linux-kernel - Re: [PATCH v3 6/6] cpufreq: schedutil: ignore sugov kthreads

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Date:   Thu, 30 Nov 2017 14:41:55 +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 6/6] cpufreq: schedutil: ignore sugov kthreads

Hi,

On 30/11/17 11:47, 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>
> 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
> - moved at the end of the stack since considered more controversial
> Changes from v1:
> - move check before policy spinlock (JuriL)
> 
> Change-Id: I4d749458229b6496dd24a8c357be42cd35a739fd
> ---
>  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 3eea8884e61b..a93ad5b0c40d 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -270,6 +270,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  	bool rt_mode;
>  	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;
>  
> @@ -356,6 +360,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>  	unsigned int next_f;
>  	bool rt_mode;
>  
> +	/* Skip updates generated by sugov kthreads */
> +	if (unlikely(current == sg_policy->thread))
> +		return;
> +
>  	raw_spin_lock(&sg_policy->update_lock);
>  
>  	sugov_get_util(&util, &max, sg_cpu->cpu);

If the DL changes (which I shall post again as soon as tip/sched/core is
bumped up to 4.15-rc1) get in first, this is going to be useless (as the
DL kthread gets ignored by the scheduling class itself). But, this looks
good to me "in the meantime".

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

Best,

Juri