netdev - Re: [PATCHv3 linux-next] hrtimer: Add notifier when clock_was

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Date:	Tue, 17 Sep 2013 15:47:26 +0800
From:	Fan Du <fan.du@...driver.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	David Miller <davem@...emloft.net>, <herbert@...dor.hengli.com.au>,
	<steffen.klassert@...unet.com>, <dborkman@...hat.com>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCHv3 linux-next] hrtimer: Add notifier when clock_was_set
 was called



On 2013年09月16日 17:01, Thomas Gleixner wrote:
> On Mon, 16 Sep 2013, Fan Du wrote:
>> On 2013年09月13日 22:32, Thomas Gleixner wrote:
>>> On Fri, 13 Sep 2013, Fan Du wrote:
>>>> (2) What I have been bugging you around here for this long time is really
>>>> the
>>>> second
>>>>       problem, I'm sorry I didn't make it clearly to you and others, which
>>>> is
>>>> below:
>>>>
>>>>       Why using wall clock time to calculate soft/hard IPsec events when
>>>> xfrm_state timer
>>>>       out happens in its timeout handler? Because even if xfrm_state using
>>>> CLOCK_BOOTTIME,
>>>>       system wall clock time changing will surely disturb soft/hard IPsec
>>>> events, which
>>>>       you raised your concern about in (*a*).
>>>
>>> No CLOCK_BOOTTIME is not affected by wall clock time changes. It's
>>> basically CLOCK_MONOTONIC, it just keeps counting the suspend time as
>>> well. So without a suspend/resume cycle CLOCK_MONOTONIC and
>>> CLOCK_BOOTTIME are the same. After a suspend/resume cycle
>>> CLOCK_BOOTTIME will be N seconds ahead of CLOCK_MONOTONIC, where N is
>>> the number of seconds spent in suspend.
>>
>> Sorry for the ambiguity. Yes, CLOCK_BOOTTIME is monotonic *and* counting
>> suspend/resume time as well as not affected by wall clock time change.
>>
>> Using CLOCK_BOOTTIME guarantees b- will timeout in a right manner, i.e., not
>> affected by wall clock time change, as well as keep the timer valid when
>> suspend/resume.
>>
>> A classic way of using timer is:
>>   a- Arm a timer with specified interval
>>   b- Wait for the timer to timeout
>>   c- After the timeout, notify the event to other place in the timer handler.
>>
>> IPsec key lifetime timer does its this way:
>>   a- Arm a timer with specified interval
>>   b- Wait for the timer to timeout
>>   c- After timeout, in the timer handler, using wall clock time to calculate
>>     which kind of event to notify user(soft or hard for both key use time,
>>     and key created time). So the problem arises at this stage if wall clock
>>     time changed.
>
> And why on earth must it use wall clock time for selecting the event
> type?

/*shivering... sorry to bother you again.*/

Yep, it's a bit twisted. This parts of codes come a long way from v2.5.67
Beyond this fact, I barely know its original design goal by doing so.
The first version of this patch is to remove the wall clock time things
by myself, it's a pity that the feedback is not very welcome at the end.
So later on adding notifier at clock_was_set is suggested by David.

> Thanks,
>
> 	tglx

-- 
浮沉随浪只记今朝笑

--fan
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