linux-kernel - Re: [PATCHv6 1/7] notify userspace about time changes

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Date:	Thu, 11 Nov 2010 21:55:17 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexander Shishkin <virtuoso@...nd.org>
Cc:	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	mtk.manpages@...il.com
Subject: Re: [PATCHv6 1/7] notify userspace about time changes

On Thursday 11 November 2010 20:29:56 Alexander Shishkin wrote:
> Certain userspace applications (like "clock" desktop applets or cron) might
> want to be notified when some other application changes the system time.
> There are several known to me reasons for this:
>  - avoiding periodic wakeups to poll time changes;
>  - changing system timekeeping policy for system-wide time management
>    programs;
>  - keeping guest applications/operating systems running in emulators
>    up to date.
> 
> 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. After that, any calls to settimeofday()/
> stime()/adjtimex() made by other processes will be signalled to this eventfd.
> Credits for suggesting the eventfd mechanism for this purpose go to Kirill
> Shutemov.
> 
> This patch adds the syscall to asm-generic/unistd.h and a simple usage
> example.

Looks reasonable to me.

It would be good to have the man page for this, too. I guess it could
be added to the existing clock_{get,set}time man page, so you can
add a patch for that.

	Arnd
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