linux-kernel - Re: [PATCH RFC] schedutil: Allow cpufreq requests to be made even when kthread kicked

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Message-ID: <20180517102024.s3dxo4uepujh5f65@vireshk-i7>
Date:   Thu, 17 May 2018 15:50:24 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Joel Fernandes <joelaf@...gle.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH RFC] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

On 17-05-18, 09:00, Juri Lelli wrote:
> Hi Joel,
> 
> On 16/05/18 15:45, Joel Fernandes (Google) wrote:
> 
> [...]
> 
> > @@ -382,13 +391,24 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
> >  static void sugov_work(struct kthread_work *work)
> >  {
> >  	struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> > +	unsigned int freq;
> > +	unsigned long flags;
> > +
> > +	/*
> > +	 * Hold sg_policy->update_lock shortly to handle the case where:
> > +	 * incase sg_policy->next_freq is read here, and then updated by
> > +	 * sugov_update_shared just before work_in_progress is set to false
> > +	 * here, we may miss queueing the new update.
> > +	 */
> > +	raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
> > +	freq = sg_policy->next_freq;
> > +	sg_policy->work_in_progress = false;
> > +	raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
> 
> OK, we queue the new request up, but still we need to let this kthread
> activation complete and then wake it up again to service the request
> already queued, right? Wasn't what Claudio proposed (service back to
> back requests all in the same kthread activation) better from an
> overhead pow?

We would need more locking stuff in the work handler in that case and
I think there maybe a chance of missing the request in that solution
if the request happens right at the end of when sugov_work returns.

-- 
viresh