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

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Date:	Tue, 14 Jun 2016 22:42:25 +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]
>              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]
>          swapper     0 [001] 1790025.330514: timer:timer_cancel: timer=0xffff8800c8346920
>          swapper     0 [001] 1790025.330515: timer:timer_start: timer=0xffff8800c8346920 function=death_by_timeout expires=4850647504 [timeout=108000000]
>              ssh  3870 [001] 1790055.307058: timer:timer_cancel: timer=0xffff8800c8346920
>              ssh  3870 [001] 1790055.307060: timer:timer_start: timer=0xffff8800c8346920 function=death_by_timeout expires=4742730003 [timeout=75000]
>          swapper     0 [001] 1790055.352146: timer:timer_cancel: timer=0xffff8800c8346920
> 
> And that goes on forever. 2834 such sequences for this particular timer
> instance in 4.5 hours. 90000 sequences total for all timers related to
> death_by_timeout in 4.5 hours
> 
> No idea what this is doing and why the heck it nees a 120 hour timeout ....

So it moves that timer on every packet for that TCP connection stream,
provided the expiration is at least 1 second behind.

If the stream hasn't had a packet in 5 days (see previous email), then
the connection state is destroyed.

Its been too long since I've read the TCP RFCs, but I can imagine
changing this will upset people.