linux-kernel - Re: [PATCH -mm] fix jiffies clocksource inittime

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Date:	Wed, 04 Apr 2007 13:53:47 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@....de>
Subject: Re: [PATCH -mm] fix jiffies clocksource inittime

On Wed, 2007-04-04 at 13:43 -0700, Andrew Morton wrote:
> On Wed, 04 Apr 2007 12:50:15 -0700
> john stultz <johnstul@...ibm.com> wrote:
> 
> > 	In debugging a problem w/ the -rt tree, I noticed that on systems that
> > mark the tsc as unstable before it is registered, the TSC would still be
> > selected and used for a short period of time. Digging in it looks to be
> > a result of the mix of the clocksource list changes and my clocksource
> > initialization changes.
> > 
> > With the -rt tree, using a bad TSC, even for a short period of time can
> > results in a hang at boot. I was not able to reproduce this hang w/
> > mainline, but I'm not completely certain that someone won't trip on it.
> > 
> > This patch resolves the issue by initializing the jiffies clocksource
> > earlier so a bad TSC won't get selected just because nothing else is yet
> > registered.
> > 
> > thanks
> > -john
> > 
> > Signed-off-by: John Stultz <johnstul@...ibm.com>
> > 
> > diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
> > index 3be8da8..4c256fd 100644
> > --- a/kernel/time/jiffies.c
> > +++ b/kernel/time/jiffies.c
> > @@ -69,4 +69,4 @@ static int __init init_jiffies_clocksour
> >  	return clocksource_register(&clocksource_jiffies);
> >  }
> >  
> > -module_init(init_jiffies_clocksource);
> > +core_initcall(init_jiffies_clocksource);
> 
> Do you think this is needed in 2.6.21?
> 
> If so: worrisome.  Startup ordering issues are fragile and tricky.  Time
> management is fragile and tricky.  The combination of the two doesn't get
> any better ;)

Its likely low-risk, however I'm pretty cautious this close to release.

> An option would be to test it in 2.6.22-rcX for a while, then backport to
> 2.6.21.x.

I'm fine with this (also getting it into the next -mm would help). But
I'd defer to others with stronger opinions.

thanks
-john

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