linux-kernel - Re: [PATCH] sched: Add schedutil overview

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Message-ID: <jhjr1nnry1u.mognet@arm.com>
Date:   Fri, 18 Dec 2020 14:25:33 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, patrick.bellasi@...bug.net,
        lenb@...nel.org, linux-kernel@...r.kernel.org,
        ionela.voinescu@....com, qperret@...gle.com,
        viresh.kumar@...aro.org
Subject: Re: [PATCH] sched: Add schedutil overview


On 18/12/20 13:40, Morten Rasmussen wrote:
> On Fri, Dec 18, 2020 at 11:33:09AM +0000, Valentin Schneider wrote:
>> (also, does this need a word about runnable rt tasks => goto max?)
>
> What is actually the intended policy there? I thought it was goto max
> unless rt was clamped, but if I read the code correctly in
> schedutil_cpu_util() the current policy is only goto max if uclamp isn't
> in use at all, including cfs.
>

Right, so the policy pretty much is: by default, if there are runnable rt
tasks, goto max freq.

When uclamp isn't used, that's hardcoded.

When uclamp is in use, the default RT uclamp.min is 1024, so it "naturally"
drives frequency selection to the max when there are runnable RT tasks
(rq-aggregated uclamp.min == 1024). That default
(uclamp_util_min_rt_default) can be tweaked.

> The write-up looks good to me.
>
> Reviewed-by: Morten Rasmussen <morten.rasmussen@....com>
>
> Morten