Re: [PATCH] tcp: properly update lost_cnt_hint during shifting

On 09/28/2011 04:17 PM, Ilpo Järvinen wrote:

On Wed, 28 Sep 2011, Yan, Zheng wrote:

quoted

lost_skb_hint is used by tcp_mark_head_lost() to mark the first
unhandled skb. lost_cnt_hint is the number of sacked packets before
the lost_skb_hint. tcp_shifted_skb() shouldn't increase lost_cnt_hint
when shifting a sacked skb that is before the lost_skb_hint, because
packets in it are already counted.

Signed-off-by: Zheng Yan <redacted>
---

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 21fab3e..f712ace 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -1390,9 +1390,14 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
 	BUG_ON(!pcount);
 
 	/* Tweak before seqno plays */
-	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
-	    !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
-		tp->lost_cnt_hint += pcount;
+	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint) {
+		if (skb == tp->lost_skb_hint)
+			tp->lost_cnt_hint += pcount;
+		else if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
+			 before(TCP_SKB_CB(skb)->seq,
+				TCP_SKB_CB(tp->lost_skb_hint)->seq))
+			tp->lost_cnt_hint += pcount;
+	}

Ah right, the hole filled case which shifts not only the newly SACKed 
skb but also the next, already SACKed skb?

I fail to see why you needed to change !before into two checks though:
 skb == tp->lost_skb_hint and before(params reversed) ? Shouldn't the 
equality that is provided by the negation cover for the == check (and the 
params reversion isn't necessary in any case)? In fact, isn't the skb == 
tp->lost_skb_hint check strictly wrong without the same TCPCB_SACKED_ACKED 
guard (though I'm not sure, I didn't check, if the hint can ever point to 
such a segment in the first place)?

Thanks you for your reply.

skb == tp->lost_skb_hint is special.

If the skb is sacked and we shift 'pcount' packets to previous skb,
these packets will not be counted by future tcp_mark_head_lost() call.
So we should increase lost_cnt_hint.

If the skb is not sacked, the skb will be sacked soon by tcp_sacktag_one(),
So we should not increase lost_cnt_hint.

I didn't think out the second case. I think the correct patch should be:
---

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 21fab3e..dcc2411 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -1390,9 +1390,15 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
 	BUG_ON(!pcount);
 
 	/* Tweak before seqno plays */
-	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
-	    !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
-		tp->lost_cnt_hint += pcount;
+	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint) {
+		if ((TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
+		    skb == tp->lost_skb_hint)
+			tp->lost_cnt_hint += pcount;
+		else if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
+			 before(TCP_SKB_CB(skb)->seq,
+				TCP_SKB_CB(tp->lost_skb_hint)->seq))
+			tp->lost_cnt_hint += pcount;
+	}
 
 	TCP_SKB_CB(prev)->end_seq += shifted;
 	TCP_SKB_CB(skb)->seq += shifted;
---

Added Cc to Nandita as they're hunting (possibly other) bug in 
tcp_mark_head_lost.