linux-kernel - Re: [PATCHv5 0/7] system time changes notification

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Date:	Fri, 17 Sep 2010 14:57:41 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alexander Shishkin <virtuoso@...nd.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kay Sievers <kay.sievers@...y.org>, Greg KH <gregkh@...e.de>,
	Chris Friesen <chris.friesen@...band.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCHv5 0/7] system time changes notification

On Fri, 17 Sep 2010, Alexander Shishkin wrote:

> Hi,
> 
> This is the fifth version of system time change notification mechanism
> for linux kernel. The need for it comes from applications which would
> like to keep track of time changes without having to wake up every
> $TIMEOUT and calling gettimeofday().
> 
> An excellent description for one of the usecases, written by Kay Sievers
> (http://kerneltrap.org/mailarchive/linux-kernel/2010/8/5/4603531):
> """
> This is the example Lennart and I thought about when we considered
> adding cron-like stuff to systemd's timer configs, but didn't want to
> do silly things like scheduled checks for the actual time, so we
> delayed this feature until such a notification becomes available.
> 
> Consider we want stuff like "wakeup every day at 3pm", the next wakeup
> might be earlier than the timer we calculated last time, on system
> time changes. We need to re-calculate it. This is necessary for all
> repeating events.
> 
> Say we want to wakeup at 3pm, now it's 4pm, so we schedule it in 23
> hours. Now the system time changes to 2pm, and we would expect to
> wakeup in one hour, but we take 25.

And that's why we have posix-timers with the ability to arm absolute
timers. They already deal with the clock being set.

man timer_settime

Thanks,

	tglx
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