Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()

On Wed, Sep 1, 2021 at 3:52 AM Paolo Abeni [off-list ref] wrote:

On Wed, 2021-09-01 at 18:39 +0800, Yunsheng Lin wrote:

quoted

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 <redacted>

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-

quoted

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 [off-list ref]
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 [off-list ref]

I will update this commit to also remove the part in MPTCP.

Let's remove this feature and replace it with something less costly.