linux-kernel - Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK

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Message-ID: <20101201003358.GJ26184@shisha.kicks-ass.net>
Date:	Wed, 1 Dec 2010 02:33:58 +0200
From:	Alexander Shishkin <virtuoso@...nd.org>
To:	Lennart Poettering <mzxreary@...inter.de>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Feng Tang <feng.tang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Tokarev <mjt@....msk.ru>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	John Stultz <johnstul@...ibm.com>,
	Chris Friesen <chris.friesen@...band.com>,
	Kay Sievers <kay.sievers@...y.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	linux-fsdevel@...r.kernel.org,
	Alexander Shishkin <virtuoso@...nd.org>
Subject: Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for
 clock changes

On Tue, Nov 23, 2010 at 11:43:46PM +0100, Lennart Poettering wrote:
> On Tue, 23.11.10 19:22, Alexander Shishkin (virtuoso@...nd.org) wrote:
> 
> > Certain userspace applications (like "clock" desktop applets or cron or
> > systemd) 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;
> >  - rearming CLOCK_REALTIME timers when said changes happen;
> >  - changing system timekeeping policy for system-wide time management
> >    programs;
> >  - keeping guest applications/operating systems running in emulators
> >    up to date.
> > 
> > This is another attempt to approach notifying userspace about system
> > clock changes. The other one is using an eventfd and a syscall [1]. In
> > the course of discussing the necessity of a syscall for this kind of
> > notifications, it was suggested that this functionality can be achieved
> > via timers [2] (and timerfd in particular [3]). This idea got quite
> > some support [4], [5], [6] and some vague criticism [7], so I decided
> > to try and go a bit further with it.
> 
> I agree with Kay, this is pretty much exactly what we want for
> systemd. (Assuming that the time jump due to system suspend is
> propagated to userspace like any other time jump with this path).

Good point, I'll add the notification to the resume point.

> So yeah, I'd be very happy if this could be merged.

Good to hear.

Regards,
--
Alex
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