[RFCv6 PATCH 03/10] sched: scheduler-driven cpu frequency selection

[Steve Muckle]

From: Michael Turquette <mturquette@xxxxxxxxxxxx>

Scheduler-driven CPU frequency selection hopes to exploit both
per-task and global information in the scheduler to improve frequency
selection policy, achieving lower power consumption, improved
responsiveness/performance, and less reliance on heuristics and
tunables. For further discussion on the motivation of this integration
see [0].

This patch implements a shim layer between the Linux scheduler and the
cpufreq subsystem. The interface accepts capacity requests from the
CFS, RT and deadline sched classes. The requests from each sched class
are summed on each CPU with a margin applied to the CFS and RT
capacity requests to provide some headroom. Deadline requests are
expected to be precise enough given their nature to not require
headroom. The maximum total capacity request for a CPU in a frequency
domain drives the requested frequency for that domain.

Policy is determined by both the sched classes and this shim layer.

Note that this algorithm is event-driven. There is no polling loop to
check cpu idle time nor any other method which is unsynchronized with
the scheduler, aside from a throttling mechanism to ensure frequency
changes are not attempted faster than the hardware can accommodate them.

Thanks to Juri Lelli <juri.lelli@xxxxxxx> for contributing design ideas,
code and test results, and to Ricky Liang <jcliang@xxxxxxxxxxxx>
for initialization and static key inc/dec fixes.

[0] http://article.gmane.org/gmane.linux.kernel/1499836

[smuckle@xxxxxxxxxx: various additions and fixes, revised commit text]

CC: Ricky Liang <jcliang@xxxxxxxxxxxx>
Signed-off-by: Michael Turquette <mturquette@xxxxxxxxxxxx>
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxx>
Signed-off-by: Steve Muckle <smuckle@xxxxxxxxxx>
---
 drivers/cpufreq/Kconfig      |  20 +++
 include/linux/cpufreq.h      |   3 +
 include/linux/sched.h        |   8 +
 kernel/sched/Makefile        |   1 +
 kernel/sched/cpufreq_sched.c | 364 +++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h         |  51 ++++++
 6 files changed, 447 insertions(+)
 create mode 100644 kernel/sched/cpufreq_sched.c

diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
index 659879a..6f2e96c 100644
--- a/drivers/cpufreq/Kconfig
+++ b/drivers/cpufreq/Kconfig
@@ -102,6 +102,14 @@ config CPU_FREQ_DEFAULT_GOV_CONSERVATIVE
 	  Be aware that not all cpufreq drivers support the conservative
 	  governor. If unsure have a look at the help section of the
 	  driver. Fallback governor will be the performance governor.
+
+config CPU_FREQ_DEFAULT_GOV_SCHED
+	bool "sched"
+	select CPU_FREQ_GOV_SCHED
+	help
+	  Use the CPUfreq governor 'sched' as default. This scales
+	  cpu frequency using CPU utilization estimates from the
+	  scheduler.
 endchoice
 
 config CPU_FREQ_GOV_PERFORMANCE
@@ -183,6 +191,18 @@ config CPU_FREQ_GOV_CONSERVATIVE
 
 	  If in doubt, say N.
 
+config CPU_FREQ_GOV_SCHED
+	bool "'sched' cpufreq governor"
+	depends on CPU_FREQ
+	select CPU_FREQ_GOV_COMMON
+	help
+	  'sched' - this governor scales cpu frequency from the
+	  scheduler as a function of cpu capacity utilization. It does
+	  not evaluate utilization on a periodic basis (as ondemand
+	  does) but instead is event-driven by the scheduler.
+
+	  If in doubt, say N.
+
 comment "CPU frequency scaling drivers"
 
 config CPUFREQ_DT
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 7f8c63d..7e4bde1 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -495,6 +495,9 @@ extern struct cpufreq_governor cpufreq_gov_ondemand;
 #elif defined(CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE)
 extern struct cpufreq_governor cpufreq_gov_conservative;
 #define CPUFREQ_DEFAULT_GOVERNOR	(&cpufreq_gov_conservative)
+#elif defined(CONFIG_CPU_FREQ_DEFAULT_GOV_SCHED)
+extern struct cpufreq_governor cpufreq_gov_sched;
+#define CPUFREQ_DEFAULT_GOVERNOR	(&cpufreq_gov_sched)
 #endif
 
