Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

From: Thomas Gleixner <tglx@linutronix.de>
To: Scott James Remnant <scott@netsplit.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Shishkin <virtuoso@slind.org>,
	linux-kernel@vger.kernel.org, Ken MacLeod <ken@bitsko.slc.ut.us>,
	Shaun Reich <predator106@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@suse.de>,
	Feng Tang <feng.tang@intel.com>, Michael Tokarev <mjt@tls.msk.ru>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	John Stultz <johnstul@us.ibm.com>,
	Chris Friesen <chris.friesen@genband.com>,
	Kay Sievers <kay.sievers@vrfy.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Artem Bityutskiy <dedekind1@gmail.com>,
	Davide Libenzi <davidel@xmailserver.org>,
	linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
	Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes
Date: Fri, 11 Mar 2011 20:56:49 +0100 (CET)	[thread overview]
Message-ID: <alpine.LFD.2.00.1103112054540.2787@localhost6.localdomain6> (raw)
In-Reply-To: <AANLkTi=_DG8uYLxDnXF=so5daUE=w-pxL67mYaFAGkkL@mail.gmail.com>

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On Fri, 11 Mar 2011, Scott James Remnant wrote:
> On Thu, Mar 10, 2011 at 12:25 AM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Wed, 9 Mar 2011 18:01:09 -0800 Scott James Remnant <scott@netsplit.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