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 wrote:

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@@ -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