netdev - Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc

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Date:   Wed, 1 Sep 2021 08:16:05 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        MPTCP Upstream <mptcp@...ts.linux.dev>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...neuler.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()

On Wed, Sep 1, 2021 at 8:06 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Sep 1, 2021 at 3:52 AM Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > On Wed, 2021-09-01 at 18:39 +0800, Yunsheng Lin wrote:
> > > Since tcp_tx_skb_cache is disabled by default in:
> > > commit 0b7d7f6b2208 ("tcp: add tcp_tx_skb_cache sysctl")
> > >
> > > Add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb() to
> > > avoid possible branch-misses.
> > >
> > > Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> >
> > Note that MPTCP is currently exploiting sk->sk_tx_skb_cache. If we get
> > this patch goes in as-is, it will break mptcp.
> >
> > One possible solution would be to let mptcp usage enable sk-
> > >sk_tx_skb_cache, but that has relevant side effects on plain TCP.
> >
> > Another options would be re-work once again the mptcp xmit path to
> > avoid using sk->sk_tx_skb_cache.
> >
>
> Hmmm, I actually wrote a revert of this feature but forgot to submit
> it last year.
>
> commit c36cfbd791f62c0f7c6b32132af59dfdbe6be21b (HEAD -> listener_scale4)
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Wed May 20 06:38:38 2020 -0700
>
>     tcp: remove sk_{tr}x_skb_cache
>
>     This reverts the following patches :
>
>     2e05fcae83c41eb2df10558338dc600dc783af47 ("tcp: fix compile error
> if !CONFIG_SYSCTL")
>     4f661542a40217713f2cee0bb6678fbb30d9d367 ("tcp: fix zerocopy and
> notsent_lowat issues")
>     472c2e07eef045145bc1493cc94a01c87140780a ("tcp: add one skb cache for tx")
>     8b27dae5a2e89a61c46c6dbc76c040c0e6d0ed4c ("tcp: add one skb cache for rx")
>
>     Having a cache of one skb (in each direction) per TCP socket is fragile,
>     since it can cause a significant increase of memory needs,
>     and not good enough for high speed flows anyway where more than one skb
>     is needed.
>
>     We want instead to add a generic infrastructure, with more flexible per-cpu
>     caches, for alien NUMA nodes.
>
>     Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> I will update this commit to also remove the part in MPTCP.
>
> Let's remove this feature and replace it with something less costly.

Paolo, can you work on MPTP side, so that my revert can be then applied ?

Thanks !

View attachment "0001-tcp-remove-sk_-tr-x_skb_cache.patch" of type "text/x-patch" (9624 bytes)