s390/topology: alternative topology for topology-less machines

If running on machines that do not provide topology information we
currently generate a "fake" topology which defines the maximum
distance between each cpu: each cpu will be put into an own drawer.

Historically this used to be the best option for (virtual) machines in
overcommited hypervisors.

For some workloads however it is better to generate a different
topology where all cpus are siblings within a package (all cpus are
core siblings). This shows performance improvements of up to 10%,
depending on the workload.

In order to keep the current behaviour, but also allow to switch to
the different core sibling topology use the existing "topology="
kernel parameter:

Specifying "topology=on" on machines without topology information will
generate the core siblings (fake) topology information, instead of the
default topology information where all cpus have the maximum distance.

On machines which provide topology information specifying
"topology=on" does not have any effect.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/early.c

@@ -404,18 +404,6 @@ static inline void save_vector_registers(void)
 #endif
 }
 
-static int __init topology_setup(char *str)
-{
-	bool enabled;
-	int rc;
-
-	rc = kstrtobool(str, &enabled);
-	if (!rc && !enabled)
-		S390_lowcore.machine_flags &= ~MACHINE_FLAG_TOPOLOGY;
-	return rc;
-}
-early_param("topology", topology_setup);
-
 static int __init disable_vector_extension(char *str)
 {
 	S390_lowcore.machine_flags &= ~MACHINE_FLAG_VX;

arch/s390/kernel/topology.c

@@ -29,12 +29,20 @@
 #define PTF_VERTICAL	(1UL)
 #define PTF_CHECK	(2UL)
 
+enum {
+	TOPOLOGY_MODE_HW,
+	TOPOLOGY_MODE_SINGLE,
+	TOPOLOGY_MODE_PACKAGE,
+	TOPOLOGY_MODE_UNINITIALIZED
+};
+
 struct mask_info {
 	struct mask_info *next;
 	unsigned char id;
 	cpumask_t mask;
 };
 
+static int topology_mode = TOPOLOGY_MODE_UNINITIALIZED;
 static void set_topology_timer(void);
 static void topology_work_fn(struct work_struct *work);
 static struct sysinfo_15_1_x *tl_info;
@@ -59,11 +67,26 @@ static cpumask_t cpu_group_map(struct mask_info *info, unsigned int cpu)
 	cpumask_t mask;
 
 	cpumask_copy(&mask, cpumask_of(cpu));
-	if (!MACHINE_HAS_TOPOLOGY)
-		return mask;
-	for (; info; info = info->next) {
-		if (cpumask_test_cpu(cpu, &info->mask))
-			return info->mask;
+	switch (topology_mode) {
+	case TOPOLOGY_MODE_HW:
+		while (info) {
+			if (cpumask_test_cpu(cpu, &info->mask)) {
+				mask = info->mask;
+				break;
+			}
+			info = info->next;
+		}
+		if (cpumask_empty(&mask))
+			cpumask_copy(&mask, cpumask_of(cpu));
+		break;
+	case TOPOLOGY_MODE_PACKAGE:
+		cpumask_copy(&mask, cpu_present_mask);
+		break;
+	default:
+		/* fallthrough */
+	case TOPOLOGY_MODE_SINGLE:
+		cpumask_copy(&mask, cpumask_of(cpu));
+		break;
 	}
 	return mask;
 }
@@ -74,7 +97,7 @@ static cpumask_t cpu_thread_map(unsigned int cpu)
 	int i;
 
 	cpumask_copy(&mask, cpumask_of(cpu));
-	if (!MACHINE_HAS_TOPOLOGY)
+	if (topology_mode != TOPOLOGY_MODE_HW)
 		return mask;
 	cpu -= cpu % (smp_cpu_mtid + 1);
 	for (i = 0; i <= smp_cpu_mtid; i++)
@@ -223,7 +246,7 @@ int topology_set_cpu_management(int fc)
 static void update_cpu_masks(void)
 {
 	struct cpu_topology_s390 *topo;
-	int cpu;
+	int cpu, id;
 
 	for_each_possible_cpu(cpu) {
 		topo = &cpu_topology[cpu];
@@ -231,12 +254,13 @@ static void update_cpu_masks(void)
 		topo->core_mask = cpu_group_map(&socket_info, cpu);
 		topo->book_mask = cpu_group_map(&book_info, cpu);
 		topo->drawer_mask = cpu_group_map(&drawer_info, cpu);
-		if (!MACHINE_HAS_TOPOLOGY) {
+		if (topology_mode != TOPOLOGY_MODE_HW) {
+			id = topology_mode == TOPOLOGY_MODE_PACKAGE ? 0 : cpu;
 			topo->thread_id = cpu;
 			topo->core_id = cpu;
-			topo->socket_id = cpu;
-			topo->book_id = cpu;
-			topo->drawer_id = cpu;
+			topo->socket_id = id;
+			topo->book_id = id;
+			topo->drawer_id = id;
 			if (cpu_present(cpu))
 				cpumask_set_cpu(cpu, &cpus_with_topology);
 		}
@@ -459,6 +483,12 @@ void __init topology_init_early(void)
 	struct sysinfo_15_1_x *info;
 
 	set_sched_topology(s390_topology);
+	if (topology_mode == TOPOLOGY_MODE_UNINITIALIZED) {
+		if (MACHINE_HAS_TOPOLOGY)
+			topology_mode = TOPOLOGY_MODE_HW;
+		else
+			topology_mode = TOPOLOGY_MODE_SINGLE;
+	}
 	if (!MACHINE_HAS_TOPOLOGY)
 		goto out;
 	tl_info = memblock_virt_alloc(PAGE_SIZE, PAGE_SIZE);
@@ -474,6 +504,26 @@ void __init topology_init_early(void)
 	__arch_update_cpu_topology();
 }
 
+static inline int topology_get_mode(int enabled)
+{
+	if (!enabled)
+		return TOPOLOGY_MODE_SINGLE;
+	return MACHINE_HAS_TOPOLOGY ? TOPOLOGY_MODE_HW : TOPOLOGY_MODE_PACKAGE;
+}
+
+static int __init topology_setup(char *str)
+{
+	bool enabled;
+	int rc;
+
+	rc = kstrtobool(str, &enabled);
+	if (rc)
+		return rc;
+	topology_mode = topology_get_mode(enabled);
+	return 0;
+}
+early_param("topology", topology_setup);
+
 static int __init topology_init(void)
 {
 	if (MACHINE_HAS_TOPOLOGY)