4 files changed, 47 insertions, 5 deletions

diff --git a/Documentation/x86_64/boot-options.txt b/Documentation/x86_64/boot-options.txt
index ffe1c062088b..a83139692cdf 100644
--- a/Documentation/x86_64/boot-options.txt
+++ b/Documentation/x86_64/boot-options.txt
@@ -122,6 +122,9 @@ 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)
+
 NUMA
 
   numa=off      Only set up a single NUMA node spanning all memory.
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,
diff --git a/arch/x86_64/kernel/smpboot.c b/arch/x86_64/kernel/smpboot.c
index 3393fc08823b..f74319a80659 100644
--- a/arch/x86_64/kernel/smpboot.c
+++ b/arch/x86_64/kernel/smpboot.c
@@ -880,6 +880,9 @@ static __init void disable_smp(void)
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
+
+int additional_cpus __initdata = -1;
+
 /*
  * cpu_possible_map should be static, it cannot change as cpu's
  * are onlined, or offlined. The reason is per-cpu data-structures
@@ -888,14 +891,38 @@ static __init void disable_smp(void)
  * cpu_present_map on the other hand can change dynamically.
  * In case when cpu_hotplug is not compiled, then we resort to current
  * behaviour, which is cpu_possible == cpu_present.
- * If cpu-hotplug is supported, then we need to preallocate for all
- * those NR_CPUS, hence cpu_possible_map represents entire NR_CPUS range.
  * - Ashok Raj
+ *
+ * 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
+ * We do this because additional CPUs waste a lot of memory.
+ * -AK
  */
 __init void prefill_possible_map(void)
 {
         int i;
-        for (i = 0; i < NR_CPUS; i++)
+        int possible;
+
+        if (additional_cpus == -1) {
+                if (disabled_cpus > 0) {
+                        additional_cpus = disabled_cpus;
+                } else {
+                        additional_cpus = num_processors / 2;
+                        if (additional_cpus == 0)
+                                additional_cpus = 2;
+                }
+        }
+        possible = num_processors + additional_cpus;
+        if (possible > NR_CPUS) 
+                possible = NR_CPUS;
+
+        printk(KERN_INFO "SMP: Allowing %d CPUs, %d hotplug CPUs\n",
+                possible,
+                max_t(int, possible - num_processors, 0));
+
+        for (i = 0; i < possible; i++)
                 cpu_set(i, cpu_possible_map);
 }
 #endif
@@ -1151,6 +1178,12 @@ void __cpu_die(unsigned int cpu)
         printk(KERN_ERR "CPU %u didn't die...\n", cpu);
 }
 
+static __init int setup_additional_cpus(char *s)
+{
+        return get_option(&s, &additional_cpus);
+}
+__setup("additional_cpus=", setup_additional_cpus);
+
 #else /* ... !CONFIG_HOTPLUG_CPU */
 
 int __cpu_disable(void)
diff --git a/include/asm-x86_64/smp.h b/include/asm-x86_64/smp.h
index 592161e979e5..cf8f969f9020 100644
--- a/include/asm-x86_64/smp.h
+++ b/include/asm-x86_64/smp.h
@@ -81,6 +81,8 @@ extern int safe_smp_processor_id(void);
 extern int __cpu_disable(void);
 extern void __cpu_die(unsigned int cpu);
 extern void prefill_possible_map(void);
+extern unsigned num_processors;
+extern unsigned disabled_cpus;
 
 #endif /* !ASSEMBLY */