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

[Yu, Fenghua]

> 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@xxxxxxxxx>
> >>>>
> >>>> +
> >>>> +/*
> >>>> + * 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.

Why is this design relying on notifier priority fragile? I don't get it.

The priority in pm_notifier() is in a well designed API. This code just
follows the API to assign lower priority for bsp_pm_callback than
cpu_hotplug_pm_callback. Unless your justification is that the API of
pm_notifier() is fragile, I think this patch should be ok.

It's not a good idea to move bsp_pm_callback() into
cpu_hotplug_pm_callback(). There is no architecture specific hook to
call x86 bsp specific hotplug in cpu_hotplug_pm_callback(). Moving
bsp_pm_callback() into cpu_hotplug_pm_callback() is hacking while 
this patch follows well defined API and has better coding.

Thanks.

-Fenghua 
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