 /*********************************************************************
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3b0de68..d910a31 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -927,6 +927,14 @@ enum cpu_idle_type {
 #define SCHED_CAPACITY_SHIFT	10
 #define SCHED_CAPACITY_SCALE	(1L << SCHED_CAPACITY_SHIFT)
 
+struct sched_capacity_reqs {
+	unsigned long cfs;
+	unsigned long rt;
+	unsigned long dl;
+
+	unsigned long total;
+};
+
 /*
  * Wake-queues are lists of tasks with a pending wakeup, whose
  * callers have already marked the task as woken internally,
diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 6768797..90ed832 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -19,3 +19,4 @@ obj-$(CONFIG_SCHED_AUTOGROUP) += auto_group.o
 obj-$(CONFIG_SCHEDSTATS) += stats.o
 obj-$(CONFIG_SCHED_DEBUG) += debug.o
 obj-$(CONFIG_CGROUP_CPUACCT) += cpuacct.o
+obj-$(CONFIG_CPU_FREQ_GOV_SCHED) += cpufreq_sched.o
diff --git a/kernel/sched/cpufreq_sched.c b/kernel/sched/cpufreq_sched.c
new file mode 100644
index 0000000..af8b5bc
--- /dev/null
+++ b/kernel/sched/cpufreq_sched.c
@@ -0,0 +1,364 @@
+/*
+ *  Copyright (C)  2015 Michael Turquette <mturquette@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/cpufreq.h>
+#include <linux/module.h>
+#include <linux/kthread.h>
+#include <linux/percpu.h>
+#include <linux/irq_work.h>
+#include <linux/delay.h>
+#include <linux/string.h>
+
+#include "sched.h"
+
+#define THROTTLE_NSEC		50000000 /* 50ms default */
+
+struct static_key __read_mostly __sched_freq = STATIC_KEY_INIT_FALSE;
+static bool __read_mostly cpufreq_driver_slow;
+
+#ifndef CONFIG_CPU_FREQ_DEFAULT_GOV_SCHED
+static struct cpufreq_governor cpufreq_gov_sched;
+#endif
+
+/*
+ * Capacity margin added to CFS and RT capacity requests to provide
+ * some head room if task utilization further increases.
+ */
+unsigned int capacity_margin = 1280;
+
+static DEFINE_PER_CPU(unsigned long, enabled);
+DEFINE_PER_CPU(struct sched_capacity_reqs, cpu_sched_capacity_reqs);
+
+/**
+ * gov_data - per-policy data internal to the governor
+ * @throttle: next throttling period expiry. Derived from throttle_nsec
+ * @throttle_nsec: throttle period length in nanoseconds
+ * @task: worker thread for dvfs transition that may block/sleep
+ * @irq_work: callback used to wake up worker thread
+ * @requested_freq: last frequency requested by the sched governor
+ *
+ * struct gov_data is the per-policy cpufreq_sched-specific data structure. A
+ * per-policy instance of it is created when the cpufreq_sched governor receives
+ * the CPUFREQ_GOV_START condition and a pointer to it exists in the gov_data
+ * member of struct cpufreq_policy.
+ *
+ * Readers of this data must call down_read(policy->rwsem). Writers must
+ * call down_write(policy->rwsem).
+ */
+struct gov_data {
+	ktime_t throttle;
+	unsigned int throttle_nsec;
+	struct task_struct *task;
+	struct irq_work irq_work;
+	unsigned int requested_freq;
+};
+
+static void cpufreq_sched_try_driver_target(struct cpufreq_policy *policy,
+					    unsigned int freq)
+{
+	struct gov_data *gd = policy->governor_data;
+
+	/* avoid race with cpufreq_sched_stop */
+	if (!down_write_trylock(&policy->rwsem))
+		return;
+
+	__cpufreq_driver_target(policy, freq, CPUFREQ_RELATION_L);
+
+	gd->throttle = ktime_add_ns(ktime_get(), gd->throttle_nsec);
+	up_write(&policy->rwsem);
+}
+
+static bool finish_last_request(struct gov_data *gd)
+{
+	ktime_t now = ktime_get();
+
+	if (ktime_after(now, gd->throttle))
+		return false;
