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

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Date:	Fri, 03 Dec 2010 16:57:52 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Valdis.Kletnieks@...edu, Lennart Poettering <mzxreary@...inter.de>,
	Alexander Shishkin <virtuoso@...nd.org>,
	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>,
	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
Subject: Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for
 clock changes

On Wed, 2010-12-01 at 17:55 -0800, john stultz wrote:
> On Thu, 2010-12-02 at 01:18 +0000, Jamie Lokier wrote:
> > Valdis.Kletnieks@...edu wrote:
> > > On Wed, 01 Dec 2010 10:43:59 GMT, Jamie Lokier said:
> > > 
> > > > So maybe CLOCK_MONOTONIC should be changed to include elapsed time
> > > > during suspend/resume, and CLOCK_MONOTONIC_RAW could remain as it is,
> > > > for programs that want that?
> > > 
> > > Wouldn't that be an API break for programs that are expecting the current
> > > behavior of CLOCK_MONOTONIC?  Yes, there should be a way to request either of
> > > them - but if there's only one way now, it should continue to act the current
> > > way, and the added way is the second option.
> > 
> > I don't know.  Can you think of any program which would break if
> > suspend/resume's clocks behaved like ordinary task scheduling - when a
> > task doesn't run for a long time because of scheduling decisions?
> > Hmm, I guess some realtime apps might like to know.
> 
> Like I mentioned earlier, CLOCK_MONOTONIC_RAW and CLOCK_MONOTONIC are
> tightly tied, so anything using CLOCK_MONOTONIC_RAW would break.
> 
> It might be possible to change both, but I still think such a change
> would be bad.

So actually, as I think more about this, I'm starting to come around to
the side that maybe CLOCK_MONOTONIC should be changed to increment
during suspend (CLOCK_MONOTONIC_RAW could also be moved forward by the
same amount, which isn't really ideal, but maybe not problematic).

There are still quite a number of problems that might be caused by such
a change. So it may still be impractical to actually do, but more and
more it does seem like it might be the better approach.

I keep thinking about it.

thanks
-john


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