x86: "nosmp" command line option should force the system into UP mode
Jan Beulich [Mon, 24 May 2010 19:13:17 +0000 (12:13 -0700)]
Bits set in cpu_possible_mask prior to the execution of
prefill_possible_map() (i.e.  when parsing ACPI or MPS tables) would
prevent the SMP alternatives logic from switching to UP mode, plus
unnecessary setup of per-CPU data for CPUs that can never come online.

Additionally, without CONFIG_HOTPLUG_CPU disabled CPUs can never come
online, and hence setting cpu_possible_mask bits for them is again a
simple waste of resources.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <201005241913.o4OJDH3Z010874@imap1.linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/smpboot.c

index 763d815..37462f1 100644 (file)
@@ -1215,9 +1215,17 @@ __init void prefill_possible_map(void)
        if (!num_processors)
                num_processors = 1;
 
-       if (setup_possible_cpus == -1)
-               possible = num_processors + disabled_cpus;
-       else
+       i = setup_max_cpus ?: 1;
+       if (setup_possible_cpus == -1) {
+               possible = num_processors;
+#ifdef CONFIG_HOTPLUG_CPU
+               if (setup_max_cpus)
+                       possible += disabled_cpus;
+#else
+               if (possible > i)
+                       possible = i;
+#endif
+       } else
                possible = setup_possible_cpus;
 
        total_cpus = max_t(int, possible, num_processors + disabled_cpus);
@@ -1230,11 +1238,23 @@ __init void prefill_possible_map(void)
                possible = nr_cpu_ids;
        }
 
+#ifdef CONFIG_HOTPLUG_CPU
+       if (!setup_max_cpus)
+#endif
+       if (possible > i) {
+               printk(KERN_WARNING
+                       "%d Processors exceeds max_cpus limit of %u\n",
+                       possible, setup_max_cpus);
+               possible = i;
+       }
+
        printk(KERN_INFO "SMP: Allowing %d CPUs, %d hotplug CPUs\n",
                possible, max_t(int, possible - num_processors, 0));
 
        for (i = 0; i < possible; i++)
                set_cpu_possible(i, true);
+       for (; i < NR_CPUS; i++)
+               set_cpu_possible(i, false);
 
        nr_cpu_ids = possible;
 }