+
+	while (1) {
+		int usec_left = ktime_to_ns(ktime_sub(gd->throttle, now));
+
+		usec_left /= NSEC_PER_USEC;
+		usleep_range(usec_left, usec_left + 100);
+		now = ktime_get();
+		if (ktime_after(now, gd->throttle))
+			return true;
+	}
+}
+
+/*
+ * we pass in struct cpufreq_policy. This is safe because changing out the
+ * policy requires a call to __cpufreq_governor(policy, CPUFREQ_GOV_STOP),
+ * which tears down all of the data structures and __cpufreq_governor(policy,
+ * CPUFREQ_GOV_START) will do a full rebuild, including this kthread with the
+ * new policy pointer
+ */
+static int cpufreq_sched_thread(void *data)
+{
+	struct sched_param param;
+	struct cpufreq_policy *policy;
+	struct gov_data *gd;
+	unsigned int new_request = 0;
+	unsigned int last_request = 0;
+	int ret;
+
+	policy = (struct cpufreq_policy *) data;
+	gd = policy->governor_data;
+
+	param.sched_priority = 50;
+	ret = sched_setscheduler_nocheck(gd->task, SCHED_FIFO, &param);
+	if (ret) {
+		pr_warn("%s: failed to set SCHED_FIFO\n", __func__);
+		do_exit(-EINVAL);
+	} else {
+		pr_debug("%s: kthread (%d) set to SCHED_FIFO\n",
+				__func__, gd->task->pid);
+	}
+
+	do {
+		set_current_state(TASK_INTERRUPTIBLE);
+		new_request = gd->requested_freq;
+		if (new_request == last_request) {
+			schedule();
+		} else {
+			/*
+			 * if the frequency thread sleeps while waiting to be
+			 * unthrottled, start over to check for a newer request
+			 */
+			if (finish_last_request(gd))
+				continue;
+			last_request = new_request;
+			cpufreq_sched_try_driver_target(policy, new_request);
+		}
+	} while (!kthread_should_stop());
+
+	return 0;
+}
+
+static void cpufreq_sched_irq_work(struct irq_work *irq_work)
+{
+	struct gov_data *gd;
+
+	gd = container_of(irq_work, struct gov_data, irq_work);
+	if (!gd)
+		return;
+
+	wake_up_process(gd->task);
+}
+
+static void update_fdomain_capacity_request(int cpu)
+{
+	unsigned int freq_new, index_new, cpu_tmp;
+	struct cpufreq_policy *policy;
+	struct gov_data *gd;
+	unsigned long capacity = 0;
+
+	/*
+	 * Avoid grabbing the policy if possible. A test is still
+	 * required after locking the CPU's policy to avoid racing
+	 * with the governor changing.
+	 */
+	if (!per_cpu(enabled, cpu))
+		return;
+
+	policy = cpufreq_cpu_get(cpu);
+	if (IS_ERR_OR_NULL(policy))
+		return;
+
+	if (policy->governor != &cpufreq_gov_sched ||
+	    !policy->governor_data)
+		goto out;
+
+	gd = policy->governor_data;
+
+	/* find max capacity requested by cpus in this policy */
+	for_each_cpu(cpu_tmp, policy->cpus) {
+		struct sched_capacity_reqs *scr;
+
+		scr = &per_cpu(cpu_sched_capacity_reqs, cpu_tmp);
+		capacity = max(capacity, scr->total);
+	}
+
+	/* Convert the new maximum capacity request into a cpu frequency */
+	freq_new = capacity * policy->max >> SCHED_CAPACITY_SHIFT;
+	if (cpufreq_frequency_table_target(policy, policy->freq_table,
+					   freq_new, CPUFREQ_RELATION_L,
+					   &index_new))
+		goto out;
+	freq_new = policy->freq_table[index_new].frequency;
+
+	if (freq_new == gd->requested_freq)
+		goto out;
+
+	gd->requested_freq = freq_new;
+
+	/*
+	 * Throttling is not yet supported on platforms with fast cpufreq
+	 * drivers.
+	 */
+	if (cpufreq_driver_slow)
+		irq_work_queue_on(&gd->irq_work, cpu);
+	else
+		cpufreq_sched_try_driver_target(policy, freq_new);
+
+out:
+	cpufreq_cpu_put(policy);
+}
+
+void update_cpu_capacity_request(int cpu, bool request)
+{
+	unsigned long new_capacity;
+	struct sched_capacity_reqs *scr;
+
