Re: [PATCH v9 05/12] x86, hotplug, suspend: Online CPU0 for suspend or hibernate

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 10/16/2012 09:47 PM, Rafael J. Wysocki wrote:
> On Tuesday 16 of October 2012 11:05:18 Srivatsa S. Bhat wrote:
>> On 10/16/2012 02:20 AM, Rafael J. Wysocki wrote:
>>> On Friday 12 of October 2012 09:09:42 Fenghua Yu wrote:
>>>> From: Fenghua Yu <fenghua.yu@intel.com>
>>>>
>>>> +
>>>> +/*
>>>> + * When bsp_check() is called in hibernate and suspend, cpu hotplug
>>>> + * is disabled already. So it's unnessary to handle race condition between
>>>> + * cpumask query and cpu hotplug.
>>>> + */
>>>> +static int bsp_check(void)
>>>> +{
>>>> +	if (cpumask_first(cpu_online_mask) != 0) {
>>>> +		pr_warn("CPU0 is offline.\n");
>>>> +		return -ENODEV;
>>>> +	}
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static int bsp_pm_callback(struct notifier_block *nb, unsigned long action,
>>>> +			   void *ptr)
>>>> +{
>>>> +	int ret = 0;
>>>> +
>>>> +	switch (action) {
>>>> +	case PM_SUSPEND_PREPARE:
>>>> +	case PM_HIBERNATION_PREPARE:
>>>> +		ret = bsp_check();
>>>> +		break;
>>>> +	default:
>>>> +		break;
>>>> +	}
>>>> +	return notifier_from_errno(ret);
>>>> +}
>>>> +
>>>
>>> I wonder if there's anything preventing CPU0 from becoming offline after you've
>>> done this check and before user space is frozen?
>>>
>>
>> Hi Rafael,
>>
>> bsp_pm_callback runs as a low priority notifier callback, specifically with lower
>> priority than the cpu_hotplug_pm_callback (as mentioned in the comment below).
>> And cpu_hotplug_pm_callback disables regular CPU hotplug (till the suspend/resume
>> sequence is complete).. So there is no chance for CPU0 to become offline after that.
>>
>> Or, are you thinking of some other scenario where CPU0 can go offline?
> 
> No, that should be fine technically, but designs relying on notifier priority
> for correctness are kind of fragile.
> 

Hmm.. I agree.

> Would it be possible to make cpu_hotplug_pm_callback() do the BSP online check?
>

Good idea! I think that could be done quite easily. And while doing that, it would
be good to rethink what to do in patch 12/12 (Debug CPU0 hotplug) to fix the bug I
pointed out in my other mail.

Regards,
Srivatsa S. Bhat

>>>> +static int __init bsp_pm_check_init(void)
>>>> +{
>>>> +	/*
>>>> +	 * Set this bsp_pm_callback as lower priority than
>>>> +	 * cpu_hotplug_pm_callback. So cpu_hotplug_pm_callback will be called
>>>> +	 * earlier to disable cpu hotplug before bsp online check.
>>>> +	 */
>>>> +	pm_notifier(bsp_pm_callback, -INT_MAX);
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +core_initcall(bsp_pm_check_init);
>