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

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Message-ID: <20190122175042.GE261387@google.com>
Date:   Tue, 22 Jan 2019 09:50:42 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Viresh Kumar <viresh.kumar@...aro.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 Tue, Jan 22, 2019 at 12:39:36PM +0530, Viresh Kumar wrote:
> On 18-01-19, 14:45, Matthias Kaehlcke wrote:
> > On Fri, Jan 18, 2019 at 03:32:34PM +0530, Viresh Kumar wrote:
> > > 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.
> > 
> > Ok, I was still stuck at the old one thread per CPU model, where a
> > slow work would block other items in the same workqueue until it
> > finishes execution. After reading a bit through
> > Documentation/core-api/workqueue.rst I agree that a system workqueue
> > is probably fast enough. It might be warranted though to use
> > system_highpri_wq here.
> 
> Is this really that high priority stuff ? I am not sure.

In terms of thermal it could be. But then again, thermal throttling is
driven by input from thermal sensors, which often are polled with
periods >= 100 ms rather than being interrupt driven, so the type of
workqueue wouldn't make a major difference here. I now think it should
be fine to use the normal workqueue unless problems are reported.