linux-kernel - Re: [RFC PATCH v4 00/19] Core scheduling v4

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Date:   Thu, 5 Mar 2020 21:45:15 +0800
From:   Aubrey Li <aubrey.intel@...il.com>
To:     Aaron Lu <aaron.lwe@...il.com>
Cc:     Phil Auld <pauld@...hat.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Dario Faggioli <dfaggioli@...e.com>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v4 00/19] Core scheduling v4

On Fri, Feb 28, 2020 at 10:54 AM Aaron Lu <aaron.lwe@...il.com> wrote:
>
> When the core wide weight is somewhat balanced, yes I definitely agree.
> But when core wide weight mismatch a lot, I'm not so sure since if these
> high weight task is spread among cores, with the feature of core
> scheduling, these high weight tasks can get better performance.

It depends.

Say TaskA(cookie 1) and TaskB(cookie1) has high weight,
TaskC(cookie 2) and Task D(cookie 2) has low weight.
And we have two cores 4 CPU.

If we dispatch
- TaskA and TaskB on Core0,
- TaskC and TaskD on Core1,

with coresched enabled, all 4 tasks can run all the time.

But if we dispatch
- TaskA on Core0.CPU0, TaskB on Core1.CPU2,
- TaskC on Core0.CPU1, TaskB on Core1.CPU3,

with coresched enabled, when TaskC is running, TaskA will be forced
off CPU and replaced with a forced idle thread.

Things get worse if TaskA and TaskB share some data and can get
benefit from the core level cache.

> So this appeared to me like a question of: is it desirable to protect/enhance
> high weight task performance in the presence of core scheduling?

This sounds to me a policy VS mechanism question. Do you have any idea
how to spread high weight task among the cores with coresched enabled?

Thanks,
-Aubrey