linux-kernel - Re: Avoid checking for cpu gone when CONFIG_HOTPLUG

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Message-Id: <20070406232500.3fd84c77.akpm@linux-foundation.org>
Date:	Fri, 6 Apr 2007 23:25:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Keshavamurthy, Anil S" <anil.s.keshavamurthy@...el.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>, Gautham R Shenoy <ego@...ibm.com>
Subject: Re: Avoid checking for cpu gone when CONFIG_HOTPLUG_CPU not defined

On Fri, 6 Apr 2007 14:41:50 -0700 "Keshavamurthy, Anil S" <anil.s.keshavamurthy@...el.com> wrote:

> Subject: Avoid checking for cpu gone when CONFIG_HOTPLUG_CPU not defined
> 
> Avoid checking for cpu gone in mm hot path when
> CONFIG_HOTPLUG_CPU is not defined.
> 
> Signed-off-by: Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>
> 
> ---
>  arch/i386/kernel/smp.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Index: work/arch/i386/kernel/smp.c
> ===================================================================
> --- work.orig/arch/i386/kernel/smp.c
> +++ work/arch/i386/kernel/smp.c
> @@ -365,10 +365,12 @@ static void flush_tlb_others(cpumask_t c
>  	BUG_ON(cpu_isset(smp_processor_id(), cpumask));
>  	BUG_ON(!mm);
>  
> +#ifdef CONFIG_HOTPLUG_CPU
>  	/* If a CPU which we ran on has gone down, OK. */
>  	cpus_and(cpumask, cpumask, cpu_online_map);
> -	if (cpus_empty(cpumask))
> +	if (unlikely(cpus_empty(cpumask)))
>  		return;
> +#endif
>  
>  	/*
>  	 * i'm not happy about this global shared spinlock in the

Fair enough.

The code you're touching in with the original CPU-hotplug-for-i386 patches.

x86_64 doesn't do it.  It handles tlb flushing differently anyway.  But I
suspect that x86_64 is just buggy, unless all callers of flush_tlb_others()
have taken care to disable preemption prior to their calculation of the
passed-in cpumask.

Shudder.  Gautham, this is code which we can cheerfully delete when we get
the freezer stuff done.  Fortunately, Anil's patch will make it nice and
easy to find again.

-
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