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(a)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?

Thanks,

Paolo