s390/hiperdispatch: Introduce hiperdispatch

When LPAR is in vertical polarization, CPUs get different polarization
values, namely vertical high, vertical medium and vertical low. These
values represent the likelyhood of the CPU getting physical runtime.
Vertical high CPUs will always get runtime and others get varying
runtime depending on the load the CEC is under.

Vertical high and vertical medium CPUs are considered the CPUs which the
current LPAR has the entitlement to run on. The vertical lows are on the
other hand are borrowed CPUs which would only be given to the LPAR by
hipervisor when the other LPARs are not utilizing them.

Using the CPU capacities, hint linux scheduler when it should prioritise
vertical high and vertical medium CPUs over vertical low CPUs.
By tracking various system statistics hiperdispatch determines when to
adjust cpu capacities.
After each adjustment, rebuilding of scheduler domains is necessary to
notify the scheduler about capacity changes but since this operation is
costly it should be done as sparsely as possible.

Acked-by: Vasily Gorbik <[email protected]>
Co-developed-by: Tobias Huschle <[email protected]>
Signed-off-by: Tobias Huschle <[email protected]>
Signed-off-by: Mete Durlu <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>

Author: NickJackolson

arch/s390/include/asm/hiperdispatch.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright IBM Corp. 2024
+ */
+
+#ifndef _ASM_HIPERDISPATCH_H
+#define _ASM_HIPERDISPATCH_H
+
+void hd_reset_state(void);
+void hd_add_core(int cpu);
+void hd_disable_hiperdispatch(void);
+int hd_enable_hiperdispatch(void);
+
+#endif /* _ASM_HIPERDISPATCH_H */

arch/s390/kernel/Makefile

@@ -51,7 +51,7 @@ obj-$(CONFIG_SYSFS)		+= nospec-sysfs.o
 CFLAGS_REMOVE_nospec-branch.o	+= $(CC_FLAGS_EXPOLINE)
 
 obj-$(CONFIG_MODULES)		+= module.o
-obj-$(CONFIG_SCHED_TOPOLOGY)	+= topology.o
+obj-$(CONFIG_SCHED_TOPOLOGY)	+= topology.o hiperdispatch.o
 obj-$(CONFIG_NUMA)		+= numa.o
 obj-$(CONFIG_AUDIT)		+= audit.o
 compat-obj-$(CONFIG_AUDIT)	+= compat_audit.o

