linux-kernel - Re: [PATCH] core: Fix maxcpus boot option broken

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Date:	Fri, 31 May 2013 14:52:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Youquan Song <youquan.song@...el.com>
Cc:	gregkh@...uxfoundation.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH] core: Fix maxcpus boot option broken

On Thu, 30 May 2013 00:00:06 -0400 Youquan Song <youquan.song@...el.com> wrote:

> maxcpus boot option to limit maximum number of CPUs on system, but this option
> is broken at recent kernel.

Can you help us identify which kernel versions have this bug? 
Identifying the offending commit would be best.  Thanks.

> Though we use maxcpus to limit CPUs number, but
> current kernel will register all of present CPUs in sysfs.
> udev will enumerate all registered cpu at sysfs, and it will bring up the CPU
> if the CPU is offline. So the maxcpus option is broken.
> 
> This patch will only register the CPU which is not over limitation of maxcpus 
> option in sysfs. So it will keep the maxcpus limitation when udev enumeration
> or other intention of bring up CPUs over the limitation by method like  
> echo 1 > /sys/devices/system/cpu/online 
> 
> ...
>
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -272,6 +272,10 @@ int __cpuinit register_cpu(struct cpu *cpu, int num)
>  {
>  	int error;
>  
> +	/* return when cpu number greater than maximum number of CPUs */
> +	if (num >= setup_max_cpus)
> +		return 0;
> +
>  	cpu->node_id = cpu_to_node(num);
>  	memset(&cpu->dev, 0x00, sizeof(struct device));
>  	cpu->dev.id = num;
> -- 
> 1.7.7.4
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