netdev - Re: [REGRESSION, bisect]cxgb4 port failure with TSO traffic after commit 10d3be569243def8("tcp-tso: do not split TSO packets at retransmit time")

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Date:	Mon, 27 Jun 2016 15:27:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Arjun V <arjun@...lsio.com>, Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Hariprasad S <hariprasad@...lsio.com>,
	Casey Leedom <leedom@...lsio.com>,
	Kumar A S <kumaras@...lsio.com>,
	Santosh Rastapur <santosh@...lsio.com>,
	Nirranjan Kirubaharan <nirranjan@...lsio.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"ycheng@...gle.com" <ycheng@...gle.com>
Subject: Re: [REGRESSION, bisect]cxgb4 port failure with TSO traffic after
 commit 10d3be569243def8("tcp-tso: do not split TSO packets at retransmit
 time")

On Mon, 2016-06-27 at 09:12 -0400, Neal Cardwell wrote:
> On Mon, Jun 27, 2016 at 8:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Patch would be :
> >
> > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> > index 8bd9911fdd16..3587efe22864 100644
> > --- a/net/ipv4/tcp_output.c
> > +++ b/net/ipv4/tcp_output.c
> > @@ -2784,6 +2784,10 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
> >                 segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
> >                 if (segs <= 0)
> >                         return;
> > +               /* In case tcp_shift_skb_data() have aggregated large skbs,
> > +                * we need to make sure not sending too big TSO packets.
> > +                */
> > +               segs = min_t(int, segs, tp->gso_segs);
> >
> >                 if (fwd_rexmitting) {
> >  begin_fwd:
> 
> Nice catch, Eric. What do you think about using tcp_tso_autosize()
> instead of tp->gso_segs? The goal would be to get autosized skbs in
> this case, instead of 64KByte skbs. In addition to helping this corner
> case of SACK reneging, this might also help things in general, since
> if we are retransmitting packets then the cwnd and hence pacing rate
> and hence autosized skb length might be smaller now than they were
> when the packets were first sent. Just a thought.

Excellent idea ;)

Here is the v2 patch.

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8bd9911fdd16..e00e972c4e6a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2751,7 +2751,7 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *skb;
 	struct sk_buff *hole = NULL;
-	u32 last_lost;
+	u32 max_segs, last_lost;
 	int mib_idx;
 	int fwd_rexmitting = 0;
 
@@ -2771,6 +2771,7 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 		last_lost = tp->snd_una;
 	}
 
+	max_segs = tcp_tso_autosize(sk, tcp_current_mss(sk));
 	tcp_for_write_queue_from(skb, sk) {
 		__u8 sacked = TCP_SKB_CB(skb)->sacked;
 		int segs;
@@ -2784,6 +2785,10 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 		segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
 		if (segs <= 0)
 			return;
+		/* In case tcp_shift_skb_data() have aggregated large skbs,
+		 * we need to make sure not sending too big TSO packets.
+		 */
+		segs = min_t(int, segs, max_segs);
 
 		if (fwd_rexmitting) {
 begin_fwd: