[PATCH] x86_64: New heuristics to find out hotpluggable CPUs.

With a NR_CPUS==128 kernel with CPU hotplug enabled we would waste 4MB
on per CPU data of all possible CPUs.  The reason was that HOTPLUG
always set up possible map to NR_CPUS cpus and then we need to allocate
that much (each per CPU data is roughly ~32k now)

The underlying problem is that ACPI didn't tell us how many hotplug CPUs
the platform supports.  So the old code just assumed all, which would
lead to this memory wastage.

This implements some new heuristics:

 - If the BIOS specified disabled CPUs in the ACPI/mptables assume they
   can be enabled later (this is bending the ACPI specification a bit,
   but seems like a obvious extension)
 - The user can overwrite it with a new additionals_cpus=NUM option
 - Otherwise use half of the available CPUs or 2, whatever is more.

Cc: ashok.raj@intel.com
Cc: len.brown@intel.com

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

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/x86_64/kernel/mpparse.c b/arch/x86_64/kernel/mpparse.c
index 4a581d1cefbd..1d61f10a92c6 100644
--- a/arch/x86_64/kernel/mpparse.c
+++ b/arch/x86_64/kernel/mpparse.c
@@ -65,7 +65,9 @@ unsigned long mp_lapic_addr = 0;
 /* Processor that is doing the boot up */
 unsigned int boot_cpu_id = -1U;
 /* Internal processor count */
-static unsigned int num_processors = 0;
+unsigned int num_processors __initdata = 0;
+
+unsigned disabled_cpus __initdata;
 
 /* Bitmask of physically existing CPUs */
 physid_mask_t phys_cpu_present_map = PHYSID_MASK_NONE;
@@ -109,8 +111,10 @@ static void __init MP_processor_info (struct mpc_config_processor *m)
         int ver, cpu;
         static int found_bsp=0;
 
-        if (!(m->mpc_cpuflag & CPU_ENABLED))
+        if (!(m->mpc_cpuflag & CPU_ENABLED)) {
+                disabled_cpus++;
                 return;
+        }
 
         printk(KERN_INFO "Processor #%d %d:%d APIC version %d\n",
                 m->mpc_apicid,