linux-kernel - Re: [PATCH 1/7] notify userspace about time changes

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Date:	Fri, 17 Sep 2010 13:07:43 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Alexander Shishkin <virtuoso@...nd.org>,
	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>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jon Hunter <jon-hunter@...com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Avi Kivity <avi@...hat.com>, John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH 1/7] notify userspace about time changes

On Fri, 17 Sep 2010, Alan Cox wrote:

> > Certain userspace applications (like "clock" desktop applets or cron) might
> > want to be notified when some other application changes the system time. It
> > might also be important for an application to be able to distinguish between
> > its own and somebody else's time changes.
> 
> A program that cannot work out if it or someone else changed the time is
> very very broken indeed !
> 
> > This patch implements a notification interface via eventfd mechanism. Proccess
> > wishing to be notified about time changes should create an eventfd and pass it
> > to time_change_notify() syscall along with notification options.
> 
> This seems complete overkill and it doesn't really help applications much
> that I can see because of suspend/resume.
> 
> What are your actual use cases ?
> 
> Clocks apps don't care because they check the actual time so notice it
> shfited. Cron and anacron appear to contain the needed internal handling.
> 
> Anything sleeping until a time occurs maybe ? In which case its a lot
> simpler and cleaner than events to provide a new itimer which wakes the
> process when the wall time hits the time specified in the timer.

We already have that. posix timers provide this.

The only case I can imagine where a notification might be interesting
is when something armed an absolute timer on CLOCK_REALTIME and time
is set backwards.

Thanks,

	tglx
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