netdev - Re: [PATCH net-next 1/2] tcp: Measure TIME-WAIT reuse delay with millisecond precision

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Message-ID: <CANn89iL5oE79_qtNUFFsyxLXoJALJCZgawsubuvn1XOcOuOzFw@mail.gmail.com>
Date: Wed, 4 Dec 2024 20:22:36 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: netdev@...r.kernel.org, Jason Xing <kerneljasonxing@...il.com>, 
	Adrien Vasseur <avasseur@...udflare.com>, Lee Valentine <lvalentine@...udflare.com>, 
	kernel-team@...udflare.com
Subject: Re: [PATCH net-next 1/2] tcp: Measure TIME-WAIT reuse delay with
 millisecond precision

On Wed, Dec 4, 2024 at 7:53 PM Jakub Sitnicki <jakub@...udflare.com> wrote:

> A low effort alternative would be to introduce a new field to hold a
> millisecond timestamp for measuring the TW reuse delay. However, this would
> cause the struct tcp_timewait_socket size to go over 256 bytes and overflow
> into another cache line.

s/tcp_timewait_socket/tcp_timewait_sock/

Can you elaborate on this ?

Due to SLUB management, note that timewait_sockets are not cache
aligned, and use 264 bytes already:

# grep tw_sock_TCP /proc/slabinfo
tw_sock_TCPv6       3596   3596    264   62    4 : tunables    0    0
  0 : slabdata     58     58      0
tw_sock_TCP            0      0    264   62    4 : tunables    0    0
  0 : slabdata      0      0      0

In any case, there is one 4 byte hole in struct inet_timewait_sock
after tw_priority