add command line init_start_cpus

Impact: new command line

so could select cpus to be started during init stage

---
 Documentation/kernel-parameters.txt |   14 ++++++++++++++
 init/main.c                         |   29 ++++++++++++++++++++++++++++-
 2 files changed, 42 insertions(+), 1 deletion(-)

Index: linux-2.6/Documentation/kernel-parameters.txt
===================================================================

Documentation/kernel-parameters.txt +4 -0

@@ -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>

init/main.c +12 -0

@@ -413,11 +413,35 @@ 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);
+

init/main.c +10 -1

+		if (!start_mask)
+			start_mask = &cpu_present_map;
+		else
+			cpumask_and(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()

@@ -426,13 +450,16 @@ 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, *start_mask) {
 		if (num_online_cpus() >= setup_max_cpus)
 			break;
 		if (!cpu_online(cpu))
 			cpu_up(cpu);
 	}
 
+	if (start_mask != &cpu_present_map)
+		kfree(start_mask);
+

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.

+static __initdata cpumask_var_t cpu_init_start_mask;

Impact: new command line

so could select cpus to be started during init stage

Quoted 3 lines · Yinghai Lu

Impact: new command line

so could select cpus to be started during init stage

maxcpus=N should do this already, at least for the first N cpus. If you really need a non continuous range of N (why?) then it would be better to fix maxcpus instead of adding a new option.

Quoted 4 lines · Andi Kleen

Quoted 2 lines · Yinghai Lu

Impact: new command line

Quoted 5 lines

Quoted 1 line

so could select cpus to be started during init stage

maxcpus=N should do this already, at least for the first N cpus. If you really need a non continuous range of N (why?) then it would be better to fix maxcpus instead of adding a new option.

it seems it is hard to make maxcpus= to support random cpus selecting and maxcpus=0 looks insane

also maxcpus=1, will disable smp, can you enable other cpus later in user space?