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

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Date:   Mon, 14 Sep 2020 13:36:13 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com, soheil@...gle.com
Subject: Re: [PATCH net-next] tcp: remove SOCK_QUEUE_SHRUNK

From: Eric Dumazet <edumazet@...gle.com>
Date: Mon, 14 Sep 2020 03:20:27 -0700

> 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>

Applied, thanks Eric.