linux-kernel - Re: [PATCH] cpufreq: schedutil: Don't consider freq reduction to busy CPU if need_freq

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Date:   Fri, 19 Feb 2021 11:38:04 +0800
From:   Yue Hu <zbestahu@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        huyue2@...ong.com, zbestahu@....com
Subject: Re: [PATCH] cpufreq: schedutil: Don't consider freq reduction to
 busy CPU if need_freq_update is set

On Thu, 18 Feb 2021 15:50:29 +0530
Viresh Kumar <viresh.kumar@...aro.org> wrote:

> On 18-02-21, 16:25, Yue Hu wrote:
> > From: Yue Hu <huyue2@...ong.com>
> > 
> > For busy CPU case, we do not need to avoid freq reduction if limits
> > change since commit 600f5badb78c ("cpufreq: schedutil: Don't skip
> > freq update when limits change").
> > 
> > Later, commit 23a881852f3e ("cpufreq: schedutil: Don't skip freq
> > update if need_freq_update is set") discarded the need_freq_update
> > check for special case of busy CPU because we won't abort a
> > frequency update anymore if need_freq_update is set.
> > 
> > That is nonlogical since we will not reduce the freq for busy CPU
> > if the computed next_f is really reduced when limits change.  
> 
> Schedutil governor will probably ask for a higher frequency than
> allowed, but cpufreq core will clamp the request between policy
> min/max before updating the frequency here.
> 
> We added the check in 600f5badb78c here earlier as there were chances
> that we will abort the operation without reaching to cpufreq core,
> which won't happen now.
> 

There's a possibility: we will use the previous freq to update if next_f
is reduced for busy CPU if need_freq_update is set in
sugov_update_next_freq(). This possibility would happen now? And this
update is what we want if it happens? This is related to another
possible patch ready to send.

Thank you.