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

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Date:	Tue, 14 Jun 2016 14:17:32 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	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 1:42 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.
>

Original TCP RFCs tell timeout is infinite ;)

Practically, conntrack has a 5 days timeout, but I really doubt anyone
expects an idle TCP flow to stay 'alive' when nothing is sent for 5
days.