linux-kernel - Re: [PATCH] sky2: Use deferrable timer for watchdog

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Message-ID: <476ADDA6.9070107@intel.com>
Date:	Thu, 20 Dec 2007 13:24:54 -0800
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
CC:	Parag Warudkar <parag.warudkar@...il.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	netdev@...r.kernel.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sky2: Use deferrable timer for watchdog

Stephen Hemminger wrote:
> On Thu, 20 Dec 2007 15:36:13 -0500
> "Parag Warudkar" <parag.warudkar@...il.com> wrote:
> 
>> On Dec 20, 2007 3:04 PM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>>> I think it is reasonable for Network driver watchdogs to use a
>>>> deferrable timer - if the machine is 100% IDLE there is no one needing
>>>> the network to be up. If there is something running even on the other
>>>> CPU - that is going to cause an IPI, reschedule, TLB invalidation etc.
>>>> which will make it very likely in practice that each CPU will be
>>>> interrupted in reasonable amount of time.
>>> this is not correct; many machines are idle waiting for network data. Think of webservers...
>> Yes, I forgot the receive case. So if a server was 100% IDLE and a web
>> server was listening for network data and we reach 0 wakeups per
>> second on the CPU where the network watchdog timer is scheduled to run
>> deferred _and_ the network link went down, it would cause the watchdog
>> to not run and redo the link until some one else wakes up that CPU
>> later.
>> So as long as we make sure we don't convert every timer to deferrable
>> we should be ok - may be this can be resolved easily by having a
>> non-deferrable "dont-allow-deferring-for-too-long" timer on each CPU
>> that just causes at least one wake up in some reasonable time delta
>> from the previous wakeup (whoever caused that one.) It is still
>> beneficial in that all deferrable timers would run at once without
>> needing to have separate wakeup for each.
>>
>>>> Of course there are theoretical cases where we could land into a
>>>> situation where a CPU in a multiprocessor machine is IDLE infinitely
>>>> and that causes the watchdog that happens to be bound to run on the
>>>> same CPU to not run. To take care of these unlikely cases I think the
>>>> timer mechanism should have a reasonable limit on how long a CPU can
>>>> go IDLE if there are deferrable timers.
>>> how about something else instead: a timer mechanism that takes a range instead..
>>> that at least has defined semantics; the deferrable semantics really are "indefinite".
>>> Lets keep at least the semantics clear and clean.
>>>
>> Would not the simpler solution of installing a non-deferrable timer
>> per cpu which will not allow the CPU to go IDLE for more than x units
>> of time at once  (or something to that effect) work? Range would
>> complicate the thing and I am not sure how many cases will know
>> reasonably correct range for their normal operation. In this instance
>> of the e1000 watchdog what range could it give and be successful at
>> what it wants to do - bring up the link in reasonable amount of time,
>> while also realizing the power savings?
>>
>> Perhaps depending on Server/Laptop/Desktop machine (may be based on
>> Preemption) we could have normal or deferrable timers but that'll
>> exclude Servers from power savings and I am not sure Data center folks
>> will like that :) .
>>
>> Parag
> 
> 
> The problem is that on a server the receiver will go deaf if the chip
> bug that the watchdog is looking for triggers.  Yes, no packets in
> and it happily will just sit there.
> 
> So for now, I am not going to apply your simple patch and work on a 
> two stage timer per arjan's suggestion for a later release.

I also think that's the right way to go for now. I'll ask jeff to hold off on the
two patches for now.

Auke

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