linux-kernel - Re: [PATCH v3 5/6] cpufreq: schedutil: relax rate-limiting while running RT/DL tasks

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Message-ID: <20171130133642.GE9903@localhost.localdomain>
Date:   Thu, 30 Nov 2017 14:36:42 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH v3 5/6] cpufreq: schedutil: relax rate-limiting while
 running RT/DL tasks

Hi,

On 30/11/17 11:47, Patrick Bellasi wrote:
> The policy in use for RT/DL tasks sets the maximum frequency when a task
> in these classes calls for a cpufreq_update_util().  However, the
> current implementation is still enforcing a frequency switch rate
> limiting when these tasks are running.
> This is potentially working against the goal to switch to the maximum OPP
> when RT tasks are running. In certain unfortunate cases it can also happen
> that a RT task almost completes its activation at a lower OPP.
> 
> This patch overrides on purpose the rate limiting configuration
> to better serve RT/DL tasks. As long as a frequency scaling operation
> is not in progress, a frequency switch is always authorized when
> running in "rt_mode", i.e. the current task in a CPU belongs to the
> RT/DL class.
> 
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-pm@...r.kernel.org
> 
> ---
> Changes from v2:
> - rebased on v4.15-rc1
> 
> Change-Id: I733d47b9e265cebb2e3e5e71a3cd468e9be002d1

Luckily this gets ignored... :)

> ---
>  kernel/sched/cpufreq_schedutil.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 40521d59630b..3eea8884e61b 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -74,7 +74,8 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
>  
>  /************************ Governor internals ***********************/
>  
> -static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> +static bool sugov_should_update_freq(struct sugov_policy *sg_policy,
> +				     u64 time, bool rt_mode)
>  {
>  	s64 delta_ns;
>  
> @@ -111,6 +112,10 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  		return true;
>  	}
>  
> +	/* Always update if a RT/DL task is running */
> +	if (rt_mode)
> +		return true;
> +
>  	delta_ns = time - sg_policy->last_freq_update_time;
>  	return delta_ns >= sg_policy->freq_update_delay_ns;
>  }
> @@ -268,11 +273,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  	sugov_set_iowait_boost(sg_cpu, time, flags);
>  	sg_cpu->last_update = time;
>  
> -	if (!sugov_should_update_freq(sg_policy, time))
> -		return;
> -
> -	busy = sugov_cpu_is_busy(sg_cpu);
> -
>  	/*
>  	 * While RT/DL tasks are running we do not want FAIR tasks to
>  	 * overvrite this CPU's flags, still we can update utilization and
> @@ -281,6 +281,11 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  	rt_mode = task_has_dl_policy(current) ||
>  		  task_has_rt_policy(current) ||
>  		  (flags & SCHED_CPUFREQ_RT_DL);
> +	if (!sugov_should_update_freq(sg_policy, time, rt_mode))
> +		return;
> +
> +	busy = sugov_cpu_is_busy(sg_cpu);
> +
>  	if (rt_mode) {
>  		next_f = policy->cpuinfo.max_freq;
>  	} else {
> @@ -379,7 +384,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>  	sugov_set_iowait_boost(sg_cpu, time, flags);
>  	sg_cpu->last_update = time;
>  
> -	if (sugov_should_update_freq(sg_policy, time)) {
> +	if (sugov_should_update_freq(sg_policy, time, rt_mode)) {
>  		next_f = rt_mode
>  			? sg_policy->policy->cpuinfo.max_freq
>  			: sugov_next_freq_shared(sg_cpu, time);

Reviewed-by: Juri Lelli <juri.lelli@...hat.com>

I wonder if we would also need some way to trigger a back to back update
as soon as a currently running one finishes and an RT/DL task asked for
an update (without waiting for the next tick).

Best,

Juri