linux-kernel - Re: [RFC PATCH] sched/fair: Introduce WAKEUP_BIAS_PREV

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Message-ID: <ZSwI4UTrXLNEWwKM@chenyu5-mobl2.ccr.corp.intel.com>
Date:   Sun, 15 Oct 2023 23:44:33 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        "Mel Gorman" <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Aaron Lu <aaron.lu@...el.com>, Tim Chen <tim.c.chen@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>, <x86@...nel.org>
Subject: Re: [RFC PATCH] sched/fair: Introduce WAKEUP_BIAS_PREV_IDLE to
 reduce migrations

On 2023-10-12 at 16:36:26 -0400, Mathieu Desnoyers wrote:
> Introduce the WAKEUP_BIAS_PREV_IDLE scheduler feature to reduce the
> task migration rate.
> 
> For scenarios where the system is under-utilized (CPUs are partly idle),
> eliminate frequent task migrations from almost idle CPU to completely
> idle CPUs by introducing a bias towards the previous CPU if it is idle
> or almost idle in select_idle_sibling(). Use 1% of the CPU capacity
> of the previously used CPU as CPU utilization "almost idle" cutoff.
>
> +
> +/*
> + * A runqueue is considered almost idle if:
> + *
> + *   cpu_util_without(cpu, p) / 1024 <= 1% * capacity_of(cpu)

util_avg is in the range [0:1024], thus cpu_util_without(cpu, p) / 1024
is <= 1, and 1% * cap is 10, so 1 <= 10 is always true.
I suppose you want to compare:
 (cpu_util_without(cpu, p) / capacity_orig_of(cpu)) <= 1% ->
    cpu_util_without(cpu, p) * 100 <= capacity_orig_of(cpu) ?

thanks,
Chenyu