netdev - Re: [PATCH net] net: fix the RTO timer retransmitting skb every 1ms if linear option is enabled

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Date: Fri, 11 Aug 2023 10:35:27 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Simon Horman <horms@...nel.org>
Cc: edumazet@...gle.com, davem@...emloft.net, dsahern@...nel.org, 
	kuba@...nel.org, pabeni@...hat.com, apetlund@...ula.no, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net] net: fix the RTO timer retransmitting skb every 1ms
 if linear option is enabled

On Fri, Aug 11, 2023 at 4:32 AM Simon Horman <horms@...nel.org> wrote:
>
> On Thu, Aug 10, 2023 at 07:21:48PM +0800, Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > In the real workload, I encountered an issue which could cause the RTO
> > timer to retransmit the skb per 1ms with linear option enabled. The amount
> > of lost-retransmitted skbs can go up to 1000+ instantly.
> >
> > The root cause is that if the icsk_rto happens to be zero in the 6th round
> > (which is the TCP_THIN_LINEAR_RETRIES value), then it will alway be zero
>
> nit: alway -> always
>
>      checkpatch.pl --codespell is your friend

Thanks for your reminder.

>
> > due to the changed calculation method in tcp_retransmit_timer() as follows:
> >
> > icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
> >
> > Above line could be converted to
> > icsk->icsk_rto = min(0 << 1, TCP_RTO_MAX) = 0
> >
> > Therefore, the timer expires so quickly without any doubt.
> >
> > I read through the RFC 6298 and found that the RTO value can be rounded
> > up to a certain value, in Linux, say TCP_RTO_MIN as default, which is
> > regarded as the lower bound in this patch as suggested by Eric.
> >
> > Fixes: 36e31b0af587 ("net: TCP thin linear timeouts")
> > Suggested-by: Eric Dumazet <edumazet@...gle.com>
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > ---
> >  net/ipv4/tcp_timer.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index d45c96c7f5a4..b2b25861355c 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
> > @@ -599,7 +599,9 @@ void tcp_retransmit_timer(struct sock *sk)
> >           tcp_stream_is_thin(tp) &&
> >           icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
> >               icsk->icsk_backoff = 0;
> > -             icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
> > +             icsk->icsk_rto = clamp(__tcp_set_rto(tp),
> > +                                         tcp_rto_min(sk),
> > +                                         TCP_RTO_MAX);
>
> nit: this indentation looks a bit odd.

Yeah, I'm always stuck with this kind of indentation issue. I'll fix
it in v2 patch.

Thanks,
Jason

>
>                 icsk->icsk_rto = clamp(__tcp_set_rto(tp),
>                                        tcp_rto_min(sk),
>                                        TCP_RTO_MAX);
>
> >       } else if (sk->sk_state != TCP_SYN_SENT ||
> >                  icsk->icsk_backoff >
> >                  READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
> > --
> > 2.37.3
> >
> >