Re: [RFCv5 PATCH 38/46] sched: scheduler-driven cpu frequency selection

[Juri Lelli <juri.lelli@arm.com>]

Hi,

On 11/08/15 03:14, Leo Yan wrote:
> On Tue, Jul 07, 2015 at 07:24:21PM +0100, Morten Rasmussen wrote:
>> From: Michael Turquette <mturquette@baylibre.com>
>>
>> Scheduler-driven cpu frequency selection is desirable as part of the
>> on-going effort to make the scheduler better aware of energy
>> consumption.  No piece of the Linux kernel has a better view of the
>> factors that affect a cpu frequency selection policy than the
>> scheduler[0], and this patch is an attempt to converge on an initial
>> solution.
>>
>> This patch implements a simple shim layer between the Linux scheduler
>> and the cpufreq subsystem. This interface accepts a capacity request
>> from the Completely Fair Scheduler and honors the max request from all
>> cpus in the same frequency domain.
>>
>> The policy magic comes from choosing the cpu capacity request from cfs
>> and is not contained in this cpufreq governor. This code is
>> intentionally dumb.
>>
>> Note that this "governor" is event-driven. There is no polling loop to
>> check cpu idle time nor any other method which is unsynchronized with
>> the scheduler.
>>
>> Thanks to Juri Lelli <juri.lelli@arm.com> for contributing design ideas,
>> code and test results.
>>
>> [0] http://article.gmane.org/gmane.linux.kernel/1499836
>>
>> Signed-off-by: Michael Turquette <mturquette@baylibre.com>
>> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
>> ---
>>  drivers/cpufreq/Kconfig      |  24 ++++
>>  include/linux/cpufreq.h      |   3 +
>>  kernel/sched/Makefile        |   1 +
>>  kernel/sched/cpufreq_sched.c | 308 +++++++++++++++++++++++++++++++++++++++++++
>>  kernel/sched/sched.h         |   8 ++
>>  5 files changed, 344 insertions(+)
>>  create mode 100644 kernel/sched/cpufreq_sched.c
>>
>> diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
>> index 659879a..9bbf44c 100644
>> --- a/drivers/cpufreq/Kconfig
>> +++ b/drivers/cpufreq/Kconfig
>> @@ -102,6 +102,15 @@ 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
>> +     select CPU_FREQ_GOV_PERFORMANCE
>> +     help
>> +       Use the CPUfreq governor 'sched' as default. This scales
>> +       cpu frequency from the scheduler as per-entity load tracking
>> +       statistics are updated.
>>  endchoice
>>
>>  config CPU_FREQ_GOV_PERFORMANCE
>> @@ -183,6 +192,21 @@ config CPU_FREQ_GOV_CONSERVATIVE
>>
>>         If in doubt, say N.
>>
>> +config CPU_FREQ_GOV_SCHED
>> +     tristate "'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 invoked from the completely fair
>> +       scheduler when updating per-entity load tracking statistics.
>> +       Latency to respond to changes in load is improved over polling
>> +       governors due to its event-driven design.
>> +
>> +       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 1f2c9a1..30241c9 100644
>> --- a/include/linux/cpufreq.h
>> +++ b/include/linux/cpufreq.h
>> @@ -494,6 +494,9 @@ 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_GOV)
> 
> s/CONFIG_CPU_FREQ_DEFAULT_GOV_SCHED_GOV/CONFIG_CPU_FREQ_DEFAULT_GOV_SCHED/
> 
>> +extern struct cpufreq_governor cpufreq_gov_sched_gov;
> 
> s/cpufreq_gov_sched_gov/cpufreq_gov_sched/
> 

Yes, right. Dietmar pointed out the same problem in reply to Mike's
original posting. I guess Mike is going to squash the fix in his
next posting.

Thanks a lot anyway! :)

