netdev - [PATCH net-next 1/3] tcp: plug skb_still_in_host

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Date:   Thu, 11 Mar 2021 12:35:04 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neil Spring <ntspring@...com>
Subject: [PATCH net-next 1/3] tcp: plug skb_still_in_host_queue() to TSQ

From: Eric Dumazet <edumazet@...gle.com>

Jakub and Neil reported an increase of RTO timers whenever
TX completions are delayed a bit more (by increasing
NIC TX coalescing parameters)

Main issue is that TCP stack has a logic preventing a packet
being retransmit if the prior clone has not yet been
orphaned or freed.

This logic came with commit 1f3279ae0c13 ("tcp: avoid
retransmits of TCP packets hanging in host queues")

Thankfully, in the case skb_still_in_host_queue() detects
the initial clone is still in flight, it can use TSQ logic
that will eventually retry later, at the moment the clone
is freed or orphaned.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Neil Spring <ntspring@...com>
Reported-by: Jakub Kicinski <kuba@...nel.org>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
 include/linux/skbuff.h |  2 +-
 net/ipv4/tcp_output.c  | 12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0503c917d77301f433122bf34a659bb855763144..483e89348f78b48235748de37ae3ea7ec9450491 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1140,7 +1140,7 @@ static inline bool skb_fclone_busy(const struct sock *sk,
 
 	return skb->fclone == SKB_FCLONE_ORIG &&
 	       refcount_read(&fclones->fclone_ref) > 1 &&
-	       fclones->skb2.sk == sk;
+	       READ_ONCE(fclones->skb2.sk) == sk;
 }
 
 /**
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index fbf140a770d8e21b936369b79abbe9857537acd8..0dbf208a4f2f17c630084e87f4a9a2ad0dc24168 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2775,13 +2775,17 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
  * a packet is still in a qdisc or driver queue.
  * In this case, there is very little point doing a retransmit !
  */
-static bool skb_still_in_host_queue(const struct sock *sk,
+static bool skb_still_in_host_queue(struct sock *sk,
 				    const struct sk_buff *skb)
 {
 	if (unlikely(skb_fclone_busy(sk, skb))) {
-		NET_INC_STATS(sock_net(sk),
-			      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
-		return true;
+		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
+		smp_mb__after_atomic();
+		if (skb_fclone_busy(sk, skb)) {
+			NET_INC_STATS(sock_net(sk),
+				      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
+			return true;
+		}
 	}
 	return false;
 }
-- 
2.31.0.rc2.261.g7f71774620-goog