headers of upper
>> >> layer protocols and multiplex messages among threads. It proved
>> >> so complex even for UDP that we did not attempt the same feature
>> >> for TCP. Also, KCM implements demultiplexing using eBPF today.
>> >
>> > Interesting, but how would userspace coordinate like that? Suppose
>> > multiple
>> > threads are woken up by a datagram being received
>>
>> This assumes a separate listener thread and worker threadpool.
>
> The listener thread still needs to synchronise with the worker that got
> activated and wait for it to recv from the socket before the listener thread
> can go back to poll().
>
> If we are really talking about threads in the same process, it might be easier
> for the listener to just read the datagram anyway and pass it on to the
> worker. That way, it can proceed immediately to the next datagram and not have
> to wait for the possibly slow worker.
>
> If it is a separate process, then I don't see another way and this might be
> necessary.
>
> By the way, what does recv with MSG_PEEK | MSG_TRUNC return? Is it the full
> datagram's size or is it the size minus the peek offset?

udp_recvmsg returns ulen if the flag is passed.

        if (flags & MSG_TRUNC)
                err = ulen;

This is computed earlier in the function as udp payload length and not
modified after.

        ulen = udp_skb_len(skb);