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

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Message-ID: <20090528164011.GA30104@linux-sh.org>
Date:	Fri, 29 May 2009 01:40:11 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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 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.
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