linux-kernel - Re: [PATCH 1/3] drivers: base: Add frequency constraint infrastructure

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Date:   Fri, 18 Jan 2019 15:32:34 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <vireshk@...nel.org>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] drivers: base: Add frequency constraint
 infrastructure

On 17-01-19, 17:03, Matthias Kaehlcke wrote:
> On Fri, Jan 11, 2019 at 02:48:34PM +0530, Viresh Kumar wrote:
> > +static void fcs_update(struct freq_constraints *fcs, struct freq_pair *freq,
> > +		       enum fc_event event)
> > +{
> > +	mutex_lock(&fcs->lock);
> > +
> > +	if (_fcs_update(fcs, freq, event)) {
> > +		if (fcs->callback)
> > +			schedule_work(&fcs->work);
> 
> IIUC the constraints aren't applied until the callback is executed. I
> wonder if a dedicated workqueue should be used instead of the system
> one, to avoid longer delays from other kernel entities that might
> 'misbehave'. Especially for thermal constraints we want a quick
> response.

I thought the system workqueue should be fast enough, it contains
multiple threads which can all run in parallel and service this work.

> > +
> > +	/* Find a CPU for which fcs already exists */
> > +	for_each_cpu(cpu, cpumask) {
> > +		cpu_dev = get_cpu_device(cpu);
> > +		if (unlikely(!cpu_dev))
> > +			continue;
> > +
> > +		if (unlikely(!first_cpu_dev))
> > +			first_cpu_dev = cpu_dev;
> 
> I'd expect setting the callback to be a one time/rare operation. Is
> there really any gain from cluttering this code with 'unlikely's?
> 
> There are other functions where it could be removed if the outcome is
> that it isn't needed/desirable in code that only runs sporadically.

I was looking to make the code as fast as possible and the use of
unlikely doesn't look that bad to me. Lets see what others have to say
on such a policy.

> > +	if (ret)
> > +		remove_cpumask_fcs(fcs, cpumask, cpu);
> 
> I think it would be clearer to pass -1 instead of 'cpu', as in
> freq_constraint_remove_cpumask_callback(), no need to backtrack and
> 'confirm' that the above for loop always stops at the last CPU in the
> cpumask (unless the function returns due to an error).

Okay.

-- 
viresh