Re: [PATCH 2/3] cpufreq: schedutil: move slow path from workqueue to SCHED_FIFO task

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Nov 11, 2016 at 11:22 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> If slow path frequency changes are conducted in a SCHED_OTHER context
> then they may be delayed for some amount of time, including
> indefinitely, when real time or deadline activity is taking place.
>
> Move the slow path to a real time kernel thread. In the future the
> thread should be made SCHED_DEADLINE. The RT priority is arbitrarily set
> to 50 for now.
>
> Hackbench results on ARM Exynos, dual core A15 platform for 10
> iterations:
>
> $ hackbench -s 100 -l 100 -g 10 -f 20
>
> Before                  After
> ---------------------------------
> 1.808                   1.603
> 1.847                   1.251
> 2.229                   1.590
> 1.952                   1.600
> 1.947                   1.257
> 1.925                   1.627
> 2.694                   1.620
> 1.258                   1.621
> 1.919                   1.632
> 1.250                   1.240
>
> Average:
>
> 1.8829                  1.5041
>
> Based on initial work by Steve Muckle.
>
> Signed-off-by: Steve Muckle <smuckle.linux@gmail.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 62 ++++++++++++++++++++++++++++++++--------
>  1 file changed, 50 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index ccb2ab89affb..045ce0a4e6d1 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -12,6 +12,7 @@
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>
>  #include <linux/cpufreq.h>
> +#include <linux/kthread.h>
>  #include <linux/slab.h>
>  #include <trace/events/power.h>
>
> @@ -35,8 +36,10 @@ struct sugov_policy {
>
>         /* The next fields are only needed if fast switch cannot be used. */
>         struct irq_work irq_work;
> -       struct work_struct work;
> +       struct kthread_work work;
>         struct mutex work_lock;
> +       struct kthread_worker worker;
> +       struct task_struct *thread;
>         bool work_in_progress;
>
>         bool need_freq_update;
> @@ -291,9 +294,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>         raw_spin_unlock(&sg_policy->update_lock);
>  }
>
> -static void sugov_work(struct work_struct *work)
> +static void sugov_work(struct kthread_work *work)
>  {
> -       struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> +       struct sugov_policy *sg_policy =
> +               container_of(work, struct sugov_policy, work);

Why this change?

>
>         mutex_lock(&sg_policy->work_lock);
>         __cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> @@ -308,7 +312,7 @@ static void sugov_irq_work(struct irq_work *irq_work)
>         struct sugov_policy *sg_policy;
>
>         sg_policy = container_of(irq_work, struct sugov_policy, irq_work);
> -       schedule_work_on(smp_processor_id(), &sg_policy->work);
> +       kthread_queue_work(&sg_policy->worker, &sg_policy->work);
>  }
>
>  /************************** sysfs interface ************************/
> @@ -362,9 +366,23 @@ static struct kobj_type sugov_tunables_ktype = {
>
>  static struct cpufreq_governor schedutil_gov;
>
> +static void sugov_policy_free(struct sugov_policy *sg_policy)
> +{
> +       if (!sg_policy->policy->fast_switch_enabled) {
> +               kthread_flush_worker(&sg_policy->worker);
> +               kthread_stop(sg_policy->thread);
> +       }
> +
> +       mutex_destroy(&sg_policy->work_lock);
> +       kfree(sg_policy);
> +}
> +
>  static struct sugov_policy *sugov_policy_alloc(struct cpufreq_policy *policy)
>  {
>         struct sugov_policy *sg_policy;
> +       struct task_struct *thread;
> +       struct sched_param param = { .sched_priority = 50 };

I'd define a symbol for the 50.  It's just one extra line of code ...

Thanks,
Rafael