netdev - Re: [PATCH net-next] tcp: drop dst in tcp_add

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Date:   Tue, 20 Nov 2018 10:21:22 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: drop dst in tcp_add_backlog()

On Tue, Nov 20, 2018 at 10:16 AM David Miller <davem@...emloft.net> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Tue, 20 Nov 2018 10:08:12 -0800
>
> > On Tue, Nov 20, 2018 at 10:01 AM David Miller <davem@...emloft.net> wrote:
> >>
> >> From: Eric Dumazet <edumazet@...gle.com>
> >> Date: Mon, 19 Nov 2018 17:45:55 -0800
> >>
> >> > Under stress, softirq rx handler often hits a socket owned by the user,
> >> > and has to queue the packet into socket backlog.
> >> >
> >> > When this happens, skb dst refcount is taken before we escape rcu
> >> > protected region. This is done from __sk_add_backlog() calling
> >> > skb_dst_force().
> >> >
> >> > Consumer will have to perform the opposite costly operation.
> >> >
> >> > AFAIK nothing in tcp stack requests the dst after skb was stored
> >> > in the backlog. If this was the case, we would have had failures
> >> > already since skb_dst_force() can end up clearing skb dst anyway.
> >> >
> >> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >>
> >> Hmmm, it seems to be used by connection completion code to setup the
> >> socket cached rx dst, right?
> >>
> >> For example tcp_finish_connect() --> icsk->icsk_af_ops->sk_rx_dst_set(sk, skb)
> >
> > We already cope with skb->dst being NULL there I believe.
> >
> > For reference look at
> >
> > commit 5037e9ef9454917b047f9f3a19b4dd179fbf7cd4    net: fix IP early demux races
>
> Well, I'm sure we "handle" it.  But I was more asking about the performance
> tradeoff, which probably is on the side of your change but I wanted to
> just be sure.

Ah sorry for misunderstanding.
Yes, that should be fine, backlog is not generally used at the
beginning of a flow.

I am working on adding coalescing support to tcp_add_backlog() to balance
time spent in softirq and time spent from process context, hoping to reduce
probability of having user threads trapped for a while in __release_sock()