Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths

From: Viresh Kumar <viresh.kumar@linaro.org>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Lists linaro-kernel <linaro-kernel@lists.linaro.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Steve Muckle <smuckle.linux@gmail.com>,
	Juri Lelli <juri.lelli@arm.com>,
	Morten Rasmussen <Morten.Rasmussen@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	eas-dev@lists.linaro.org
Subject: Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths
Date: Wed, 12 Apr 2017 19:56:28 +0530	[thread overview]
Message-ID: <20170412142628.GH5910@vireshk-i7> (raw)
In-Reply-To: <CAJZ5v0iQ4X=Vt54EvJce9R9rPhtVqvznte8=HoQ_UOAZ1fQj2g@mail.gmail.com>

On 11-04-17, 16:00, Rafael J. Wysocki wrote:
> On Tue, Apr 11, 2017 at 12:35 PM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > On 29-03-17, 23:28, Rafael J. Wysocki wrote:
> >> On Thursday, March 09, 2017 05:15:15 PM Viresh Kumar wrote:
> >> > @@ -216,7 +216,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >> >     if (flags & SCHED_CPUFREQ_RT_DL) {
> >> >             next_f = policy->cpuinfo.max_freq;
> >> >     } else {
> >> > -           sugov_get_util(&util, &max);
> >> > +           sugov_get_util(&util, &max, hook->cpu);
> >>
> >> Why is this not racy?
> >
> > Why would reading the utilization values be racy? The only dynamic value here is
> > "util_avg" and I am not sure if reading it is racy.
> >
> > But, this whole routine has races which I ignored as we may end up updating
> > frequency simultaneously from two threads.
> 
> Those races aren't there if we don't update cross-CPU, which is my point. :-)

Of course. There are no races without this series.

> >> >             sugov_iowait_boost(sg_cpu, &util, &max);
> >> >             next_f = get_next_freq(sg_policy, util, max);
> >> >     }
> >> > @@ -272,7 +272,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >> >     unsigned long util, max;
> >> >     unsigned int next_f;
> >> >
> >> > -   sugov_get_util(&util, &max);
> >> > +   sugov_get_util(&util, &max, hook->cpu);
> >> >
> >>
> >> And here?
> >>
> >> >     raw_spin_lock(&sg_policy->update_lock);
> >
> > The lock prevents the same here though.
> >
> > So, if we are going to use this series, then we can use the same update-lock in
> > case of single cpu per policies as well.
> 
> No, we can't.
> 
> The lock is unavoidable in the mulit-CPU policies case, but there's no
> way I will agree on using a lock in the single-CPU case.

How do you suggest to avoid the locking here then ? Some atomic
variable read/write as done in cpufreq_governor.c ?

-- 
viresh