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some systems that have disable cpus entries because same
BIOS will support 2 sockets and 4 sockets and more at
same time, BIOS just leave some disable entries, but
those system do not support cpu hotplug. we don't need
treat disabled_cpus as hotplug cpus.
so we can make nr_cpu_ids smaller and save more space
(pcpu data allocations), and could make some systems run
with logical flat instead of physical flat apic mode

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
arch/x86/kernel/smpboot.c | 39 +++++++++++++++++++++++++++++++++++++--
1 file changed, 37 insertions(+), 2 deletions(-)

Index: linux-2.6/arch/x86/kernel/smpboot.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/smpboot.c
+++ linux-2.6/arch/x86/kernel/smpboot.c
@@ -47,6 +47,7 @@
#include <linux/bootmem.h>
#include <linux/err.h>
#include <linux/nmi.h>
+#include <linux/dmi.h>
#include <linux/tboot.h>

#include <asm/acpi.h>
@@ -1180,6 +1181,24 @@ static int __init _setup_possible_cpus(c
}
early_param("possible_cpus", _setup_possible_cpus);

+static __initdata int treat_disabled_cpus_as_hotplug;
+static __init int hotplug_cpus_check(const struct dmi_system_id *d)
+{
+ printk(KERN_NOTICE "%s detected: treat disabled cpus as hotplug ones\n", d->ident);
+ treat_disabled_cpus_as_hotplug = 1;
+
+ return 0;
+}
+
+static struct dmi_system_id hotplug_cpus_dmi_table[] __initdata = {
+ { hotplug_cpus_check, "WHICH VENDOR WHAT SYSTEM",
+ { DMI_MATCH(DMI_BIOS_VENDOR, "WHAT VENDOR"),
+ DMI_MATCH(DMI_BIOS_VERSION, "WHAT VER"),
+ }
+ },
+ { } /* NULL entry stops DMI scanning */
+};
+

/*
* cpu_possible_mask should be static, it cannot change as cpu's
@@ -1206,8 +1225,24 @@ __init void prefill_possible_map(void)
if (!num_processors)
num_processors = 1;

- if (setup_possible_cpus == -1)
- possible = num_processors + disabled_cpus;
+ if (setup_possible_cpus == -1) {
+ possible = num_processors;
+ /*
+ * do we have better way to detect hotplug cpus?
+ *
+ * some systems that have disable cpus entries because same
+ * BIOS will support 2 sockets and 4 sockets and more at
+ * same time, BIOS just leave some disable entries, but
+ * those system do not support cpu hotplug. we don't need
+ * treat disabled_cpus as hotplug cpus.
+ * so we can make nr_cpu_ids smaller and save more space
+ * (pcpu data allocations), and could make some systems run
+ * with logical flat instead of physical flat apic mode
+ */
+ dmi_check_system(hotplug_cpus_dmi_table);
+ if (treat_disabled_cpus_as_hotplug)
+ possible += disabled_cpus;
+ }
else
possible = setup_possible_cpus;


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