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

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Message-ID: <eaee3c892545e072095e7b296ddde598f1e966d9.camel@redhat.com>
Date: Tue, 13 Feb 2024 14:02:51 +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 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?

Thanks,

Paolo