linux-kernel - Re: [PATCH v3] Documentation: sched: Document util clamp feature

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Message-ID: <Y52Ri28ThsM4iU8X@debian.me>
Date:   Sat, 17 Dec 2022 16:53:15 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Jonathan Corbet <corbet@....net>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Lukasz Luba <lukasz.luba@....com>,
        Xuewen Yan <xuewen.yan94@...il.com>, Wei Wang <wvw@...gle.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>,
        Hank <han.lin@...iatek.com>, Paul Bone <pbone@...illa.com>,
        Russell King <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v3] Documentation: sched: Document util clamp feature

On Fri, Dec 16, 2022 at 11:57:16PM +0000, Qais Yousef wrote:
> +Another example is in Android where tasks are classified as background,
> +foreground, top-app, etc. Util clamp can be used to constrain how much
> +resources background tasks are consuming by capping the performance point they
> +can run at. This constraint helps reserve resources for important tasks, like
> +the ones belonging to the currently active app (top-app group). Beside this
> +helps in limiting how much power they consume. This can be more obvious in
> +heterogeneous systems (e.g. Arm big.LITTLE); the constraint will help bias the
> +background tasks to stay on the little cores which will ensure that:
> +
> +        1. The big cores are free to run top-app tasks immediately. top-app
> +           tasks are the tasks the user is currently interacting with, hence
> +           the most important tasks in the system.
> +        2. They don't run on a power hungry core and drain battery even if they
> +           are CPU intensive tasks.
> +
> +.. note::
> +  **little cores**:
> +    CPUs with capacity < 1024
> +
> +  **big cores**:
> +    CPUs with capacity = 1024

Processing capacity (CPU frequency) in MHz? This is the first time I
hear Arm big.LITTLE architecture. CC'ing several Arm folks and
linux-arm-kernel list for I'm unsure on this.

> +
> +By making these uclamp performance requests, or rather hints, user space can
> +ensure system resources are used optimally to deliver the best possible user
> +experience.
> +
> +Another use case is to help with **overcoming the ramp up latency inherit in
> +how scheduler utilization signal is calculated**.

IMO the bold text isn't needed (why did you highlight the phrase above)?

> +When task @p is running, **the scheduler should try its best to ensure it
> +starts at 40% performance level**. If the task runs for a long enough time so
> +that its actual utilization goes above 80%, the utilization, or performance
> +level, will be capped.

Same here.

> +**Generally it is advised to perceive the input as performance level or point
> +which will imply both task placement and frequency selection**.

Same here too.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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