+	/* The rq lock serializes access to the CPU's sched_capacity_reqs. */
+	lockdep_assert_held(&cpu_rq(cpu)->lock);
+
+	scr = &per_cpu(cpu_sched_capacity_reqs, cpu);
+
+	new_capacity = scr->cfs + scr->rt;
+	new_capacity = new_capacity * capacity_margin
+		/ SCHED_CAPACITY_SCALE;
+	new_capacity += scr->dl;
+
+	if (new_capacity == scr->total)
+		return;
+
+	scr->total = new_capacity;
+	if (request)
+		update_fdomain_capacity_request(cpu);
+}
+
+static inline void set_sched_freq(void)
+{
+	static_key_slow_inc(&__sched_freq);
+}
+
+static inline void clear_sched_freq(void)
+{
+	static_key_slow_dec(&__sched_freq);
+}
+
+static int cpufreq_sched_policy_init(struct cpufreq_policy *policy)
+{
+	struct gov_data *gd;
+	int cpu;
+
+	for_each_cpu(cpu, policy->cpus)
+		memset(&per_cpu(cpu_sched_capacity_reqs, cpu), 0,
+		       sizeof(struct sched_capacity_reqs));
+
+	gd = kzalloc(sizeof(*gd), GFP_KERNEL);
+	if (!gd)
+		return -ENOMEM;
+
+	gd->throttle_nsec = policy->cpuinfo.transition_latency ?
+			    policy->cpuinfo.transition_latency :
+			    THROTTLE_NSEC;
+	pr_debug("%s: throttle threshold = %u [ns]\n",
+		  __func__, gd->throttle_nsec);
+
+	if (cpufreq_driver_is_slow()) {
+		cpufreq_driver_slow = true;
+		gd->task = kthread_create(cpufreq_sched_thread, policy,
+					  "kschedfreq:%d",
+					  cpumask_first(policy->related_cpus));
+		if (IS_ERR_OR_NULL(gd->task)) {
+			pr_err("%s: failed to create kschedfreq thread\n",
+			       __func__);
+			goto err;
+		}
+		get_task_struct(gd->task);
+		kthread_bind_mask(gd->task, policy->related_cpus);
+		wake_up_process(gd->task);
+		init_irq_work(&gd->irq_work, cpufreq_sched_irq_work);
+	}
+
+	policy->governor_data = gd;
+	set_sched_freq();
+
+	return 0;
+
+err:
+	kfree(gd);
+	return -ENOMEM;
+}
+
+static int cpufreq_sched_policy_exit(struct cpufreq_policy *policy)
+{
+	struct gov_data *gd = policy->governor_data;
+
+	clear_sched_freq();
+	if (cpufreq_driver_slow) {
+		kthread_stop(gd->task);
+		put_task_struct(gd->task);
+	}
+
+	policy->governor_data = NULL;
+
+	kfree(gd);
+	return 0;
+}
+
+static int cpufreq_sched_start(struct cpufreq_policy *policy)
+{
+	int cpu;
+
+	for_each_cpu(cpu, policy->cpus)
+		per_cpu(enabled, cpu) = 1;
+
+	return 0;
+}
+
+static int cpufreq_sched_stop(struct cpufreq_policy *policy)
+{
+	int cpu;
+
+	for_each_cpu(cpu, policy->cpus)
+		per_cpu(enabled, cpu) = 0;
+
+	return 0;
+}
+
+static int cpufreq_sched_setup(struct cpufreq_policy *policy,
+			       unsigned int event)
+{
+	switch (event) {
+	case CPUFREQ_GOV_POLICY_INIT:
+		return cpufreq_sched_policy_init(policy);
+	case CPUFREQ_GOV_POLICY_EXIT:
+		return cpufreq_sched_policy_exit(policy);
+	case CPUFREQ_GOV_START:
+		return cpufreq_sched_start(policy);
+	case CPUFREQ_GOV_STOP:
+		return cpufreq_sched_stop(policy);
+	case CPUFREQ_GOV_LIMITS:
+		break;
+	}
+	return 0;
+}
+
+#ifndef CONFIG_CPU_FREQ_DEFAULT_GOV_SCHED
+static
+#endif
+struct cpufreq_governor cpufreq_gov_sched = {
+	.name			= "sched",
+	.governor		= cpufreq_sched_setup,
+	.owner			= THIS_MODULE,
+};
+
+static int __init cpufreq_sched_init(void)
+{
+	int cpu;
+
+	for_each_cpu(cpu, cpu_possible_mask)
+		per_cpu(enabled, cpu) = 0;
+	return cpufreq_register_governor(&cpufreq_gov_sched);
+}
+
+/* Try to make this the default governor */
+fs_initcall(cpufreq_sched_init);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a5a6b3e..a88dbec 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1383,6 +1383,57 @@ unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
 }
 #endif
 
