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

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Message-ID: <20101202011832.GO22787@shareable.org>
Date:	Thu, 2 Dec 2010 01:18:33 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Valdis.Kletnieks@...edu
Cc:	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>,
	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
Subject: Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

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.

Currently CLOCK_MONOTONIC jumps forwards by 4 seconds on
suspend/resume anyway (as seen by userspace), on my x86 laptop running
2.6.37-rc3.  So it does already jump a bit...

But see my other reply; maybe there's no need to change it.  A
reliable, immediate notification that CLOCK_MONOTONIC's relationship
to real time has been disrupted by an unknown amount would be
sufficient for the problems I have in mind.

-- Jamie

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