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    SubjectRe: [PATCH v5 12/12] x86/topology.c: debug CPU0 hotplug
    On 01/12/2012 01:04 AM, Fenghua Yu wrote:
    > From: Fenghua Yu <fenghua.yu@intel.com>
    >
    > CONFIG_DEBUG_HOTPLUG_CPU0 is for debuging the CPU0 hotplug feature. The switch
    > offlines CPU0 as soon as possible and boots userspace up with CPU0 offlined.
    > User can online CPU0 back after boot time. The default value of the switch is
    > off.
    >
    > 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.
    >
    > It's safe and early place to take down CPU0 after all hotplug notifiers
    > are installed and SMP is booted.
    >
    > Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
    > ---
    > arch/x86/Kconfig | 15 +++++++++++++++
    > arch/x86/kernel/topology.c | 39 +++++++++++++++++++++++++++++++++++++++
    > 2 files changed, 54 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    > index c2cf195..36b6a2c 100644
    > --- a/arch/x86/Kconfig
    > +++ b/arch/x86/Kconfig
    > @@ -1748,6 +1748,21 @@ config BOOTPARAM_HOTPLUG_CPU0
    > You still can enable the CPU0 hotplug feature at boot by kernel
    > parameter cpu0_hotplug.
    >
    > +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.
    > +
    > config COMPAT_VDSO
    > def_bool y
    > prompt "Compat VDSO support"
    > diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
    > index 8cec15c..3f697dd 100644
    > --- a/arch/x86/kernel/topology.c
    > +++ b/arch/x86/kernel/topology.c
    > @@ -49,6 +49,45 @@ static int __init enable_cpu0_hotplug(char *str)
    > __setup("cpu0_hotplug", enable_cpu0_hotplug);
    > #endif
    >
    > +#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
    > +/*
    > + * This function offlines CPU0 as early as possible and allows userspace to
    > + * boot up without CPU0. CPU0 can be onlined back by user after boot.
    > + *
    > + * This is only called for debugging CPU0 offline feature.
    > + */
    > +static void __init _debug_hotplug_cpu(int num)
    > +{
    > + int ret;
    > + struct sys_device *dev = get_cpu_sysdev(num);
    > +
    > + /*
    > + * Take hotpluggable online CPU0 down.
    > + */
    > + if (num || !per_cpu(cpu_devices, num).cpu.hotpluggable || !dev ||
    > + !cpu_online(num)) {
    > + pr_debug("CPU0 can't be offlined.\n");
    > + return;
    > + }
    The usage of parameter num seems a little inconsistent.
    Suggest either to remove the num parameter or to get rid of the assumption
    that num is always zero.

    > +
    > + cpu_hotplug_driver_lock();
    > +
    > + ret = cpu_down(num);
    > + if (!ret)
    > + kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
    > + else
    > + pr_debug("Can't offline CPU%d.\n", num);
    > +
    > + cpu_hotplug_driver_unlock();
    > +}
    > +static int __init debug_hotplug_cpu(void)
    > +{
    > + _debug_hotplug_cpu(0);
    > + return 0;
    > +}
    > +device_initcall_sync(debug_hotplug_cpu);
    > +#endif /* CONFIG_DEBUG_HOTPLUG_CPU0 */
    > +
    > int __ref arch_register_cpu(int num)
    > {
    > /*



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