[PATCH] add init_start_cpus to config boot cpus -v2

[Yinghai Lu <yinghai@kernel.org>]

Impact: new command line

so could select cpus to be started during init stage.

example:
init_start_cpus=2,4,6
to start core 0 only on every node or thread 0 or all cores

init_start_cpus=0
not start other APs, and later let user to use
 echo 1 > /sys/devices/cpu/cpu2
to start them in user space.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
 Documentation/kernel-parameters.txt |   14 ++++++++++++++
 init/main.c                         |   19 ++++++++++++++++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

Index: linux-2.6/Documentation/kernel-parameters.txt
===================================================================
--- linux-2.6.orig/Documentation/kernel-parameters.txt
+++ linux-2.6/Documentation/kernel-parameters.txt
@@ -918,6 +918,20 @@ and is between 256 and 4096 characters.
 		forcesac
 		soft
 
+	init_start_cpus=	[KNL,SMP] set CPUs that will be started during
+			init stage.
+			Format:
+			<cpu number>,...,<cpu number>
+			or
+			<cpu number>-<cpu number>
+			(must be a positive range in ascending order)
+			or a mixture
+			<cpu number>,...,<cpu number>-<cpu number>
+
+			This option can be used to specify one or more CPUs
+			to be started during init stage.
+			<cpu number> begins at 0 and the maximum value is
+			"number of CPUs in system - 1".
 
 	intel_iommu=	[DMAR] Intel IOMMU driver (DMAR) option
 		off
Index: linux-2.6/init/main.c
===================================================================
--- linux-2.6.orig/init/main.c
+++ linux-2.6/init/main.c
@@ -413,11 +413,28 @@ static void __init setup_per_cpu_areas(v
 }
 #endif /* CONFIG_HAVE_SETUP_PER_CPU_AREA */
 
+static __initdata cpumask_var_t cpu_init_start_mask;
+
+/* Setup the mask of cpus configured for init start */
+static int __init init_start_cpu_setup(char *str)
+{
+	cpulist_parse(str, *cpu_init_start_mask);
+
+	return 1;
+}
+
+__setup("init_start_cpus=", init_start_cpu_setup);
+
 /* Called by boot processor to activate the rest. */
 static void __init smp_init(void)
 {
 	unsigned int cpu;
 
+	if (cpumask_empty(cpu_init_start_mask))
+		cpumask_copy(cpu_init_start_mask, &cpu_present_map);
+	else
+		cpumask_and(cpu_init_start_mask, cpu_init_start_mask,
+					 &cpu_present_map);
 	/*
 	 * Set up the current CPU as possible to migrate to.
 	 * The other ones will be done by cpu_up/cpu_down()
@@ -426,7 +443,7 @@ static void __init smp_init(void)
 	cpu_set(cpu, cpu_active_map);
 
 	/* FIXME: This should be done in userspace --RR */
-	for_each_present_cpu(cpu) {
+	for_each_cpu_mask_nr(cpu, *cpu_init_start_mask) {
 		if (num_online_cpus() >= setup_max_cpus)
 			break;
 		if (!cpu_online(cpu))