arch/s390/kernel/hiperdispatch.c

@@ -0,0 +1,199 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright IBM Corp. 2024
+ */
+
+#define KMSG_COMPONENT "hd"
+#define pr_fmt(fmt) KMSG_COMPONENT ": " fmt
+
+/*
+ * Hiperdispatch:
+ * Dynamically calculates the optimum number of high capacity COREs
+ * by considering the state the system is in. When hiperdispatch decides
+ * that a capacity update is necessary, it schedules a topology update.
+ * During topology updates the CPU capacities are always re-adjusted.
+ *
+ * There is two places where CPU capacities are being accessed within
+ * hiperdispatch.
+ * -> hiperdispatch's reoccuring work function reads CPU capacities to
+ *    determine high capacity CPU count.
+ * -> during a topology update hiperdispatch's adjustment function
+ *    updates CPU capacities.
+ * These two can run on different CPUs in parallel which can cause
+ * hiperdispatch to make wrong decisions. This can potentially cause
+ * some overhead by leading to extra rebuild_sched_domains() calls
+ * for correction. Access to capacities within hiperdispatch has to be
+ * serialized to prevent the overhead.
+ *
+ * Hiperdispatch decision making revolves around steal time.
+ * HD_STEAL_THRESHOLD value is taken as reference. Whenever steal time
+ * crosses the threshold value hiperdispatch falls back to giving high
+ * capacities to entitled CPUs. When steal time drops below the
+ * threshold boundary, hiperdispatch utilizes all CPUs by giving all
+ * of them high capacity.
+ *
+ * The theory behind HD_STEAL_THRESHOLD is related to the SMP thread
+ * performance. Comparing the throughput of;
+ * - single CORE, with N threads, running N tasks
+ * - N separate COREs running N tasks,
+ * using individual COREs for individual tasks yield better
+ * performance. This performance difference is roughly ~30% (can change
+ * between machine generations)
+ *
+ * Hiperdispatch tries to hint scheduler to use individual COREs for
+ * each task, as long as steal time on those COREs are less than 30%,
+ * therefore delaying the throughput loss caused by using SMP threads.
+ */
+
+#include <linux/cpumask.h>
+#include <linux/kernel_stat.h>
+#include <linux/ktime.h>
+#include <linux/workqueue.h>
+#include <asm/hiperdispatch.h>
+#include <asm/smp.h>
+#include <asm/topology.h>
+
+#define HD_DELAY_FACTOR			(4)
+#define HD_DELAY_INTERVAL		(HZ / 4)
+#define HD_STEAL_THRESHOLD		30
+
+static cpumask_t hd_vl_coremask;	/* Mask containing all vertical low COREs */
+static cpumask_t hd_vmvl_cpumask;	/* Mask containing vertical medium and low CPUs */
+static int hd_high_capacity_cores;	/* Current CORE count with high capacity */
+static int hd_entitled_cores;		/* Total vertical high and medium CORE count */
+static int hd_online_cores;		/* Current online CORE count */
+
+static unsigned long hd_previous_steal;	/* Previous iteration's CPU steal timer total */
+
+static void hd_capacity_work_fn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(hd_capacity_work, hd_capacity_work_fn);
+
+void hd_reset_state(void)
+{
+	cpumask_clear(&hd_vl_coremask);
+	cpumask_clear(&hd_vmvl_cpumask);
+	hd_entitled_cores = 0;
+	hd_online_cores = 0;
+}
+
+void hd_add_core(int cpu)
+{
+	const struct cpumask *siblings;
+	int polarization;
+
+	hd_online_cores++;
+	polarization = smp_cpu_get_polarization(cpu);
+	siblings = topology_sibling_cpumask(cpu);
+	switch (polarization) {
+	case POLARIZATION_VH:
+		hd_entitled_cores++;
+		break;
+	case POLARIZATION_VM:
+		hd_entitled_cores++;
+		cpumask_or(&hd_vmvl_cpumask, &hd_vmvl_cpumask, siblings);
+		break;
+	case POLARIZATION_VL:
+		cpumask_set_cpu(cpu, &hd_vl_coremask);
+		cpumask_or(&hd_vmvl_cpumask, &hd_vmvl_cpumask, siblings);
+		break;
+	}
+}
+
+static void hd_update_capacities(void)
+{
+	int cpu, upscaling_cores;
+	unsigned long capacity;
+
+	upscaling_cores = hd_high_capacity_cores - hd_entitled_cores;
+	capacity = upscaling_cores > 0 ? CPU_CAPACITY_HIGH : CPU_CAPACITY_LOW;
+	hd_high_capacity_cores = hd_entitled_cores;
+	for_each_cpu(cpu, &hd_vl_coremask) {
+		smp_set_core_capacity(cpu, capacity);
+		if (capacity != CPU_CAPACITY_HIGH)
+			continue;
+		hd_high_capacity_cores++;
+		upscaling_cores--;
+		if (upscaling_cores == 0)
+			capacity = CPU_CAPACITY_LOW;
+	}
+}
+
+void hd_disable_hiperdispatch(void)
+{
+	cancel_delayed_work_sync(&hd_capacity_work);
+	hd_high_capacity_cores = hd_online_cores;
+	hd_previous_steal = 0;
+}
+
+int hd_enable_hiperdispatch(void)
+{
+	if (hd_entitled_cores == 0)
+		return 0;
+	if (hd_online_cores <= hd_entitled_cores)
+		return 0;
+	mod_delayed_work(system_wq, &hd_capacity_work, HD_DELAY_INTERVAL * HD_DELAY_FACTOR);
+	hd_update_capacities();
+	return 1;
+}
+
+static unsigned long hd_calculate_steal_percentage(void)
+{
+	unsigned long time_delta, steal_delta, steal, percentage;
+	static ktime_t prev;
+	int cpus, cpu;
+	ktime_t now;
+
+	cpus = 0;
+	steal = 0;
+	percentage = 0;
+	for_each_cpu(cpu, &hd_vmvl_cpumask) {
+		steal += kcpustat_cpu(cpu).cpustat[CPUTIME_STEAL];
+		cpus++;
+	}
+	/*
+	 * If there is no vertical medium and low CPUs steal time
+	 * is 0 as vertical high CPUs shouldn't experience steal time.
+	 */
+	if (cpus == 0)
+		return percentage;
+	now = ktime_get();
+	time_delta = ktime_to_ns(ktime_sub(now, prev));
+	if (steal > hd_previous_steal && hd_previous_steal != 0) {
+		steal_delta = (steal - hd_previous_steal) * 100 / time_delta;
+		percentage = steal_delta / cpus;
+	}
+	hd_previous_steal = steal;
+	prev = now;
+	return percentage;
+}
+
+static void hd_capacity_work_fn(struct work_struct *work)
+{
+	unsigned long steal_percentage, new_cores;
+
+	mutex_lock(&smp_cpu_state_mutex);
+	/*
+	 * If online cores are less or equal to entitled cores hiperdispatch
+	 * does not need to make any adjustments, call a topology update to
+	 * disable hiperdispatch.
+	 * Normally this check is handled on topology update, but during cpu
+	 * unhotplug, topology and cpu mask updates are done in reverse
+	 * order, causing hd_enable_hiperdispatch() to get stale data.
+	 */
+	if (hd_online_cores <= hd_entitled_cores) {
+		topology_schedule_update();
+		mutex_unlock(&smp_cpu_state_mutex);
+		return;
+	}
+	steal_percentage = hd_calculate_steal_percentage();
+	if (steal_percentage < HD_STEAL_THRESHOLD)
+		new_cores = hd_online_cores;
+	else
+		new_cores = hd_entitled_cores;
+	if (hd_high_capacity_cores != new_cores) {
+		hd_high_capacity_cores = new_cores;
+		topology_schedule_update();
+	}
+	mutex_unlock(&smp_cpu_state_mutex);
+	schedule_delayed_work(&hd_capacity_work, HD_DELAY_INTERVAL);
+}

