linux-kernel - Re: [RFC 5/9] sched: cpufreq: remove smp_processor

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Date:   Tue, 11 Apr 2017 16:00:28 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steve Muckle <smuckle.linux@...il.com>,
        Juri Lelli <juri.lelli@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        eas-dev@...ts.linaro.org
Subject: Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths

On Tue, Apr 11, 2017 at 12:35 PM, Viresh Kumar <viresh.kumar@...aro.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. :-)

>> >             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.

Thanks,
Rafael