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Date:	Sun, 19 Oct 2014 23:39:23 -0700
From:	Neil Zhang <zhangwm@...vell.com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] drivers: base: update cpu offline info when do hotplug

Greg,

-----Original Message-----
From: Greg KH [mailto:gregkh@...uxfoundation.org] 
Sent: 2014年10月20日 12:44
To: Neil Zhang
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: base: update cpu offline info when do hotplug

On Mon, Oct 20, 2014 at 11:29:08AM +0800, Neil Zhang wrote:
> The current per-cpu offline info won't be updated if it is hotplugged 
> in/out by a kernel governer.
> Let's update it via cpu notifier.
> 
> Signed-off-by: Neil Zhang <zhangwm@...vell.com>
> ---
>  drivers/base/cpu.c |   25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c index 
> 006b1bc..9d61824 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -418,10 +418,35 @@ static void __init 
> cpu_dev_register_generic(void)  #endif  }
>  
> +static int device_hotplug_notifier(struct notifier_block *nfb,
> +				      unsigned long action, void *hcpu) {
> +	unsigned int cpu = (unsigned long)hcpu;
> +	struct device *dev = get_cpu_device(cpu);
> +	int ret;
> +
> +	switch (action & ~CPU_TASKS_FROZEN) {
> +	case CPU_ONLINE:
> +		dev->offline = false;
> +		ret = NOTIFY_OK;
> +		break;
> +	case CPU_DYING:
> +		dev->offline = true;
> +		ret = NOTIFY_OK;
> +		break;
> +	default:
> +		ret = NOTIFY_DONE;
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
>  void __init cpu_dev_init(void)
>  {
>  	if (subsys_system_register(&cpu_subsys, cpu_root_attr_groups))
>  		panic("Failed to register CPU subsystem");
>  
>  	cpu_dev_register_generic();
> +	cpu_notifier(device_hotplug_notifier, 0);
>  }

How much noise is this going to cause on a big/little system that constantly hot unplug/plugs processors all of the time?

Can you explain more what kind of noise will be introduced on a big/little system?
As I know IKS on arm will use cpu_suspend way to power down a core.
But I don't know well about other architectures.
Please give your suggestions.
Thanks!

greg k-h

Best Regards,
Neil Zhang

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