[PATCH] [RFC] wrong cpus started

[Mark Rustad <mrustad@gmail.com>]

I have noticed that recent kernels, 2.6.16 through at least 2.6.19, hyperthreading
can no longer be effectively disabled by using maxcpus. This means that anyone that
was using maxcpus to disable hyperthreading when running Linux will no longer be
using all of their physical cpus.

The following patch restores the earlier behavior, but I am fairly sure that some
CPU hotplug environments would not want this change. I can imagine making the two
lines moved in the patch below to instead be conditional on CONFIG_HOTPLUG_CPU and
!CONFIG_HOTPLUG_CPU respectively. Can someone more familiar with hotplug CPU come
up with a better fix?

--- a/arch/i386/kernel/mpparse.c	2006-11-29 15:57:37.000000000 -0600
+++ b/arch/i386/kernel/mpparse.c	2007-06-06 16:41:06.635814335 -0500
@@ -180,9 +180,6 @@ static void __devinit MP_processor_info 
 	}
 	apic_version[m->mpc_apicid] = ver;
 
-	phys_cpu = apicid_to_cpu_present(apicid);
-	physids_or(phys_cpu_present_map, phys_cpu_present_map, phys_cpu);
-
 	if (num_processors >= NR_CPUS) {
 		printk(KERN_WARNING "WARNING: NR_CPUS limit of %i reached."
 			"  Processor ignored.\n", NR_CPUS);
@@ -195,6 +192,9 @@ static void __devinit MP_processor_info 
 		return;
 	}
 
+	phys_cpu = apicid_to_cpu_present(apicid);
+	physids_or(phys_cpu_present_map, phys_cpu_present_map, phys_cpu);
+
 	cpu_set(num_processors, cpu_possible_map);
 	num_processors++;
 
-- 
Mark Rustad, mrustad@gmail.com
-
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