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Subject[PATCH v8 03/12] x86, topology: Don't offline CPU0 if any PIC irq can not be migrated out of it
Date
From: Fenghua Yu <fenghua.yu@intel.com>

If CONFIG_BOOTPARAM_HOTPLUG_CPU is turned on, CPU0 hotplug feature is enabled
by default.

If CONFIG_BOOTPARAM_HOTPLUG_CPU is not turned on, CPU0 hotplug feature is not
enabled by default. The kernel parameter cpu0_hotplug can enable CPU0 hotplug
feature at boot.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
---
arch/x86/kernel/topology.c | 41 ++++++++++++++++++++++++++++++++++-------
1 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 76ee977..fa87fd6 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -30,23 +30,50 @@
#include <linux/mmzone.h>
#include <linux/init.h>
#include <linux/smp.h>
+#include <linux/irq.h>
#include <asm/cpu.h>

static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);

#ifdef CONFIG_HOTPLUG_CPU
+
+#ifdef CONFIG_BOOTPARAM_HOTPLUG_CPU0
+static int cpu0_hotpluggable = 1;
+#else
+static int cpu0_hotpluggable;
+static int __init enable_cpu0_hotplug(char *str)
+{
+ cpu0_hotpluggable = 1;
+ return 1;
+}
+
+__setup("cpu0_hotplug", enable_cpu0_hotplug);
+#endif
+
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.
+ * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
+ * depends on BSP. PIC interrupts depend on BSP.
*
- * Also certain PCI quirks require not to enable hotplug control
- * for all CPU's.
+ * If the BSP depencies are under control, one can tell kernel to
+ * enable BSP hotplug. This basically adds a control file and
+ * one can attempt to offline BSP.
*/
- if (num)
+ if (num == 0 && cpu0_hotpluggable) {
+ unsigned int irq;
+ /*
+ * We won't take down the boot processor on i386 if some
+ * interrupts only are able to be serviced by the BSP in PIC.
+ */
+ for_each_active_irq(irq) {
+ if (!IO_APIC_IRQ(irq) && irq_has_action(irq)) {
+ cpu0_hotpluggable = 0;
+ break;
+ }
+ }
+ }
+ if (num || cpu0_hotpluggable)
per_cpu(cpu_devices, num).cpu.hotpluggable = 1;

return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
--
1.7.2


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