linux-kernel - Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to related

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Message-ID: <CAJZ5v0iN0msbi=ad34nS8eVfGGS1UD-qUeHPeAvq=yW3-4HKCQ@mail.gmail.com>
Date:   Tue, 8 May 2018 22:36:40 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to
 related_cpus unnecessarily"

On Tue, May 8, 2018 at 12:34 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 08-05-18, 11:02, Quentin Perret wrote:
>> The sugov kthreads are DL tasks so they're not impacted by EAS. But even
>> if you take EAS out of the picture, those kthreads are assigned to a
>> "random" CPU at boot time and stay there forever (because that's how DL
>> works). Is this what we want ?
>
> Okay, I didn't knew that DL threads don't migrate at all. I don't
> think that's what we want then specially for big LITTLE platforms. But
> for the rest, I don't know. Take example of Qcom krait. Each CPU has a
> separate policy, why shouldn't we allow other CPUs to run the kthread?

Because that makes things more complex and harder to debug in general.

What's the exact reason why non-policy CPUs should ever run the sugov
kthread for the given policy?