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

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

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