linux-kernel - RE: TCP keepalive timer problem

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Message-ID: <0939B589FC103041945B9F13274963E303BA96BB@CORPUSMX90A.corp.emc.com>
Date:	Thu, 27 Aug 2009 21:55:35 -0400
From:	<Li_Xin2@....com>
To:	<eric.dumazet@...il.com>, <andi@...stfloor.org>
Cc:	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: RE: TCP keepalive timer problem

 > Yep, so to recap we have two changes :

> 1) The one I sent (taking into account the time of last ACK we
received) to compute the
> timer delays.

> 2) The one you suggest, avoiding to send a probe if we have packets in
flight, relying
> on normal retransmits timers.

I agree with these two changes, but I think the patch for point 2 given
by Eric is not correct:

elapsed = keepalive_time_when(tp);

 if (tp->packets_out || tcp_send_head(sk))
 	if (tcp_retries2_time < keepalive_time_when(tp))
 		goto resched;
 elapsed = tcp_time_stamp - tp->rcv_tstamp;

The problem is: you should not always compare tcp_retries2_time with
keepalive_time_when. If the first keepalive probe is already sent, you
should compare with keepalive_intvl_when
 ( or keepalive_intvl_when * (
tp->keepalive_probes?:sysctl_tcp_keepalive_probes -
icsk->icsk_probes_out ) if keepalive probe is already sent, and the
elapsed time also needs to take that into account.

elapsed =
icsk->icsk_probes_out?keepalive_intvl_when(tp):keepalive_time_when(tp);

 if (tp->packets_out || tcp_send_head(sk))
 	if (tcp_retries2_time < (icsk->icsk_probes_out?
keepalive_intvl_when(tp)  : keepalive_time_when(tp))
 		goto resched;
 elapsed = tcp_time_stamp - tp->rcv_tstamp;

How do you think?
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