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

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Date:   Tue, 11 Jul 2017 12:08:55 -0700
From:   Saravana Kannan <skannan@...eaurora.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>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        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 07/04/2017 10:34 AM, 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;
> +

This seems super race-y. Especially when combined with rate_limit_us. 
Deciding to not update the frequency for a policy just because the call 
back happened in the context of the kthread is not right. Especially 
when it's combined with the remote CPU call backs patches Viresh is 
putting out (which I think is a well intended patch series).

-Saravana

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