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

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Date:   Wed, 01 Sep 2021 12:52:05 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org, MPTCP Upstream <mptcp@...ts.linux.dev>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...neuler.org, edumazet@...gle.com,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org
Subject: Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in
 sk_stream_alloc_skb()

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.

Cheers,

Paolo