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

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Date:	Mon, 16 Sep 2013 11:01:00 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Fan Du <fan.du@...driver.com>
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 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?

Thanks,

	tglx