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

On Sat, Nov 12, 2016 at 2:31 AM, Saravana Kannan skannan@codeaurora.org wrote:

On 11/11/2016 02:16 PM, Rafael J. Wysocki wrote:

...

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 ...

Hold on a sec. I thought during LPC someone (Peter?) made a point that when RT thread run, we should bump the frequency to max? So, schedutil is going to trigger schedutil to bump up the frequency to max, right?

No, it isn't, or at least that is unlikely.

sugov_update_commit() sets sg_policy->work_in_progress before queuing the IRQ work and it is not cleared until the frequency changes in sugov_work().

OTOH, sugov_should_update_freq() checks sg_policy->work_in_progress upfront and returns false when it is set, so the governor won't see its own worker threads run, unless I'm overlooking something highly non-obvious.

Thanks, Rafael