[S390] cpu topology: introduce kernel parameter

Introduce a topology=[on|off] kernel parameter which allows to switch
cpu topology on/off. Default will be off, since it looks like that for
some workloards this doesn't behave very well (on s390).

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/kernel-parameters.txt

@@ -2252,6 +2252,14 @@ and is between 256 and 4096 characters. It is defined in the file
 			See comment before function dc390_setup() in
 			drivers/scsi/tmscsim.c.
 
+	topology=	[S390]
+			Format: {off | on}
+			Specify if the kernel should make use of the cpu
+			topology informations if the hardware supports these.
+			The scheduler will make use of these informations and
+			e.g. base its process migration decisions on it.
+			Default is off.
+
 	tp720=		[HW,PS2]
 
 	trix=		[HW,OSS] MediaTrix AudioTrix Pro

arch/s390/kernel/topology.c

@@ -58,6 +58,7 @@ struct core_info {
 	cpumask_t mask;
 };
 
+static int topology_enabled;
 static void topology_work_fn(struct work_struct *work);
 static struct tl_info *tl_info;
 static struct core_info core_info;
@@ -78,7 +79,7 @@ cpumask_t cpu_coregroup_map(unsigned int cpu)
 	cpumask_t mask;
 
 	cpus_clear(mask);
-	if (!machine_has_topology)
+	if (!topology_enabled || !machine_has_topology)
 		return cpu_possible_map;
 	spin_lock_irqsave(&topology_lock, flags);
 	while (core) {
@@ -263,6 +264,15 @@ static void topology_interrupt(__u16 code)
 	schedule_work(&topology_work);
 }
 
+static int __init early_parse_topology(char *p)
+{
+	if (strncmp(p, "on", 2))
+		return 0;
+	topology_enabled = 1;
+	return 0;
+}
+early_param("topology", early_parse_topology);
+
 static int __init init_topology_update(void)
 {
 	int rc;