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

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Date:	Wed, 4 Apr 2007 13:43:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	john stultz <johnstul@...ibm.com>
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, 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 ;)

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


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