Re: [PATCH net-next] tcp: better retrans tracking for defer-accept

On Sun, 2012-10-28 at 01:29 +0300, Julian Anastasov wrote:

	Hello,

On Sat, 27 Oct 2012, Eric Dumazet wrote:

quoted

From: Eric Dumazet <redacted>

For passive TCP connections using TCP_DEFER_ACCEPT facility,
we incorrectly increment req->retrans each time timeout triggers
while no SYNACK is sent.

SYNACK are not sent for TCP_DEFER_ACCEPT that were established (for wich
we received the ACK from client). Only the last SYNACK is
sent so that we can receive again an ACK from client, to move the
req into accept queue. We plan to change this later to avoid
the useless retransmit (and potential problem as this SYNACK could be
lost)

TCP_INFO later gives wrong information to user, claiming imaginary
retransmits.

Decouple req->retrans field into two independent fields :

num_retrans : number of retransmit
num_timeout : number of timeouts

num_timeout is the counter that is incremented at each timeout,
regardless of actual SYNACK being sent or not, and used to
compute the exponential timeout.

Introduce inet_rtx_syn_ack() helper to increment num_retrans
only if ->rtx_syn_ack() succeeded.

Use inet_rtx_syn_ack() from tcp_check_req() to increment num_retrans
when we re-send a SYNACK in answer to a (retransmitted) SYN.
Prior to this patch, we were not counting these retransmits.

Change tcp_v[46]_rtx_synack() to increment TCP_MIB_RETRANSSEGS
only if a synack packet was successfully queued.

Reported-by: Yuchung Cheng <redacted>
Signed-off-by: Eric Dumazet <redacted>
Cc: Julian Anastasov <ja@ssi.bg>
Cc: Vijay Subramanian <redacted>
Cc: Elliott Hughes <redacted>
Cc: Neal Cardwell <ncardwell@google.com>
---

quoted

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index ba48e79..b236ef0 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c

@@ -340,7 +340,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	}
 
 	req->expires	= 0UL;
-	req->retrans	= 0;
+	req->num_retrans = 0;

	also req->num_timeout = 0; ?

This is not needed here.

We construct a temporary req only to be able to use get_cookie_sock()
and build a full socket with :
 icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);

By definition this req wont ever be used in SYN_RECV state, and never be
part of the timer wheel.

We init req->num_retrans to 0 because this field will be the source of

newtp->total_retrans = req->num_retrans;

in tcp_v{4|6}_syn_recv_sock()

quoted

 
 	/*
 	 * We need to lookup the route here to get at the correct

quoted

diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 182ab9a..4016197 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c

@@ -214,7 +214,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 		ireq6->iif = inet6_iif(skb);
 
 	req->expires = 0UL;
-	req->retrans = 0;
+	req->num_retrans = 0;

	also req->num_timeout = 0; ?

Same thing : this is not needed

quoted

 	ireq->ecn_ok		= ecn_ok;
 	ireq->snd_wscale	= tcp_opt.snd_wscale;
 	ireq->sack_ok		= tcp_opt.sack_ok;

quoted

@@ -1866,7 +1869,7 @@ static void get_openreq6(struct seq_file *seq,
 		   0,0, /* could print option size, but that is af dependent. */
 		   1,   /* timers active (only the expire timer) */
 		   jiffies_to_clock_t(ttd),
-		   req->retrans,
+		   req->num_timeout,

	num_timeout already noted by Neal

Yes, thanks. source patch was fine, I messed the rebase.

quoted

 		   from_kuid_munged(seq_user_ns(seq), uid),
 		   0,  /* non standard timer */
 		   0, /* open_requests have no inode */

	I don't see any problems with such patch, just
check if it is for correct tree, I see some atomic operations
with qlen_young in patch.

Arg, my net-next tree got some stuff I forgot to revert, sorry, will
cleanup.

	One thing to finally decide: should we use limit for
retransmissions or for timeout, is the following better?:

	if (!rskq_defer_accept) {
		*expire = req->num_retrans >= thresh;
			       ^^^^^^^^^^^
		*resend = 1;
		return;
	}

Not sure it matters and if this decision is part of this patch.

If a retransmit fails, it seems we zap the request anyway ?

inet_rtx_syn_ack() returns an error and inet_rsk(req)->acked is false ->
we remove the req from queue.

We dont remove the req only if we got a listen queue overflow in
tcp_check_req() : we set acked to 1 in this case.

listen_overflow:
	if (!sysctl_tcp_abort_on_overflow) {
		inet_rsk(req)->acked = 1;
		return NULL;
	}

Using number of timeouts seems better to me. There is no point holding a
req forever if we fail to retransmit SYNACKS.
Client probably gave up.

Thanks a lot for the review !