Date: Thu, 10 Mar 2011 10:08:18 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Kay Sievers <kay.sievers@vrfy.org>
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>,
        "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
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On Thu, 10 Mar 2011, Kay Sievers wrote:

> On Thu, Mar 10, 2011 at 01:25, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Wed,  9 Mar 2011 16:36:51 +0200
> > Alexander Shishkin <virtuoso@slind.org> wrote:
> >
> >> Changes since v3:
> >>  - changed timerfd_settime() semantics (see below)
> >> Changes since v2:
> >>  - replaced sysfs interface with a syscall
> >>  - added sysctl/procfs handle to set a limit to the number of users
> >>  - fixed issues pointed out by Greg.
> >> Changes since v1:
> >>  - updated against 2.6.36-rc1,
> >>  - added notification/filtering options,
> >>  - added Documentation/ABI/sysfs-kernel-time-notify interface description.
> 
> > 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?
> 
> We want it for systemd, to provide cron-like functionality but without
> the need to stupidly wake up every minute and check the system time
> for possible jumps.
> 
> It also sounds useful for a generic resume (after system suspend)
> notification for applications, which isn't really possible today.

Note, that we have CLOCK_BOOTTIME pending for .39 which aims at the
same problem. It's basically CLOCK_MONOTONIC adjusted by the time we
were in suspend. So while CLOCK_MONOTONIC timers are not aware of the
time spent in suspend CLOCK_BOOTTIME timers are. The reason for
implementing CLOCK_BOOTTIME was basically the same problem.

Thanks,

	tglx
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