netdev - Re: [PATCH] tcp: fixing TLP's FIN recovery

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Message-ID: <CAB_+Fg5akovVdbRPd-aRT9nMvx4gL3OJSmvi3rczftLoo=EfGw@mail.gmail.com>
Date:	Mon, 9 Jun 2014 00:04:14 -0700
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Per Hurtig <per.hurtig@....se>, Netdev <netdev@...r.kernel.org>,
	Anna Brunström <anna.brunstrom@....se>,
	mohammad.rajiullah@....se, Neal Cardwell <ncardwell@...gle.com>,
	sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH] tcp: fixing TLP's FIN recovery

On Sun, Jun 8, 2014 at 9:35 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Sun, 2014-06-08 at 09:41 +0200, Per Hurtig wrote:
>>
>> On sön  8 jun 2014 04:58:25, Eric Dumazet wrote:
>> > On Sat, 2014-06-07 at 16:34 +0200, Per Hurtig wrote:
>> >> Fix to a problem observed when losing a FIN segment that does not
>> >> contain data.  In such situations, TLP is unable to recover from
>> >> *any* tail loss and instead adds at least PTO ms to the
>> >> retransmission process, i.e., RTO = RTO + PTO.
>> >>
>> >> Signed-off-by: Per Hurtig <per.hurtig@....se>
>> >> ---
>> >>   net/ipv4/tcp_output.c | 6 ++++--
>> >>   1 file changed, 4 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> >> index d463c35..6573765 100644
>> >> --- a/net/ipv4/tcp_output.c
>> >> +++ b/net/ipv4/tcp_output.c
>> >> @@ -2130,8 +2130,10 @@ void tcp_send_loss_probe(struct sock *sk)
>> >>    if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
>> >>            goto rearm_timer;
>> >>
>> >> -  /* Probe with zero data doesn't trigger fast recovery. */
>> >> -  if (skb->len > 0)
>> >> +  /* Probe with zero data doesn't trigger fast recovery, if FIN
>> >> +   * flag is not set.
>> >> +   */
>> >> +  if ((skb->len > 0) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
>> >>            err = __tcp_retransmit_skb(sk, skb);
>> >>
>> >>    /* Record snd_nxt for loss detection. */
>> >
>> >
>> > You know, I believe the test was exactly to avoid sending data less FIN
>> > packets.
>> >
>> > If you write :
>> >
>> >      if (A  || !A)
>> >
>> > Better remove the condition, completely ;)
>> >
>> Obviously, but I don't think that FINs are the only segments
>> who are targeted by this condition (or targeted at all given
>> the implications of this statement). Furthermore, the comment above
>> the if statement would probably have mentioned FINs explicity
>> and not zero sized segments in general if this were the case.
>>
>
>
> I see no other possibilities than FIN segments here, or the WARN_ON(!
> tcp_skb_pcount(skb)) right before would trigger.
>
> If we believe it could trigger, then we need to remove the WARN_ON(),
> because its far more disruptive than waiting a bit more for the RTO.
> Remember : RTO is conservative.
>
> The if (skb->len > 0) only is true for FIN with no data.
>
> This was exactly the intent : Not sending FIN at this stage.
>
> If pure FIN is OK here, just remove the comment and test, this is so
> confusing and useless.

I agree - if pure FIN can indeed trigger recovery, let's just remove
the test and comment.

Nandita
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