[PATCH] x86_64: Don't reserve hotplug CPUs by default

Most users don't need it so no need to waste memory.
This means an user has to specify the appropiate number of
hotplug CPUs on the command line with additional_cpus=...
or fix their BIOS to follow the convention in
Documentation/x86-64/cpu-hotplug-spec
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

Documentation/x86_64/boot-options.txt

@@ -125,7 +125,7 @@ SMP
   cpumask=MASK   only use cpus with bits set in mask
 
   additional_cpus=NUM Allow NUM more CPUs for hotplug
-		 (defaults are specified by the BIOS or half the available CPUs)
+		 (defaults are specified by the BIOS, see Documentation/x86_64/cpu-hotplug-spec)
 
 NUMA
 

arch/x86_64/kernel/smpboot.c

@@ -927,8 +927,8 @@ int additional_cpus __initdata = -1;
  *
  * Three ways to find out the number of additional hotplug CPUs:
  * - If the BIOS specified disabled CPUs in ACPI/mptables use that.
- * - otherwise use half of the available CPUs or 2, whatever is more.
  * - The user can overwrite it with additional_cpus=NUM
+ * - Otherwise don't reserve additional CPUs.
  * We do this because additional CPUs waste a lot of memory.
  * -AK
  */
@@ -938,13 +938,10 @@ __init void prefill_possible_map(void)
 	int possible;
 
  	if (additional_cpus == -1) {
- 		if (disabled_cpus > 0) {
+ 		if (disabled_cpus > 0)
  			additional_cpus = disabled_cpus;
- 		} else {
- 			additional_cpus = num_processors / 2;
- 			if (additional_cpus == 0)
- 				additional_cpus = 2;
- 		}
+ 		else
+			additional_cpus = 0;
  	}
 	possible = num_processors + additional_cpus;
 	if (possible > NR_CPUS)