linux-kernel - RE: [PATCH v9 05/12] x86, hotplug, suspend: Online CPU0 for suspend or hibernate

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Date:	Wed, 17 Oct 2012 18:29:12 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Brown, Len" <len.brown@...el.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Chen Gong <gong.chen@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH v9 05/12] x86, hotplug, suspend: Online CPU0 for suspend
 or hibernate

> From: Srivatsa S. Bhat [mailto:srivatsa.bhat@...ux.vnet.ibm.com]
> 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@...el.com>
> >>
> >> 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@...el.com>
> >> ---
> >>  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.

Bhat is right here. There is no functionality issue here.

bsp_check() is protected by cpu_hotplug_disable which is set before
bsp_pm_callback and cleared after it. There is no chance for CPU0
to be offline after that.

Thanks.

-Fenghua