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Date: Tue, 16 Oct 2012 14:17:43 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To: Fenghua Yu <fenghua.yu@...el.com>
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>,
Asit K Mallick <asit.k.mallick@...el.com>,
Tony Luck <tony.luck@...el.com>,
Arjan Dan De Ven <arjan@...ux.intel.com>,
Suresh B Siddha <suresh.b.siddha@...el.com>,
Len Brown <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 12/12] x86, topology: Debug CPU00 hotplug
On 10/12/2012 09:39 PM, Fenghua Yu wrote:
> From: Fenghua Yu <fenghua.yu@...el.com>
> +config DEBUG_HOTPLUG_CPU0
> + def_bool n
> + prompt "Debug CPU0 hotplug"
> + depends on HOTPLUG_CPU && EXPERIMENTAL
> + ---help---
> + Enabling this option offlines CPU0 (if CPU0 can be offlined) as
> + soon as possible and boots up userspace with CPU0 offlined. User
> + can online CPU0 back after boot time.
> +
> + To debug CPU0 hotplug, you need to enable CPU0 offline/online
> + feature by either turning on CONFIG_BOOTPARAM_HOTPLUG_CPU0 during
> + compilation or giving cpu0_hotplug kernel parameter at boot.
> +
> + If unuser, say N.
s/unuser/unsure
> +#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
> +/*
> + * This function offlines a CPU as early as possible and allows userspace to
> + * boot up without the CPU. The CPU can be onlined back by user after boot.
> + *
> + * This is only called for debugging CPU offline/online feature.
> + */
> +int __ref _debug_hotplug_cpu(int cpu, int action)
> +{
> + struct device *dev = get_cpu_device(cpu);
> + int ret;
> +
> + if (!cpu_is_hotpluggable(cpu))
> + return -EINVAL;
> +
> + cpu_hotplug_driver_lock();
> +
> + switch (action) {
> + case 0:
> + ret = cpu_down(cpu);
> + if (!ret) {
> + pr_info("CPU %u is now offline\n", cpu);
> + kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
> + } else
> + pr_debug("Can't offline CPU%d.\n", cpu);
> + break;
> + case 1:
> + ret = cpu_up(cpu);
> + if (!ret)
> + kobject_uevent(&dev->kobj, KOBJ_ONLINE);
> + else
> + pr_debug("Can't online CPU%d.\n", cpu);
> + break;
> + default:
> + ret = -EINVAL;
> + }
> +
> + cpu_hotplug_driver_unlock();
> +
> + return ret;
> +}
> +
> +static int __init debug_hotplug_cpu(void)
> +{
> + _debug_hotplug_cpu(0, 0);
> + return 0;
> +}
> +
> +late_initcall_sync(debug_hotplug_cpu);
> +#endif /* CONFIG_DEBUG_HOTPLUG_CPU0 */
> +
> int __ref arch_register_cpu(int num)
> {
> struct cpuinfo_x86 *c = &cpu_data(num);
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index adde775..120cee1 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -21,6 +21,7 @@
> #include <asm/suspend.h>
> #include <asm/debugreg.h>
> #include <asm/fpu-internal.h> /* pcntxt_mask */
> +#include <asm/cpu.h>
>
> #ifdef CONFIG_X86_32
> static struct saved_context saved_context;
> @@ -263,6 +264,43 @@ static int bsp_pm_callback(struct notifier_block *nb, unsigned long action,
> case PM_HIBERNATION_PREPARE:
> ret = bsp_check();
> break;
> +#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
> + case PM_RESTORE_PREPARE:
> + /*
> + * When system resumes from hibernation, online CPU0 because
> + * 1. it's required for resume and
> + * 2. the CPU was online before hibernation
> + */
> + if (!cpu_online(0))
> + _debug_hotplug_cpu(0, 1);
This won't work. CPU hotplug is disabled by cpu_hotplug_pm_callback() during
PM_HIBERNATION_PREPARE and is enabled back only during PM_POST_HIBERNATION.
So calls to cpu_up() or cpu_down() will fail in the meantime.
Regards,
Srivatsa S. Bhat
> + break;
> + case PM_POST_RESTORE:
> + /*
> + * When a resume really happens, this code won't be called.
> + *
> + * This code is called only when user space hibernation software
> + * prepares for snapshot device during boot time. So we just
> + * call _debug_hotplug_cpu() to restore to CPU0's state prior to
> + * preparing the snapshot device.
> + *
> + * This works for normal boot case in our CPU0 hotplug debug
> + * mode, i.e. CPU0 is offline and user mode hibernation
> + * software initializes during boot time.
> + *
> + * If CPU0 is online and user application accesses snapshot
> + * device after boot time, this will offline CPU0 and user may
> + * see different CPU0 state before and after accessing
> + * the snapshot device. But hopefully this is not a case when
> + * user debugging CPU0 hotplug. Even if users hit this case,
> + * they can easily online CPU0 back.
> + *
> + * To simplify this debug code, we only consider normal boot
> + * case. Otherwise we need to remember CPU0's state and restore
> + * to that state and resolve racy conditions etc.
> + */
> + _debug_hotplug_cpu(0, 0);
> + break;
> +#endif
> default:
> break;
> }
>
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