netdev - Re: [PATCH v3] tcp: add support for SO_PEEK

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Message-ID: <20072ba530b34729589a3d527c420a766b49e205.camel@redhat.com>
Date: Tue, 13 Feb 2024 16:28:36 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: kuba@...nel.org, passt-dev@...st.top, sbrivio@...hat.com,
 lvivier@...hat.com,  dgibson@...hat.com, jmaloy@...hat.com,
 netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

On Tue, 2024-02-13 at 14:34 +0100, Eric Dumazet wrote:
> On Tue, Feb 13, 2024 at 2:02 PM Paolo Abeni <pabeni@...hat.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@...hat.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?

Thanks!

Paolo