[PATCH] x86 boot: Wait for boot cpu to show up if nr_cpus limit is about to hit

[Vivek Goyal <vgoyal@redhat.com>]

nr_cpus allows one to specify number of possible cpus in the system.
Current assumption seems to be that first cpu to show up is boot cpu
and this assumption will be broken in kdump scenario where we can be
booting on a non boot cpu with nr_cpus=1.

It might happen that first cpu we parse is not the cpu we boot on and
later we ignore boot cpu. Though code later seems to recognize this
anomaly and forcibly sets boot cpu in physical cpu map with following
warning.

if (!physid_isset(hard_smp_processor_id(), phys_cpu_present_map)) {
        printk(KERN_WARNING
                "weird, boot CPU (#%d) not listed by the BIOS.\n",
                hard_smp_processor_id());

        physid_set(hard_smp_processor_id(), phys_cpu_present_map);
}

This patch waits for boot cpu to show up and starts ignoring the cpus
once we have hit (nr_cpus - 1) number of cpus. So effectively we are
reserving one slot out of nr_cpus for boot cpu explicitly.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 arch/x86/kernel/apic/apic.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

Index: linux-2.6/arch/x86/kernel/apic/apic.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/apic.c	2011-07-08 10:44:48.003709709 -0400
+++ linux-2.6/arch/x86/kernel/apic/apic.c	2011-07-08 12:02:01.272223921 -0400
@@ -1943,10 +1943,28 @@ void disconnect_bsp_APIC(int virt_wire_s
 
 void __cpuinit generic_processor_info(int apicid, int version)
 {
-	int cpu;
+	int cpu, max = nr_cpu_ids;
+	bool boot_cpu_detected = physid_isset(boot_cpu_physical_apicid,
+				phys_cpu_present_map);
+
+	/*
+	 * If boot cpu has not been detected yet, then only allow upto
+	 * nr_cpu_ids - 1 processors and keep one slot free for boot cpu
+	 */
+	if (!boot_cpu_detected && num_processors >= nr_cpu_ids - 1 &&
+	    apicid != boot_cpu_physical_apicid) {
+		int thiscpu = max + disabled_cpus - 1;
+
+		pr_warning(
+			"ACPI: NR_CPUS/possible_cpus limit of %i almost"
+			" reached. Keeping one slot for boot cpu."
+			"  Processor %d/0x%x ignored.\n", max, thiscpu, apicid);
+
+		disabled_cpus++;
+		return;
+	}
 
 	if (num_processors >= nr_cpu_ids) {
-		int max = nr_cpu_ids;
 		int thiscpu = max + disabled_cpus;
 
 		pr_warning(