From: Arnd Bergmann <arnd@arndb.de>
To: Alexander Shishkin <virtuoso@slind.org>
Subject: Re: [PATCHv6 1/7] notify userspace about time changes
Date: Thu, 11 Nov 2010 21:55:17 +0100
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Cc: linux-kernel@vger.kernel.org, John Stultz <johnstul@us.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Kay Sievers <kay.sievers@vrfy.org>,
        Greg KH <gregkh@suse.de>, Chris Friesen <chris.friesen@genband.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Thomas Gleixner <tglx@linutronix.de>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Jon Hunter <jon-hunter@ti.com>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>, Avi Kivity <avi@redhat.com>,
        John Kacur <jkacur@redhat.com>, mtk.manpages@gmail.com
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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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