linux-kernel - Re: [patch 13/20] timer: Switch to a non cascading wheel

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Date:	Tue, 14 Jun 2016 22:34:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Arjan van de Ven <arjanvandeven@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel

On Tue, Jun 14, 2016 at 08:05:49PM +0200, Thomas Gleixner wrote:
> On Tue, 14 Jun 2016, Arjan van de Ven wrote:
> 
> > evaluating a 120 hours timer ever 37 hours to see if it should fire...
> > not too horrid.
> 
> Well that thing is doing weird stuff anyway:
> 
>    swapper     0 [001] 1789995.305532: timer:timer_start: timer=0xffff8800c8346920 function=death_by_timeout expires=4850639994 [timeout=108000000]

You're running a HZ=250 kernel, right?

inet/netfilter/nf_conntrack_proto_tcp.c:

#define SECS * HZ
#define MINS * 60 SECS
#define HOURS * 60 MINS
#define DAYS * 24 HOURS

static unsigned int tcp_timeouts[TCP_CONNTRACK_TIMEOUT_MAX] __read_mostly = {
	[TCP_CONNTRACK_SYN_SENT]	= 2 MINS,
	[TCP_CONNTRACK_SYN_RECV]	= 60 SECS,
	[TCP_CONNTRACK_ESTABLISHED]	= 5 DAYS,

					  ^^^^^^ that
ends up being 108000000 for HZ == 250.


	[TCP_CONNTRACK_FIN_WAIT]	= 2 MINS,
	[TCP_CONNTRACK_CLOSE_WAIT]	= 60 SECS,
	[TCP_CONNTRACK_LAST_ACK]	= 30 SECS,
	[TCP_CONNTRACK_TIME_WAIT]	= 2 MINS,
	[TCP_CONNTRACK_CLOSE]		= 10 SECS,
	[TCP_CONNTRACK_SYN_SENT2]	= 2 MINS,
/* RFC1122 says the R2 limit should be at least 100 seconds.
   Linux uses 15 packets as limit, which corresponds
   to ~13-30min depending on RTO. */
	[TCP_CONNTRACK_RETRANS]		= 5 MINS,
	[TCP_CONNTRACK_UNACK]		= 5 MINS,
};


>              ssh  3870 [001] 1790025.284704: timer:timer_cancel: timer=0xffff8800c8346920
>              ssh  3870 [001] 1790025.284707: timer:timer_start: timer=0xffff8800c8346920 function=death_by_timeout expires=4742722493 [timeout=75000]

And that one would then be one of the 5 MINS ones.