linux-kernel - Re: [PATCH] scheduler: fix x86 regression in native_sched

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Message-ID: <alpine.LFD.0.99999.0712070646210.3221@localhost.localdomain>
Date:	Fri, 7 Dec 2007 06:51:55 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stefano Brivio <stefano.brivio@...imi.it>
cc:	Ingo Molnar <mingo@...e.hu>, Robert Love <rml@...h9.net>,
	linux-kernel@...r.kernel.org, Dave Jones <davej@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Michael Buesch <mb@...sch.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] scheduler: fix x86 regression in native_sched_clock

On Fri, 7 Dec 2007, Stefano Brivio wrote:

> This patch fixes a regression introduced by:
> 
> commit bb29ab26863c022743143f27956cc0ca362f258c
> Author: Ingo Molnar <mingo@...e.hu>
> Date:   Mon Jul 9 18:51:59 2007 +0200
> 
> This caused the jiffies counter to leap back and forth on cpufreq changes
> on my x86 box. I'd say that we can't always assume that TSC does "small
> errors" only, when marked unstable. On cpufreq changes these errors can be
> huge.

Hmrpf. sched_clock() is used for the time stamp of the printks. We
need to find some better solution other than killing off the tsc
access completely.

Ingo ???

Thanks,

     tglx
 
> The original bug report can be found here:
> http://bugzilla.kernel.org/show_bug.cgi?id=9475
> 
> 
> Signed-off-by: Stefano Brivio <stefano.brivio@...imi.it>
> 
> ---
> 
> diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
> index 9ebc0da..d29cd9c 100644
> --- a/arch/x86/kernel/tsc_32.c
> +++ b/arch/x86/kernel/tsc_32.c
> @@ -98,13 +98,8 @@ unsigned long long native_sched_clock(void)
>  
>  	/*
>  	 * Fall back to jiffies if there's no TSC available:
> -	 * ( But note that we still use it if the TSC is marked
> -	 *   unstable. We do this because unlike Time Of Day,
> -	 *   the scheduler clock tolerates small errors and it's
> -	 *   very important for it to be as fast as the platform
> -	 *   can achive it. )
>  	 */
> -	if (unlikely(!tsc_enabled && !tsc_unstable))
> +	if (unlikely(!tsc_enabled))
>  		/* No locking but a rare wrong value is not a big deal: */
>  		return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
>  
> 
> -- 
> Ciao
> Stefano
> 
--
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