linux-kernel - Re: [PATCH v2 3/3] sched/fair: do not scribble cache-hot CPU in select_idle

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Message-ID: <CADjb_WTFrPAgf5h1af4GuMFJ8UrDjzCPb_K+SZFo5s80xcniVA@mail.gmail.com>
Date:   Thu, 30 Nov 2023 14:43:00 +0800
From:   Chen Yu <yu.chen.surf@...il.com>
To:     Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Cc:     Chen Yu <yu.c.chen@...el.com>, Tim Chen <tim.c.chen@...el.com>,
        Aaron Lu <aaron.lu@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2 3/3] sched/fair: do not scribble cache-hot CPU in select_idle_cpu()

Hi Madadi,

On Thu, Nov 30, 2023 at 1:26 AM Madadi Vineeth Reddy
<vineethr@...ux.ibm.com> wrote:
>
> Hi Chen Yu,
>
[snip...]

>
> As per my understanding, if the task which tagged a particular CPU as cache hot has woken up before
> the cache_hot_timeout, we still don't allow that task to run on that particular CPU. Is this correct?
>

Not exactly. When we reached select_idle_cpu(), we have already
checked if the wakee's previous CPU
is idle or not in select_idle_sibling(), if it is idle, we don't check
the cache-hot tag and return the wakee's
previous CPU directly in select_idle_sibling().

thanks,
Chenyu

> If correct, then why don't we let the task to select the CPU if it's the one that tagged it?
>
> Thanks and Regards
> Madadi Vineeth Reddy