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

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Message-ID: <20171130160234.GG31247@e110439-lin>
Date:   Thu, 30 Nov 2017 16:02:34 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Juri Lelli <juri.lelli@...hat.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

On 30-Nov 14:41, Juri Lelli wrote:
> 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".

Just to better understand, you mean that the DL kthread does not send
out schedutil updates?

If that's the case I agree we can discard this patch... that's also
one of the reasons why I move it at the end of this series.

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

-- 
#include <best/regards.h>

Patrick Bellasi