arch/s390/kernel/topology.c

@@ -24,6 +24,7 @@
 #include <linux/mm.h>
 #include <linux/nodemask.h>
 #include <linux/node.h>
+#include <asm/hiperdispatch.h>
 #include <asm/sysinfo.h>
 
 #define PTF_HORIZONTAL	(0UL)
@@ -47,6 +48,7 @@ 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;
+static int cpu_management;
 
 static DECLARE_WORK(topology_work, topology_work_fn);
 
@@ -144,6 +146,7 @@ static void add_cpus_to_mask(struct topology_core *tl_core,
 			cpumask_set_cpu(cpu, &book->mask);
 			cpumask_set_cpu(cpu, &socket->mask);
 			smp_cpu_set_polarization(cpu, tl_core->pp);
+			smp_cpu_set_capacity(cpu, CPU_CAPACITY_HIGH);
 		}
 	}
 }
@@ -270,6 +273,7 @@ void update_cpu_masks(void)
 			topo->drawer_id = id;
 		}
 	}
+	hd_reset_state();
 	for_each_online_cpu(cpu) {
 		topo = &cpu_topology[cpu];
 		pkg_first = cpumask_first(&topo->core_mask);
@@ -278,8 +282,10 @@ void update_cpu_masks(void)
 			for_each_cpu(sibling, &topo->core_mask) {
 				topo_sibling = &cpu_topology[sibling];
 				smt_first = cpumask_first(&topo_sibling->thread_mask);
-				if (sibling == smt_first)
+				if (sibling == smt_first) {
 					topo_package->booted_cores++;
+					hd_add_core(sibling);
+				}
 			}
 		} else {
 			topo->booted_cores = topo_package->booted_cores;
@@ -303,8 +309,10 @@ static void __arch_update_dedicated_flag(void *arg)
 static int __arch_update_cpu_topology(void)
 {
 	struct sysinfo_15_1_x *info = tl_info;
-	int rc = 0;
+	int rc, hd_status;
 
+	hd_status = 0;
+	rc = 0;
 	mutex_lock(&smp_cpu_state_mutex);
 	if (MACHINE_HAS_TOPOLOGY) {
 		rc = 1;
@@ -314,7 +322,11 @@ static int __arch_update_cpu_topology(void)
 	update_cpu_masks();
 	if (!MACHINE_HAS_TOPOLOGY)
 		topology_update_polarization_simple();
+	if (cpu_management == 1)
+		hd_status = hd_enable_hiperdispatch();
 	mutex_unlock(&smp_cpu_state_mutex);
+	if (hd_status == 0)
+		hd_disable_hiperdispatch();
 	return rc;
 }
 
@@ -374,8 +386,6 @@ void topology_expect_change(void)
 	set_topology_timer();
 }
 
-static int cpu_management;
-
 static int set_polarization(int polarization)
 {
 	int rc = 0;