Re: [PATCH] cpufreq: queue policy->update work to rt thread to reduce its schedule latency

[Gaowei Pu <pugaowei@oppo.com>]

Hi Tim,

On 2024/7/19 6:03, Tim Chen wrote:
> On Wed, 2024-07-17 at 14:33 +0800, Gaowei Pu wrote:
>> Currently we encountered a problem that the cpufreq boost latency
>> is about 10 milliseconds or worse when we boost through cpufreq QOS request
>> under high workload scenarios, while the boost latency mainly consumed by
>> schedule latency of policy->update work.
> 
> What is the tail latency now after your change?
> 
>>
>> We should ensure the low schedule latency of cpu frequency limits work
>> to meet performance and power demands. so queue the policy->update work
>> to rt thread to reduce its schedule latency.
> 
> If my understanding is correct, kthread has a default nice
> value of 0 and is not a rt thread. 
> 
> I think the gain you see is
> your patch created a dedicated kthread work queue on CPU 0.
> The work from policy change no longer have to compete time with other
> requests coming from schedule_work(). 
It's not just other requests coming from schedule_work(), also some normal
cfs tasks running on the same cpu.

In order to not competing time with the above threads, i change the thread
policy to rt and prio set to 98 to reduce the schedule latency.
> 
> If the policy change really needs to get ahead
> of other tasks, I think you need a dedicated
> workqueue with alloc_workqueue() using WQ_HIGHPRI flag.I think the cpufreq boost or limit action should be trigger in time to meet
performance and power demands. An dedicated workqueue with highpri will be
better but maybe not good enough because cfs pick or preempt policy is not
purely based on thread nice value. So i think the final solution is rt thread
and the policy change work deserves it :)
thanks.
> 
> Tim
> 
>>
>> Signed-off-by: Gaowei Pu <pugaowei@oppo.com>
>> ---
>>  drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++------
>>  include/linux/cpufreq.h   |  4 +++-
>>  2 files changed, 21 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index a45aac17c20f..e6e42a3ba9ab 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -1193,7 +1193,7 @@ void refresh_frequency_limits(struct cpufreq_policy *policy)
>>  }
>>  EXPORT_SYMBOL(refresh_frequency_limits);
>>  
>> -static void handle_update(struct work_struct *work)
>> +static void handle_update(struct kthread_work *work)
>>  {
>>  	struct cpufreq_policy *policy =
>>  		container_of(work, struct cpufreq_policy, update);
>> @@ -1209,7 +1209,7 @@ static int cpufreq_notifier_min(struct notifier_block *nb, unsigned long freq,
>>  {
>>  	struct cpufreq_policy *policy = container_of(nb, struct cpufreq_policy, nb_min);
>>  
>> -	schedule_work(&policy->update);
>> +	kthread_queue_work(policy->worker, &policy->update);
>>  	return 0;
>>  }
>>  
>> @@ -1218,7 +1218,7 @@ static int cpufreq_notifier_max(struct notifier_block *nb, unsigned long freq,
>>  {
>>  	struct cpufreq_policy *policy = container_of(nb, struct cpufreq_policy, nb_max);
>>  
>> -	schedule_work(&policy->update);
>> +	kthread_queue_work(policy->worker, &policy->update);
>>  	return 0;
>>  }
>>  
>> @@ -1301,15 +1301,25 @@ static struct cpufreq_policy *cpufreq_policy_alloc(unsigned int cpu)
>>  		goto err_min_qos_notifier;
>>  	}
>>  
>> +	policy->worker = kthread_create_worker_on_cpu(cpu, 0, "policy_worker%d", cpu);
>> +	if (IS_ERR(policy->worker)) {
>> +		dev_err(dev, "Failed to create policy_worker%d\n", cpu);
>> +		goto err_max_qos_notifier;
>> +	}
>> +
>> +	sched_set_fifo_low(policy->worker->task);
>>  	INIT_LIST_HEAD(&policy->policy_list);
>>  	init_rwsem(&policy->rwsem);
>>  	spin_lock_init(&policy->transition_lock);
>>  	init_waitqueue_head(&policy->transition_wait);
>> -	INIT_WORK(&policy->update, handle_update);
>> +	kthread_init_work(&policy->update, handle_update);
>>  
>>  	policy->cpu = cpu;
>>  	return policy;
>>  
>> +err_max_qos_notifier:
>> +	freq_qos_remove_notifier(&policy->constraints, FREQ_QOS_MAX,
>> +				 &policy->nb_max);
>>  err_min_qos_notifier:
>>  	freq_qos_remove_notifier(&policy->constraints, FREQ_QOS_MIN,
>>  				 &policy->nb_min);
>> @@ -1353,7 +1363,9 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)
>>  				 &policy->nb_min);
>>  
>>  	/* Cancel any pending policy->update work before freeing the policy. */
>> -	cancel_work_sync(&policy->update);
>> +	kthread_cancel_work_sync(&policy->update);
>> +	if (policy->worker)
>> +		kthread_destroy_worker(policy->worker);
>>  
>>  	if (policy->max_freq_req) {
>>  		/*
>> @@ -1802,7 +1814,7 @@ static unsigned int cpufreq_verify_current_freq(struct cpufreq_policy *policy, b
>>  
>>  		cpufreq_out_of_sync(policy, new_freq);
>>  		if (update)
>> -			schedule_work(&policy->update);
>> +			kthread_queue_work(policy->worker, &policy->update);
>>  	}
>>  
>>  	return new_freq;
>> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
>> index 20f7e98ee8af..73029daddfc5 100644
>> --- a/include/linux/cpufreq.h
>> +++ b/include/linux/cpufreq.h
>> @@ -20,6 +20,7 @@
>>  #include <linux/spinlock.h>
>>  #include <linux/sysfs.h>
>>  #include <linux/minmax.h>
>> +#include <linux/kthread.h>
>>  
>>  /*********************************************************************
>>   *                        CPUFREQ INTERFACE                          *
>> @@ -77,8 +78,9 @@ struct cpufreq_policy {
>>  	void			*governor_data;
>>  	char			last_governor[CPUFREQ_NAME_LEN]; /* last governor used */
>>  
>> -	struct work_struct	update; /* if update_policy() needs to be
>> +	struct kthread_work	update; /* if update_policy() needs to be
>>  					 * called, but you're in IRQ context */
>> +	struct kthread_worker *worker;
>>  
>>  	struct freq_constraints	constraints;
>>  	struct freq_qos_request	*min_freq_req;
>