[PATCH net] Revert "tcp: change pingpong threshold to 3"

[PATCH net] Revert "tcp: change pingpong threshold to 3"

[Wei Wang]

This reverts commit 4a41f453bedfd5e9cd040bad509d9da49feb3e2c.

This to-be-reverted commit was meant to apply a stricter rule for the
stack to enter pingpong mode. However, the condition used to check for
interactive session "before(tp->lsndtime, icsk->icsk_ack.lrcvtime)" is
jiffy based and might be too coarse, which delays the stack entering
pingpong mode.
We revert this patch so that we no longer use the above condition to
determine interactive session, and also reduce pingpong threshold to 1.

Reported-by: LemmyHuang <hlm3280@163.com>
Suggested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Wei Wang <weiwan@google.com>
---
 include/net/inet_connection_sock.h | 10 +---------
 net/ipv4/tcp_output.c              | 15 ++++++---------
 2 files changed, 7 insertions(+), 18 deletions(-)

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 85cd695e7fd1..ee88f0f1350f 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -321,7 +321,7 @@ void inet_csk_update_fastreuse(struct inet_bind_bucket *tb,
 
 struct dst_entry *inet_csk_update_pmtu(struct sock *sk, u32 mtu);
 
-#define TCP_PINGPONG_THRESH	3
+#define TCP_PINGPONG_THRESH	1
 
 static inline void inet_csk_enter_pingpong_mode(struct sock *sk)
 {
@@ -338,14 +338,6 @@ static inline bool inet_csk_in_pingpong_mode(struct sock *sk)
 	return inet_csk(sk)->icsk_ack.pingpong >= TCP_PINGPONG_THRESH;
 }
 
-static inline void inet_csk_inc_pingpong_cnt(struct sock *sk)
-{
-	struct inet_connection_sock *icsk = inet_csk(sk);
-
-	if (icsk->icsk_ack.pingpong < U8_MAX)
-		icsk->icsk_ack.pingpong++;
-}
-
 static inline bool inet_csk_has_ulp(struct sock *sk)
 {
 	return inet_sk(sk)->is_icsk && !!inet_csk(sk)->icsk_ulp_ops;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c38e07b50639..d06e72e141ac 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -167,16 +167,13 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
 	if (tcp_packets_in_flight(tp) == 0)
 		tcp_ca_event(sk, CA_EVENT_TX_START);
 
-	/* If this is the first data packet sent in response to the
-	 * previous received data,
-	 * and it is a reply for ato after last received packet,
-	 * increase pingpong count.
-	 */
-	if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
-	    (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
-		inet_csk_inc_pingpong_cnt(sk);
-
 	tp->lsndtime = now;
+
+	/* If it is a reply for ato after last received
+	 * packet, enter pingpong mode.
+	 */
+	if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
+		inet_csk_enter_pingpong_mode(sk);
 }
 
 /* Account for an ACK we sent. */
-- 
2.37.0.170.g444d1eabd0-goog

Re: [PATCH net] Revert "tcp: change pingpong threshold to 3"

[Neal Cardwell]

On Wed, Jul 20, 2022 at 7:32 PM Wei Wang <weiwan@google.com> wrote:
>
> This reverts commit 4a41f453bedfd5e9cd040bad509d9da49feb3e2c.
>
> This to-be-reverted commit was meant to apply a stricter rule for the
> stack to enter pingpong mode. However, the condition used to check for
> interactive session "before(tp->lsndtime, icsk->icsk_ack.lrcvtime)" is
> jiffy based and might be too coarse, which delays the stack entering
> pingpong mode.
> We revert this patch so that we no longer use the above condition to
> determine interactive session, and also reduce pingpong threshold to 1.
>
> Reported-by: LemmyHuang <hlm3280@163.com>
> Suggested-by: Neal Cardwell <ncardwell@google.com>
> Signed-off-by: Wei Wang <weiwan@google.com>
> ---

Thanks, Wei. The substance of the change looks good to me. I would
suggest adding a Fixes: tag as the first footer in the commit message
to make sure this gets backported to the appropriate stable branches.
Otherwise this looks great.

Thanks!
neal

Re: [PATCH net] Revert "tcp: change pingpong threshold to 3"

[Wei Wang]

On Thu, Jul 21, 2022 at 5:21 AM Neal Cardwell <ncardwell@google.com> wrote:
>
> On Wed, Jul 20, 2022 at 7:32 PM Wei Wang <weiwan@google.com> wrote:
> >
> > This reverts commit 4a41f453bedfd5e9cd040bad509d9da49feb3e2c.
> >
> > This to-be-reverted commit was meant to apply a stricter rule for the
> > stack to enter pingpong mode. However, the condition used to check for
> > interactive session "before(tp->lsndtime, icsk->icsk_ack.lrcvtime)" is
> > jiffy based and might be too coarse, which delays the stack entering
> > pingpong mode.
> > We revert this patch so that we no longer use the above condition to
> > determine interactive session, and also reduce pingpong threshold to 1.
> >
> > Reported-by: LemmyHuang <hlm3280@163.com>
> > Suggested-by: Neal Cardwell <ncardwell@google.com>
> > Signed-off-by: Wei Wang <weiwan@google.com>
> > ---
>
> Thanks, Wei. The substance of the change looks good to me. I would
> suggest adding a Fixes: tag as the first footer in the commit message
> to make sure this gets backported to the appropriate stable branches.
> Otherwise this looks great.

Thanks. Will send v2 with Fixes tag.

>
> Thanks!
> neal