netdev - Re: [PATCH net-next] tcp: remove SOCK_QUEUE

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Date:   Mon, 14 Sep 2020 09:43:49 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net-next] tcp: remove SOCK_QUEUE_SHRUNK

On Mon, Sep 14, 2020 at 6:20 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> SOCK_QUEUE_SHRUNK is currently used by TCP as a temporary state
> that remembers if some room has been made in the rtx queue
> by an incoming ACK packet.
>
> This is later used from tcp_check_space() before
> considering to send EPOLLOUT.
>
> Problem is: If we receive SACK packets, and no packet
> is removed from RTX queue, we can send fresh packets, thus
> moving them from write queue to rtx queue and eventually
> empty the write queue.
>
> This stall can happen if TCP_NOTSENT_LOWAT is used.
>
> With this fix, we no longer risk stalling sends while holes
> are repaired, and we can fully use socket sndbuf.
>
> This also removes a cache line dirtying for typical RPC
> workloads.
>
> Fixes: c9bee3b7fdec ("tcp: TCP_NOTSENT_LOWAT socket option")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
> ---

Nice. It's beautiful when a fix is also a simplification...

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal