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

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Feb 14, 2024 at 10:34:50AM +1100, David Gibson wrote:
> On Tue, Feb 13, 2024 at 04:49:01PM +0100, Eric Dumazet wrote:
> > On Tue, Feb 13, 2024 at 4:28 PM Paolo Abeni <pabeni@redhat.com> wrote:
> > >
> > > On Tue, 2024-02-13 at 14:34 +0100, Eric Dumazet wrote:
> > > > On Tue, Feb 13, 2024 at 2:02 PM Paolo Abeni <pabeni@redhat.com> wrote:
> > > > >
> > > > > On Tue, 2024-02-13 at 13:24 +0100, Eric Dumazet wrote:
> > > > > > On Tue, Feb 13, 2024 at 11:49 AM Paolo Abeni <pabeni@redhat.com> wrote:
> > > > > >
> > > > > > > > @@ -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().
> > > > >
> > > > > Let me double check I read the above correctly: are you concerned by
> > > > > the 'skb_queue_walk(&sk->sk_receive_queue, skb) {' loop that could
> > > > > touch a lot of skbs/cachelines before reaching the relevant skb?
> > > > >
> > > > > The end goal here is allowing an user-space application to read
> > > > > incrementally/sequentially the received data while leaving them in
> > > > > receive buffer.
> > > > >
> > > > > I don't see a better option than MSG_PEEK, am I missing something?
> > > >
> > > >
> > > > This sk_peek_offset protocol, needing  sk_peek_offset_bwd() in the non
> > > > MSG_PEEK case is very strange IMO.
> > > >
> > > > Ideally, we should read/write over sk_peek_offset only when MSG_PEEK
> > > > is used by the caller.
> > > >
> > > > That would only touch non fast paths.
> > > >
> > > > Since the API is mono-threaded anyway, the caller should not rely on
> > > > the fact that normal recvmsg() call
> > > > would 'consume' sk_peek_offset.
> > >
> > > Storing in sk_peek_seq the tcp next sequence number to be peeked should
> > > avoid changes in the non MSG_PEEK cases.
> > >
> > > AFAICS that would need a new get_peek_off() sock_op and a bit somewhere
> > > (in sk_flags?) to discriminate when sk_peek_seq is actually set. Would
> > > that be acceptable?
> > 
> > We could have a parallel SO_PEEK_OFFSET option, reusing the same socket field.
> > 
> > The new semantic would be : Supported by TCP (so far), and tcp
> > recvmsg() only reads/writes this field when MSG_PEEK is used.
> > Applications would have to clear the values themselves.
> 
> Those semantics would likely defeat the purpose of using SO_PEEK_OFF
> for our use case, since we'd need an additional setsockopt() for every
> non-PEEK recv() (which are all MSG_TRUNC in our case).

Btw, Eric,

If you're concerned about the extra access added to the "regular" TCP
path, would you be happier with the original approach Jon proposed:
that allowed a user to essentially supply an offset to an individial
MSG_PEEK recvmsg() by inserting a dummy entry as msg_iov[0] with a
NULL pointer and length to skip.

It did the job for us, although I admit it's a little ugly, which I
presume is why Paolo suggested we investigate SO_PEEK_OFF instead.  I
think the SO_PEEK_OFF approach is more elegant, but maybe the
performance impact outweighs that.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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