Re: [PATCH 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

From: Steven Rostedt <rostedt@goodmis.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Patrick Bellasi <patrick.bellasi@arm.com>,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Vincent Guittot <vincent.guittot@linaro.org>,
	John Stultz <john.stultz@linaro.org>,
	Juri Lelli <juri.lelli@arm.com>, Todd Kjos <tkjos@android.com>,
	Tim Murray <timmurray@google.com>,
	Andres Oportus <andresoportus@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Chris Redpath <chris.redpath@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>
Subject: Re: [PATCH 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter
Date: Mon, 6 Mar 2017 09:29:59 -0500	[thread overview]
Message-ID: <20170306092959.561fdf26@gandalf.local.home> (raw)
In-Reply-To: <20170303034125.GH13760@vireshk-i7>

On Fri, 3 Mar 2017 09:11:25 +0530
Viresh Kumar <viresh.kumar@linaro.org> wrote:

> On 02-03-17, 15:45, Patrick Bellasi wrote:
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index e2ed46d..739b29d 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -3653,6 +3653,7 @@ static inline unsigned long rlimit_max(unsigned int limit)
> >  #define SCHED_CPUFREQ_RT	(1U << 0)
> >  #define SCHED_CPUFREQ_DL	(1U << 1)
> >  #define SCHED_CPUFREQ_IOWAIT	(1U << 2)
> > +#define SCHED_CPUFREQ_IDLE	(1U << 3)
> >  
> >  #define SCHED_CPUFREQ_RT_DL	(SCHED_CPUFREQ_RT | SCHED_CPUFREQ_DL)
> >  
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index fd46593..084a98b 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -281,6 +281,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  
> >  	raw_spin_lock(&sg_policy->update_lock);
> >  
> > +	/* CPU is entering IDLE, reset flags without triggering an update */
> > +	if (flags & SCHED_CPUFREQ_IDLE) {  
> 
> Will "flags == SCHED_CPUFREQ_IDLE" generate better assembly ?
> 

Even if it does, a bit check and an equal check are pretty negligible
in difference wrt execution time. I would choose whatever is the most
readable to humans.

 flags == SCHED_CPUFREQ_IDLE

will tell me (as a reviewer) that we expect no other flag to be set.

 flags & SCHED_CPUFREQ_IDLE

will tell me that we only care about the IDLE flag.

Which ever is the more meaningful is what should be used.

-- Steve

> > +		sg_cpu->flags = 0;
> > +		goto done;
> > +	}
> > +
> >  	sg_cpu->util = util;
> >  	sg_cpu->max = max;
> >  	sg_cpu->flags = flags;
> > @@ -293,6 +299,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  		sugov_update_commit(sg_policy, time, next_f);
> >  	}
> >  
> > +done:
> >  	raw_spin_unlock(&sg_policy->update_lock);
> >  }
> >  
> > diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> > index 0c00172..a844c91 100644
> > --- a/kernel/sched/idle_task.c
> > +++ b/kernel/sched/idle_task.c
> > @@ -29,6 +29,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> >  	put_prev_task(rq, prev);
> >  	update_idle_core(rq);
> >  	schedstat_inc(rq->sched_goidle);
> > +
> > +	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
> > +	cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_IDLE);
> > +
> >  	return rq->idle;
> >  }
> >  
> > -- 
> > 2.7.4  
> 