[PATCH 6/8] x86, topology.c: Enable CPU0 online/offline

From: Fenghua Yu
Date: Wed Oct 05 2011 - 12:56:48 EST

From: Fenghua Yu <fenghua.yu@xxxxxxxxx>

Enable CPU0 online/offline by setting up its sysfs control file. By default,
this feature is disabled. You can enabled it by bsp_hotplug kernel option.

Resume from hibernate, standby, or suspend needs CPU0 online. According to
arch/x86/kernel/topology.c, some PCI quirks(??) need CPU0 online as well. Anyone
knows which PCI quirks depends on CPU0 online?

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Documentation/kernel-parameters.txt | 7 +++++++
arch/x86/kernel/topology.c | 22 +++++++++++++++-------
2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 854ed5c..bf011ee 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1805,6 +1805,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.

nox2apic [X86-64,APIC] Do not enable x2APIC mode.

+ bsp_hotplug [X86] BSP (aka CPU0) is hotpluggable.
+ Suspend/resume depends on BSP. It's said some PCI
+ quirks depend on BSP too. But not sure which quirks.
+ If don't care the dependencies, you can turn on
+ bsp_ hotplug. Suspend will fail if BSP is offlined and
+ you need to online BSP before suspend/resume.
nptcg= [IA-64] Override max number of concurrent global TLB
purges which is reported from either PAL_VM_SUMMARY or
diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 8927486..ed2da5c 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -34,18 +34,26 @@
static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);

+static int __init enable_bsp_hotplug(char *str)
+ bsp_hotpluggable = 1;
+ return 0;
+__setup("bsp_hotplug", enable_bsp_hotplug);
int __ref arch_register_cpu(int num)
- * CPU0 cannot be offlined due to several
- * restrictions and assumptions in kernel. This basically
- * doesn't add a control file, one cannot attempt to offline
- * BSP.
+ * Suspend/resume depends on BSP. Certain PCI quirks (which PCI
+ * quirks??) require not to enable hotplug control for all CPU's.
- * Also certain PCI quirks require not to enable hotplug control
- * for all CPU's.
+ * If one don't care those BSP depencies, tell kernel to
+ * enable BSP hotplug. This basically adds a control file and
+ * one can attempt to offline BSP.
- if (num)
+ if (num || bsp_hotpluggable)
per_cpu(cpu_devices, num).cpu.hotpluggable = 1;

return register_cpu(&per_cpu(cpu_devices, num).cpu, num);

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