linux-kernel - Re: [tip:timers/core] timerfd: Allow timers to be cancelled when clock was set

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Date:	Wed, 11 May 2011 12:23:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	kay.sievers@...y.org, johnstul@...ibm.com, davidel@...ilserver.org,
	virtuoso@...nd.org, tglx@...utronix.de, chris.friesen@...band.com,
	kirill@...temov.name
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] timerfd: Allow timers to be cancelled when
 clock was set

On Mon, 2011-05-02 at 19:44 +0000, tip-bot for Thomas Gleixner wrote:
> Commit-ID:  99ee5315dac6211e972fa3f23bcc9a0343ff58c4
> Gitweb:     http://git.kernel.org/tip/99ee5315dac6211e972fa3f23bcc9a0343ff58c4
> Author:     Thomas Gleixner <tglx@...utronix.de>
> AuthorDate: Wed, 27 Apr 2011 14:16:42 +0200
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Mon, 2 May 2011 21:39:15 +0200
> 
> timerfd: Allow timers to be cancelled when clock was set
> 
> Some applications must be aware of clock realtime being set
> backward. A simple example is a clock applet which arms a timer for
> the next minute display. If clock realtime is set backward then the
> applet displays a stale time for the amount of time which the clock
> was set backwards. Due to that applications poll the time because we
> don't have an interface.

Shouldn't that clock applet use CLOCK_MONOTONIC for the timer anyway?

Furthermore, weren't there patches to provide clkadjust notifiers? In
which case we can have the app woken up and it can fiddle its own
timers.

> Extend the timerfd interface by adding a flag which puts the timer
> onto a different internal realtime clock. All timers on this clock are
> expired whenever the clock was set.

I don't much like adding random clocks like that, the extra timer base
makes many timer operations more expensive, and I really don't see this
being worth it :/

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