linux-kernel - Re: [PATCH v2 1/6] cpufreq: schedutil: ignore sugov kthreads

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Message-ID: <20170705050056.GN3532@vireshk-i7>
Date:   Wed, 5 Jul 2017 10:30:56 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
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>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Todd Kjos <tkjos@...roid.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 1/6] cpufreq: schedutil: ignore sugov kthreads

On 04-07-17, 18:34, Patrick Bellasi wrote:
> In system where multiple CPUs shares the same frequency domain a small
> workload on a CPU can still be subject to frequency spikes, generated by
> the activation of the sugov's kthread.
> 
> Since the sugov kthread is a special RT task, which goal is just that to
> activate a frequency transition, it does not make sense for it to bias
> the schedutil's frequency selection policy.
> 
> This patch exploits the information related to the current task to silently
> ignore cpufreq_update_this_cpu() calls, coming from the RT scheduler, while
> the sugov kthread is running.
> 
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....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 v1:
> - move check before policy spinlock (JuriL)
> ---
>  kernel/sched/cpufreq_schedutil.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index c982dd0..eaba6d6 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -218,6 +218,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  	unsigned int next_f;
>  	bool busy;
>  
> +	/* Skip updates generated by sugov kthreads */
> +	if (unlikely(current == sg_policy->thread))
> +		return;
> +
>  	sugov_set_iowait_boost(sg_cpu, time, flags);
>  	sg_cpu->last_update = time;
>  
> @@ -290,6 +294,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>  	unsigned long util, max;
>  	unsigned int next_f;
>  
> +	/* Skip updates generated by sugov kthreads */
> +	if (unlikely(current == sg_policy->thread))
> +		return;
> +
>  	sugov_get_util(&util, &max);

Yes we discussed this last time as well (I looked again at those discussions and
am still confused a bit), but wanted to clarify one more time.

After the 2nd patch of this series is applied, why will we still have this
problem? As we concluded it last time, the problem wouldn't happen until the
time the sugov RT thread is running (Hint: work_in_progress). And once the sugov
RT thread is gone, one of the other scheduling classes will take over and should
update the flag pretty quickly.

Are we worried about the time between the sugov RT thread finishes and when the
CFS or IDLE sched class call the util handler again? If yes, then we will still
have that problem for any normal RT/DL task. Isn't it ?

-- 
viresh