linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

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Message-Id: <1243530425.29511.467.camel@jstultz-laptop>
Date:	Thu, 28 May 2009 10:07:05 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Daniel Walker <dwalker@...o99.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	John Stultz <johnstul@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().

On Fri, 2009-05-29 at 01:40 +0900, Paul Mundt wrote:
> On Thu, May 28, 2009 at 06:32:09PM +0200, Peter Zijlstra wrote:
> > On Thu, 2009-05-28 at 09:13 -0700, Daniel Walker wrote:
> > > On Thu, 2009-05-28 at 14:59 +0200, Peter Zijlstra wrote:
> > > > CPU0                           CPU1
> > > > 
> > > > clock = ACCESS_ONCE(sched_clocksource);
> > > > 
> > > >                                unload module
> > > >                                  clocksource_unregister()
> > > >                                    sched_clocksource = jiffies
> > > >                                  unmap data/text
> > > > 
> > > > cyc2ns(clock, clocksource_read(clock)) <--- fireworks
> > > > 
> > > > 
> > > 
> > > Do any module based clocksources even exist right now?
> > > clocksource_unregister only seems to be used 3 times..
> > 
> > Good point, it appears its not even exported.
> > 
> > Thomas mentioned modules, I assumed.
> > 
> The drivers/clocksource/ drivers in theory could be modular anyways.
> Handling this transition properly is at least one less barrier to modular
> clocksources, so I think it's progress regardless. I don't remember what
> all of the other issues were though, John probably remembers.

Yea. Its always been something I'd want to have as a capability, but we
haven't actually had a user of. I still think its something we should
allow, even if there aren't upstream users, as if you only have to
backport a module to an old distro kernel to get some oddball bit of
hardware working, that can be really useful.

So I think the locking fix is the way to go.

thanks
-john



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