linux-kernel - Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK

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Date:	Fri, 11 Mar 2011 20:56:49 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Scott James Remnant <scott@...split.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Shishkin <virtuoso@...nd.org>,
	linux-kernel@...r.kernel.org, Ken MacLeod <ken@...sko.slc.ut.us>,
	Shaun Reich <predator106@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Feng Tang <feng.tang@...el.com>,
	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, linux-api@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock
 changes

On Fri, 11 Mar 2011, Scott James Remnant wrote:
> On Thu, Mar 10, 2011 at 12:25 AM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > On Wed, 9 Mar 2011 18:01:09 -0800 Scott James Remnant <scott@...split.com> wrote:
> >
> >> > It would be helpful to know if the identified users of this feature
> >> > actually find it useful and adequate. __I guess the most common
> >> > application is the 1,001 desktop clock widgets. __Do you have any
> >> > feedback from any of the owners of those?
> >> >
> >> cron is another obvious one (or init systems attempting to replace
> >> cron). Having to wakeup and check the time every minute can be
> >> non-conducive to power savings, it would be better if we could just
> >> sleep until the next alarm and be woken up if the time changes in
> >> between.
> >>
> >> (That being said, we also need to poll for and/or check for timezone
> >> changes - but those are entirely userspace, so we can deal with that
> >> separately)
> >
> > Sure, there will be lots of applications.
> >
> > But what I'm asking isn't "it is a good feature".  I'm asking "is the
> > feature implemented well".  Ideally someone would get down and modify
> > cron to use the interface in this patch.
> >
> So I've just been thinking today - and I'm actually not sure whether
> this is needed at all for this case.
> 
> A good cron implementation is going to set timers according to
> CLOCK_REALTIME; in the case where the clock changes forwards, those
> timers will fire as part of the clock changing already no? And in the
> case where the clock changes backwards, you don't want to re-run old
> ones anyway.
> 
> Even the hourly/daily cases are actually at a fixed time, so would be
> triggered - and a decent implementation wouldn't trigger a given
> script more than once.

Yeah, I was wondering about today as well. Though when you set back
your clock several days, stuff might be surprised if it's not woken up
for several days :)

Thanks,

	tglx