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

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  Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote  paths Viresh Kumar <viresh.kumar@linaro.org> - 2017-04-11 12:40 +0200
    Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths "Rafael J. Wysocki" <rafael@kernel.org> - 2017-04-11 16:10 +0200
      Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote  paths Viresh Kumar <viresh.kumar@linaro.org> - 2017-04-12 16:30 +0200
        Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths "Rafael J. Wysocki" <rafael@kernel.org> - 2017-04-13 01:00 +0200

#1621127 — Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths

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.

> >  		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.

-- 
viresh

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#1621320 — 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@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. :-)

>> >             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

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#1622178

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

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#1622600 — Re: [RFC 5/9] sched: cpufreq: remove smp_processor_id() in remote paths

On Wed, Apr 12, 2017 at 4:26 PM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> 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 ?

That is a very good question. :-)

I need to look at the scheduler code that invokes those things and see
what happens in there.  Chances are there already is some sufficient
mutual exclusion in place.

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