linux-kernel - Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

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Date:   Wed, 9 May 2018 08:45:30 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Claudio Scordino <claudio@...dence.eu.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Joel Fernandes <joelaf@...gle.com>, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency
 requests

On 08/05/18 21:54, Joel Fernandes wrote:

[...]

> Just for discussion sake, is there any need for work_in_progress? If we can
> queue multiple work say kthread_queue_work can handle it, then just queuing
> works whenever they are available should be Ok and the kthread loop can
> handle them. __cpufreq_driver_target is also protected by the work lock if
> there is any concern that can have races... only thing is rate-limiting of
> the requests, but we are doing a rate limiting, just not for the "DL
> increased utilization" type requests (which I don't think we are doing at the
> moment for urgent DL requests anyway).
> 
> Following is an untested diff to show the idea. What do you think?
> 
> thanks,
> 
> - Joel
> 
> ----8<---
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index d2c6083304b4..862634ff4bf3 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -38,7 +38,6 @@ struct sugov_policy {
>  	struct			mutex work_lock;
>  	struct			kthread_worker worker;
>  	struct task_struct	*thread;
> -	bool			work_in_progress;
>  
>  	bool			need_freq_update;
>  };
> @@ -92,16 +91,8 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  	    !cpufreq_can_do_remote_dvfs(sg_policy->policy))
>  		return false;
>  
> -	if (sg_policy->work_in_progress)
> -		return false;
> -
>  	if (unlikely(sg_policy->need_freq_update)) {
>  		sg_policy->need_freq_update = false;
> -		/*
> -		 * This happens when limits change, so forget the previous
> -		 * next_freq value and force an update.
> -		 */
> -		sg_policy->next_freq = UINT_MAX;
>  		return true;
>  	}
>  
> @@ -129,7 +120,6 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>  		policy->cur = next_freq;
>  		trace_cpu_frequency(next_freq, smp_processor_id());
>  	} else {
> -		sg_policy->work_in_progress = true;
>  		irq_work_queue(&sg_policy->irq_work);

Isn't this potentially introducing unneeded irq pressure (and doing the
whole wakeup the kthread thing), while the already active kthread could
simply handle multiple back-to-back requests before going to sleep?

Best,

- Juri