Best,

- Juri

>> +#define CPUFREQ_DEFAULT_GOVERNOR     (&cpufreq_gov_sched)
>>  #endif
>>
>>  /*********************************************************************
>> 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..5020f24
>> --- /dev/null
>> +++ b/kernel/sched/cpufreq_sched.c
>> @@ -0,0 +1,308 @@
>> +/*
>> + *  Copyright (C)  2015 Michael Turquette <mturquette@linaro.org>
>> + *
>> + * 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 "sched.h"
>> +
>> +#define THROTTLE_NSEC                50000000 /* 50ms default */
>> +
>> +static DEFINE_PER_CPU(unsigned long, pcpu_capacity);
>> +static DEFINE_PER_CPU(struct cpufreq_policy *, pcpu_policy);
>> +
>> +/**
>> + * 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
>> + * @freq: new frequency stored in *_sched_update_cpu and used in *_sched_thread
>> + *
>> + * 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;
>> +     struct cpufreq_policy *policy;
>> +     unsigned int 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);
>> +}
>> +
>> +/*
>> + * 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;
>> +     int ret;
>> +
>> +     policy = (struct cpufreq_policy *) data;
>> +     if (!policy) {
>> +             pr_warn("%s: missing policy\n", __func__);
>> +             do_exit(-EINVAL);
>> +     }
>> +
>> +     gd = policy->governor_data;
>> +     if (!gd) {
>> +             pr_warn("%s: missing governor data\n", __func__);
>> +             do_exit(-EINVAL);
>> +     }
>> +
>> +     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);
>> +     }
>> +
>> +     ret = set_cpus_allowed_ptr(gd->task, policy->related_cpus);
>> +     if (ret) {
>> +             pr_warn("%s: failed to set allowed ptr\n", __func__);
>> +             do_exit(-EINVAL);
>> +     }
>> +
>> +     /* main loop of the per-policy kthread */
>> +     do {
>> +             set_current_state(TASK_INTERRUPTIBLE);
>> +             schedule();
>> +             if (kthread_should_stop())
>> +                     break;
>> +
>> +             cpufreq_sched_try_driver_target(policy, gd->freq);
>> +     } while (!kthread_should_stop());
>> +
>> +     do_exit(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);
>> +}
>> +
>> +/**
>> + * cpufreq_sched_set_capacity - interface to scheduler for changing capacity values
>> + * @cpu: cpu whose capacity utilization has recently changed
>> + * @capacity: the new capacity requested by cpu
>> + *
>> + * cpufreq_sched_sched_capacity is an interface exposed to the scheduler so
>> + * that the scheduler may inform the governor of updates to capacity
>> + * utilization and make changes to cpu frequency. Currently this interface is
>> + * designed around PELT values in CFS. It can be expanded to other scheduling
>> + * classes in the future if needed.
>> + *
>> + * cpufreq_sched_set_capacity raises an IPI. The irq_work handler for that IPI
>> + * wakes up the thread that does the actual work, cpufreq_sched_thread.
>> + *
>> + * This functions bails out early if either condition is true:
>> + * 1) this cpu did not the new maximum capacity for its frequency domain
>> + * 2) no change in cpu frequency is necessary to meet the new capacity request
>> + */
>> +void cpufreq_sched_set_cap(int cpu, unsigned long capacity)
>> +{
>> +     unsigned int freq_new, cpu_tmp;
>> +     struct cpufreq_policy *policy;
>> +     struct gov_data *gd;
>> +     unsigned long capacity_max = 0;
>> +
>> +     /* update per-cpu capacity request */
>> +     __this_cpu_write(pcpu_capacity, capacity);
>> +
>> +     policy = cpufreq_cpu_get(cpu);
>> +     if (IS_ERR_OR_NULL(policy)) {
>> +             return;
>> +     }
>> +
>> +     if (!policy->governor_data)
>> +             goto out;
>> +
>> +     gd = policy->governor_data;
>> +
>> +     /* bail early if we are throttled */
>> +     if (ktime_before(ktime_get(), gd->throttle))
>> +             goto out;
>> +
>> +     /* find max capacity requested by cpus in this policy */
>> +     for_each_cpu(cpu_tmp, policy->cpus)
>> +             capacity_max = max(capacity_max, per_cpu(pcpu_capacity, cpu_tmp));
>> +
>> +     /*
>> +      * We only change frequency if this cpu's capacity request represents a
>> +      * new max. If another cpu has requested a capacity greater than the
>> +      * previous max then we rely on that cpu to hit this code path and make
>> +      * the change. IOW, the cpu with the new max capacity is responsible
>> +      * for setting the new capacity/frequency.
