[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: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/Documentation/x86_64/boot-options.txt b/Documentation/x86_64/boot-options.txt
index e566affeed7f49e7b3a66f4d516a9c380805d63a..72ab9b99b22c822f160f7e95fdd5d2d4613c3ee9 100644 (file)
--- a/Documentation/x86_64/boot-options.txt
+++ b/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
 

diff --git a/arch/x86_64/kernel/smpboot.c b/arch/x86_64/kernel/smpboot.c
index ecbd7b83acc1ff7b9024d6de6fd1762ed58320f0..043b1d262a3051bd0fcdea03f15d6c80838234bc 100644 (file)
--- a/arch/x86_64/kernel/smpboot.c
+++ b/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)