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Date: Mon, 04 Feb 2013 07:07:41 -0800 From: Eric Dumazet <eric.dumazet@...il.com> To: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> Cc: Neal Cardwell <ncardwell@...gle.com>, Pasi Kärkkäinen <pasik@....fi>, David Miller <davem@...emloft.net>, Hannes Frederic Sowa <hannes@...essinduktion.org>, Stephen Hemminger <stephen@...workplumber.org>, Netdev <netdev@...r.kernel.org>, Yuchung Cheng <ycheng@...gle.com> Subject: Re: [PATCH] tcp: frto should not set snd_cwnd to 0 On Mon, 2013-02-04 at 14:14 +0200, Ilpo Järvinen wrote: > On Sun, 3 Feb 2013, Eric Dumazet wrote: > > > From: Eric Dumazet <edumazet@...gle.com> > > > > Commit 9dc274151a548 (tcp: fix ABC in tcp_slow_start()) > > uncovered a bug in FRTO code : > > tcp_process_frto() is setting snd_cwnd to 0 if the number > > of in flight packets is 0. > > > > As Neal pointed out, if no packet is in flight we lost our > > chance to disambiguate whether a loss timeout was spurious. > > > > We should assume it was a proper loss. > > > > Reported-by: Pasi Kärkkäinen <pasik@....fi> > > Signed-off-by: Neal Cardwell <ncardwell@...gle.com> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com> > > Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> > > Cc: Yuchung Cheng <ycheng@...gle.com> > > --- > > net/ipv4/tcp_input.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > > index 8aca4ee..680c422 100644 > > --- a/net/ipv4/tcp_input.c > > +++ b/net/ipv4/tcp_input.c > > @@ -3484,7 +3484,8 @@ static bool tcp_process_frto(struct sock *sk, int flag) > > ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED))) > > tp->undo_marker = 0; > > > > - if (!before(tp->snd_una, tp->frto_highmark)) { > > + if (!before(tp->snd_una, tp->frto_highmark) || > > + !tcp_packets_in_flight(tp)) { > > I think this condition becomes now too broad because there is transient > during FRTO. I think the patch below would be enough to resolve this, > what do you think? > > -- > [PATCH 1/1] tcp: fix for zero packets_in_flight was too broad > > There are transients during normal FRTO procedure during which > the packets_in_flight can go to zero between write_queue state > updates and firing the resulting segments out. As FRTO processing > occurs during that window the check must be more precise to > not match "spuriously" :-). More specificly, e.g., when > packets_in_flight is zero but FLAG_DATA_ACKED is true the problematic > branch that set cwnd into zero would not be taken and new segments > might be sent out later. > > Only compile tested. > > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> > Cc: Neal Cardwell <ncardwell@...gle.com> > Cc: Eric Dumazet <eric.dumazet@...il.com> > Cc: Pasi Kärkkäinen <pasik@....fi> > --- > net/ipv4/tcp_input.c | 8 ++++++-- > 1 files changed, 6 insertions(+), 2 deletions(-) > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > index 680c422..500c2da 100644 > --- a/net/ipv4/tcp_input.c > +++ b/net/ipv4/tcp_input.c > @@ -3484,8 +3484,7 @@ static bool tcp_process_frto(struct sock *sk, int flag) > ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED))) > tp->undo_marker = 0; > > - if (!before(tp->snd_una, tp->frto_highmark) || > - !tcp_packets_in_flight(tp)) { > + if (!before(tp->snd_una, tp->frto_highmark)) { > tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag); > return true; > } > @@ -3505,6 +3504,11 @@ static bool tcp_process_frto(struct sock *sk, int flag) > } > } else { > if (!(flag & FLAG_DATA_ACKED) && (tp->frto_counter == 1)) { > + if (!tcp_packets_in_flight(tp)) { > + tcp_enter_frto_loss(sk, 2, flag); > + return true; > + } > + > /* Prevent sending of new data. */ > tp->snd_cwnd = min(tp->snd_cwnd, > tcp_packets_in_flight(tp)); Thanks Ilpo. I'll be able to test your patch under load only in ~8 hours. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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