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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 1 Sep 2021 08:06:52 -0700
Message-ID: <CANn89iJFeM=DgcQpDbaE38uhxTEL6REMWPnVFt7Am7Nuf4wpMw@mail.gmail.com>
Subject: Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()
To:     Paolo Abeni <pabeni@redhat.com>
Cc:     Yunsheng Lin <linyunsheng@huawei.com>,
        David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        MPTCP Upstream <mptcp@lists.linux.dev>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linuxarm@openeuler.org,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>
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On Wed, Sep 1, 2021 at 3:52 AM Paolo Abeni <pabeni@redhat.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@huawei.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@google.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@google.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.