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

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Message-ID: <YnTCjFE2+/JEgglV@samus.usersys.redhat.com>
Date:   Fri, 6 May 2022 08:39:08 +0200
From:   Artem Savkov <asavkov@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
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 10:56:54AM -0700, Josh Poimboeuf wrote:
> 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.

But cascading is still there in the handler so it will fire off quite
close to original timer in any case.


-- 
 Artem