>> +      *
>> +      * If this cpu is not the new maximum then bail
>> +      */
>> +     if (capacity_max > capacity)
>> +             goto out;
>> +
>> +     /* Convert the new maximum capacity request into a cpu frequency */
>> +     freq_new = capacity * policy->max >> SCHED_CAPACITY_SHIFT;
>> +
>> +     /* No change in frequency? Bail and return current capacity. */
>> +     if (freq_new == policy->cur)
>> +             goto out;
>> +
>> +     /* store the new frequency and perform the transition */
>> +     gd->freq = freq_new;
>> +
>> +     if (cpufreq_driver_might_sleep())
>> +             irq_work_queue_on(&gd->irq_work, cpu);
>> +     else
>> +             cpufreq_sched_try_driver_target(policy, freq_new);
>> +
>> +out:
>> +     cpufreq_cpu_put(policy);
>> +     return;
>> +}
>> +
>> +static int cpufreq_sched_start(struct cpufreq_policy *policy)
>> +{
>> +     struct gov_data *gd;
>> +     int cpu;
>> +
>> +     /* prepare per-policy private data */
>> +     gd = kzalloc(sizeof(*gd), GFP_KERNEL);
>> +     if (!gd) {
>> +             pr_debug("%s: failed to allocate private data\n", __func__);
>> +             return -ENOMEM;
>> +     }
>> +
>> +     /* initialize per-cpu data */
>> +     for_each_cpu(cpu, policy->cpus) {
>> +             per_cpu(pcpu_capacity, cpu) = 0;
>> +             per_cpu(pcpu_policy, cpu) = policy;
>> +     }
>> +
>> +     /*
>> +      * Don't ask for freq changes at an higher rate than what
>> +      * the driver advertises as transition latency.
>> +      */
>> +     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_might_sleep()) {
>> +             /* init per-policy kthread */
>> +             gd->task = kthread_run(cpufreq_sched_thread, policy, "kcpufreq_sched_task");
>> +             if (IS_ERR_OR_NULL(gd->task)) {
>> +                     pr_err("%s: failed to create kcpufreq_sched_task thread\n", __func__);
>> +                     goto err;
>> +             }
>> +             init_irq_work(&gd->irq_work, cpufreq_sched_irq_work);
>> +     }
>> +
>> +     policy->governor_data = gd;
>> +     gd->policy = policy;
>> +     return 0;
>> +
>> +err:
>> +     kfree(gd);
>> +     return -ENOMEM;
>> +}
>> +
>> +static int cpufreq_sched_stop(struct cpufreq_policy *policy)
>> +{
>> +     struct gov_data *gd = policy->governor_data;
>> +
>> +     if (cpufreq_driver_might_sleep()) {
>> +             kthread_stop(gd->task);
>> +     }
>> +
>> +     policy->governor_data = NULL;
>> +
>> +     /* FIXME replace with devm counterparts? */
>> +     kfree(gd);
>> +     return 0;
>> +}
>> +
>> +static int cpufreq_sched_setup(struct cpufreq_policy *policy, unsigned int event)
>> +{
>> +     switch (event) {
>> +             case CPUFREQ_GOV_START:
>> +                     /* Start managing the frequency */
>> +                     return cpufreq_sched_start(policy);
>> +
>> +             case CPUFREQ_GOV_STOP:
>> +                     return cpufreq_sched_stop(policy);
>> +
>> +             case CPUFREQ_GOV_LIMITS:        /* unused */
>> +             case CPUFREQ_GOV_POLICY_INIT:   /* unused */
>> +             case CPUFREQ_GOV_POLICY_EXIT:   /* unused */
>> +                     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)
>> +{
>> +     return cpufreq_register_governor(&cpufreq_gov_sched);
>> +}
>> +
>> +static void __exit cpufreq_sched_exit(void)
>> +{
>> +     cpufreq_unregister_governor(&cpufreq_gov_sched);
>> +}
>> +
>> +/* Try to make this the default governor */
>> +fs_initcall(cpufreq_sched_init);
>> +
>> +MODULE_LICENSE("GPL v2");
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index c395559..30aa0c4 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1476,6 +1476,13 @@ unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>>  }
>>  #endif
>>
>> +#ifdef CONFIG_CPU_FREQ_GOV_SCHED
>> +void cpufreq_sched_set_cap(int cpu, unsigned long util);
>> +#else
>> +static inline void cpufreq_sched_set_cap(int cpu, unsigned long util)
>> +{ }
>> +#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));
>> @@ -1484,6 +1491,7 @@ static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
>>  #else
>>  static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta) { }
>>  static inline void sched_avg_update(struct rq *rq) { }
>> +static inline void gov_cfs_update_cpu(int cpu) {}
>>  #endif
>>
>>  extern void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period);
>> --
>> 1.9.1
>>
>> --
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