+#ifdef CONFIG_CPU_FREQ_GOV_SCHED
+extern unsigned int capacity_margin;
+extern struct static_key __sched_freq;
+
+static inline bool sched_freq(void)
+{
+	return static_key_false(&__sched_freq);
+}
+
+DECLARE_PER_CPU(struct sched_capacity_reqs, cpu_sched_capacity_reqs);
+void update_cpu_capacity_request(int cpu, bool request);
+
+static inline void set_cfs_cpu_capacity(int cpu, bool request,
+					unsigned long capacity)
+{
+	if (per_cpu(cpu_sched_capacity_reqs, cpu).cfs != capacity) {
+		per_cpu(cpu_sched_capacity_reqs, cpu).cfs = capacity;
+		update_cpu_capacity_request(cpu, request);
+	}
+}
+
+static inline void set_rt_cpu_capacity(int cpu, bool request,
+				       unsigned long capacity)
+{
+	if (per_cpu(cpu_sched_capacity_reqs, cpu).rt != capacity) {
+		per_cpu(cpu_sched_capacity_reqs, cpu).rt = capacity;
+		update_cpu_capacity_request(cpu, request);
+	}
+}
+
+static inline void set_dl_cpu_capacity(int cpu, bool request,
+				       unsigned long capacity)
+{
+	if (per_cpu(cpu_sched_capacity_reqs, cpu).dl != capacity) {
+		per_cpu(cpu_sched_capacity_reqs, cpu).dl = capacity;
+		update_cpu_capacity_request(cpu, request);
+	}
+}
+#else
+static inline bool sched_freq(void) { return false; }
+static inline void set_cfs_cpu_capacity(int cpu, bool request,
+					unsigned long capacity)
+{ }
+static inline void set_rt_cpu_capacity(int cpu, bool request,
+				       unsigned long capacity)
+{ }
+static inline void set_dl_cpu_capacity(int cpu, bool request,
+				       unsigned long capacity)
+{ }
+#endif
+
 static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
 {
 	rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL, cpu_of(rq));
-- 
2.4.10

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