linux-kernel - Re: [PATCH] [RFCv2] notify userspace about time changes

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Date:	Wed, 18 Aug 2010 16:09:56 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Alexander Shishkin <virtuoso@...nd.org>
Cc:	lkml@...r.kernel.org, "Kirill A. Shutemov" <kirill@...temov.name>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>, "H. Peter Anvin" <hpa@...or.com>,
	John Kacur <jkacur@...hat.com>,
	Chris Friesen <chris.friesen@...band.com>,
	Kay Sievers <kay.sievers@...y.org>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFCv2] notify userspace about time changes

On Wed, 2010-08-18 at 16:55 +0300, Alexander Shishkin wrote:
> Changes since v1:
>  - updated against 2.6.36-rc1,
>  - added notification/filtering options,
>  - added Documentation/ABI/sysfs-kernel-time-notify interface description.
> 
> 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.
> 
> This patch implements a notification interface via eventfd mechanism. Proccess
> wishing to be notified about time changes should create an eventfd and echo
> its file descriptor along with notification options to /sys/kernel/time_notify.
> 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.

Hey Alexander, 
	Glad to see this work continue! One thing did strike me as odd in
reading over this: Does adjtimex really make sense to trigger a
notification?  Its not actually changing the time, but alters the freq
that time runs. This freq adjustment is transparent to applications or
timers (unlike something like settimeofday).  So I'm not sure I see why
it is included here.

What use case did you have in mind for it?

thanks
-john


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