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Subject[patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time
From: Chuck Ebbert <>

x86: allow number of additional hotplug CPUs to be set at compile time

The default number of additional CPU IDs for hotplugging is determined
by asking ACPI or mptables how many "disabled" CPUs there are in the
system, but many systems get this wrong so that e.g. a uniprocessor
machine gets an extra CPU allocated and never switches to single CPU

And sometimes CPU hotplugging is enabled only for suspend/hibernate
anyway, so the additional CPU IDs are not wanted. Allow the number
to be set to zero at compile time.

Also, force the number of extra CPUs to zero if hotplugging is disabled
which allows removing some conditional code.

Tested on uniprocessor x86_64 that ACPI claims has a disabled processor,
with CPU hotplugging configured.

("After" has the number of additional CPUs set to 0)
Before: NR_CPUS: 512, nr_cpu_ids: 2, nr_node_ids 1
After: NR_CPUS: 512, nr_cpu_ids: 1, nr_node_ids 1

Signed-off-by: Chuck Ebbert <>

Index: linux-2.6.26.noarch/arch/x86/Kconfig
--- linux-2.6.26.noarch.orig/arch/x86/Kconfig
+++ linux-2.6.26.noarch/arch/x86/Kconfig
@@ -1366,6 +1366,24 @@ config HOTPLUG_CPU
Say N if you want to disable CPU hotplug and don't need to

+ def_bool y
+ prompt "Allocate extra CPUs for hotplugging after boot" if HOTPLUG_CPU
+ ---help---
+ Say yes here to use the default, which allows as many CPUs as are marked
+ "disabled" by ACPI or MPTABLES to be hotplugged after bootup.
+ Say no if you do not want to allow CPUs to be added after booting, for
+ example if you only need CPU hotplugging enabled for suspend/resume.
+ This value may be overridden at boot time with the "additional_cpus"
+ kernel parameter, if CPU_HOTPLUG is enabled.
+ int
+ default -1
def_bool y
prompt "Compat VDSO support"
Index: linux-2.6.26.noarch/arch/x86/kernel/smpboot.c
--- linux-2.6.26.noarch.orig/arch/x86/kernel/smpboot.c
+++ linux-2.6.26.noarch/arch/x86/kernel/smpboot.c
@@ -1254,7 +1254,7 @@ void __init native_smp_cpus_done(unsigne

-static int additional_cpus __initdata = -1;
+static int additional_cpus __initdata = CONFIG_HOTPLUG_ADDITIONAL_CPUS;

* cpu_possible_map should be static, it cannot change as cpu's
@@ -1282,16 +1282,13 @@ __init void prefill_possible_map(void)
if (!num_processors)
num_processors = 1;

if (additional_cpus == -1) {
if (disabled_cpus > 0)
additional_cpus = disabled_cpus;
additional_cpus = 0;
- additional_cpus = 0;
possible = num_processors + additional_cpus;
if (possible > NR_CPUS)
possible = NR_CPUS;

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