netdev - Re: [PATCH net-next] tcp: cache RTAX_QUICKACK metric in a hot cache line

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Message-ID: <CADVnQynrSn-oJ4tyhXkZScEirnsd2n3PJeJgG5NFhuNiKjy5Fg@mail.gmail.com>
Date: Wed, 12 Mar 2025 09:24:24 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuniyu@...zon.com>, Simon Horman <horms@...nel.org>, 
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] tcp: cache RTAX_QUICKACK metric in a hot cache line

On Wed, Mar 12, 2025 at 4:39 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> tcp_in_quickack_mode() is called from input path for small packets.
>
> It calls __sk_dst_get() which reads sk->sk_dst_cache which has been
> put in sock_read_tx group (for good reasons).
>
> Then dst_metric(dst, RTAX_QUICKACK) also needs extra cache line misses.
>
> Cache RTAX_QUICKACK in icsk->icsk_ack.dst_quick_ack to no longer pull
> these cache lines for the cases a delayed ACK is scheduled.
>
> After this patch TCP receive path does not longer access sock_read_tx
> group.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---

Great! Indeed, from double-checking the code, it looks like with this
change there are no fetches of dst RTAX_ values outside of the
connection initialization code paths. That's great. Thanks, Eric!

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

neal