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

[Rafael J. Wysocki]

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>
> >>
> >> Because x86 BIOS requires CPU0 to resume from sleep, suspend or hibernate can't
> >> be executed if CPU0 is detected offline. To make suspend or hibernate and
> >> further resume succeed, CPU0 must be online.
> >>
> >> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> >> ---
> >>  arch/x86/power/cpu.c |   44 ++++++++++++++++++++++++++++++++++++++++++++
> >>  1 files changed, 44 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> >> index 218cdb1..adde775 100644
> >> --- a/arch/x86/power/cpu.c
> >> +++ b/arch/x86/power/cpu.c
> >> @@ -237,3 +237,47 @@ void restore_processor_state(void)
> >>  #ifdef CONFIG_X86_32
> >>  EXPORT_SYMBOL(restore_processor_state);
> >>  #endif
> >> +
> >> +/*
> >> + * 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.

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

> >> +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);

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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