linux-kernel - Re: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Date:   Mon, 24 Jul 2017 16:31:22 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        smuckle.linux@...il.com, juri.lelli@....com,
        Morten.Rasmussen@....com, patrick.bellasi@....com,
        eas-dev@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks

On 21-07-17, 15:03, Peter Zijlstra wrote:
> On Thu, Jul 13, 2017 at 12:14:37PM +0530, Viresh Kumar wrote:

> > @@ -42,6 +42,7 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> >  		return;
> >  
> >  	data->func = func;
> > +	data->cpu = cpu;
> >  	rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
> >  }
> >  EXPORT_SYMBOL_GPL(cpufreq_add_update_util_hook);
> 
> redundant.

Actually we will still need it. We pass hook->cpu to sugov_get_util()
in the 2nd patch of this series and there is no work around possible
around that.

> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 29a397067ffa..ed9c589e5386 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -218,6 +218,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >  	unsigned int next_f;
> >  	bool busy;
> >  
> > +	/* Remote callbacks aren't allowed for policies which aren't shared */
> > +	if (smp_processor_id() != hook->cpu)
> > +		return;
> > +
> >  	sugov_set_iowait_boost(sg_cpu, time, flags);
> >  	sg_cpu->last_update = time;
> >  
> > @@ -290,6 +294,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  	unsigned long util, max;
> >  	unsigned int next_f;
> >  
> > +	/* Don't allow remote callbacks */
> > +	if (smp_processor_id() != hook->cpu)
> > +		return;
> > +
> >  	sugov_get_util(&util, &max);
> >  
> >  	raw_spin_lock(&sg_policy->update_lock);
> 
> 
> Given the whole rq->lock thing, I suspect we could actually not do these
> two.

You meant sugov_get_util() and raw_spin_lock()? Why?

The locking is required here in the shared-policy case to make sure
only one CPU is updating the frequency for the entire policy. And we
can't really avoid that even with the rq->lock guarantees from the
scheduler for the target CPU. 

> That would then continue to process the iowait and other accounting
> stuff, but stall the moment we call into the actual driver, which will
> then drop the request on the floor as per the first few hunks.

I am not sure I understood your comment completely though.

> This seems ok. Except of course you'll have conflicts with Juri's patch
> set, but that should be trivial to sort out.

Yeah, I wouldn't mind rebasing if his series gets in first.

-- 
viresh