netdev - Re: [PATCH v5 2/2] net: make tcp keepalive timer upper bound

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Date:   Thu, 5 May 2022 10:56:54 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Artem Savkov <asavkov@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] net: make tcp keepalive timer upper bound

On Thu, May 05, 2022 at 03:18:11PM +0200, Artem Savkov wrote:
> Make sure TCP keepalive timer does not expire late. Switching to upper
> bound timers means it can fire off early but in case of keepalive
> tcp_keepalive_timer() handler checks elapsed time and resets the timer
> if it was triggered early. This results in timer "cascading" to a
> higher precision and being just a couple of milliseconds off it's
> original mark.
> This adds minimal overhead as keepalive timers are never re-armed and
> are usually quite long.
> 
> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  net/ipv4/inet_connection_sock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 1e5b53c2bb26..bb2dbfb6f5b5 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -589,7 +589,7 @@ EXPORT_SYMBOL(inet_csk_delete_keepalive_timer);
>  
>  void inet_csk_reset_keepalive_timer(struct sock *sk, unsigned long len)
>  {
> -	sk_reset_timer(sk, &sk->sk_timer, jiffies + len);
> +	sk_reset_timer(sk, &sk->sk_timer, jiffies + upper_bound_timeout(len));
>  }
>  EXPORT_SYMBOL(inet_csk_reset_keepalive_timer);

As I mentioned before, there might be two sides to the same coin,
depending on whether the keepalive is detecting vs preventing the
disconnect.  So this might possibly fix one case, while breaking
another.

Hopefully a networking expert can chime in.

-- 
Josh