[PATCH] sched: add CPU_DOWN_PREPARE notifier

Add a CPU_DOWN_PREPARE hotplug CPU notifier.  This is needed so we can dettach
all sched-domains before a CPU goes down, thus we can build domains from
online cpumasks, and not have to check for the possibility of a CPU coming up
or going down.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/notifier.h

@@ -64,10 +64,11 @@ extern int notifier_call_chain(struct notifier_block **n, unsigned long val, voi
 
 #define NETLINK_URELEASE	0x0001	/* Unicast netlink socket released */
 
-#define CPU_ONLINE	0x0002 /* CPU (unsigned)v is up */
-#define CPU_UP_PREPARE	0x0003 /* CPU (unsigned)v coming up */
-#define CPU_UP_CANCELED	0x0004 /* CPU (unsigned)v NOT coming up */
-#define CPU_DEAD	0x0006 /* CPU (unsigned)v dead */
+#define CPU_ONLINE		0x0002 /* CPU (unsigned)v is up */
+#define CPU_UP_PREPARE		0x0003 /* CPU (unsigned)v coming up */
+#define CPU_UP_CANCELED		0x0004 /* CPU (unsigned)v NOT coming up */
+#define CPU_DOWN_PREPARE	0x0005 /* CPU (unsigned)v going down */
+#define CPU_DEAD		0x0006 /* CPU (unsigned)v dead */
 
 #endif /* __KERNEL__ */
 #endif /* _LINUX_NOTIFIER_H */

kernel/cpu.c

@@ -124,6 +124,15 @@ int cpu_down(unsigned int cpu)
 		goto out;
 	}
 
+	err = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
+						(void *)(long)cpu);
+	if (err == NOTIFY_BAD) {
+		printk("%s: attempt to take down CPU %u failed\n",
+				__FUNCTION__, cpu);
+		err = -EINVAL;
+		goto out;
+	}
+
 	/* Ensure that we are not runnable on dying cpu */
 	old_allowed = current->cpus_allowed;
 	tmp = CPU_MASK_ALL;