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

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 <[email protected]>

---
 arch/x86/kernel/apic/apic.c |   22 ++++++++++++++++++++--

 1 file changed, 20 insertions(+), 2 deletions(-)

Index: linux-2.6/arch/x86/kernel/apic/apic.c
===================================================================

arch/x86/kernel/apic/apic.c       2011-07-08 12:02:01.272223921 -0400 renamed from arch/x86/kernel/apic/apic.c  2011-07-08 10:44:48.003709709 -0400 +20 -2

@@ -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(
--
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