linux-kernel - Re: Scheduling tasks on idle cpu

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Message-ID: <20220411052641.5370437f@rorschach.local.home>
Date:   Mon, 11 Apr 2022 05:26:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Qais Yousef' <qais.yousef@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "parth@...ux.ibm.com" <parth@...ux.ibm.com>,
        "chris.hyser@...cle.com" <chris.hyser@...cle.com>,
        "pkondeti@...eaurora.org" <pkondeti@...eaurora.org>,
        "Valentin.Schneider@....com" <Valentin.Schneider@....com>,
        "patrick.bellasi@...bug.net" <patrick.bellasi@...bug.net>,
        "pjt@...gle.com" <pjt@...gle.com>, "pavel@....cz" <pavel@....cz>,
        "tj@...nel.org" <tj@...nel.org>,
        "qperret@...gle.com" <qperret@...gle.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        Wei Wang <wvw@...gle.com>
Subject: Re: Scheduling tasks on idle cpu

On Mon, 11 Apr 2022 08:26:33 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> Does that actually happen?
> I've seen the following:
>   34533 [017]: sys_futex(uaddr: 1049104, op: 85, val: 1, utime: 1, uaddr2: 1049100, val3: 4000001)
>   34533 [017]: sched_migrate_task: pid=34512 prio=120 orig_cpu=14 dest_cpu=17
>   34533 [017]: sched_wakeup: pid=34512 prio=120 success=1 target_cpu=017
> and pid 34512 doesn't get scheduled until pid 34533 finally sleeps.
> This is in spite of there being 5 idle cpu.

What's the topology? I believe the scheduler will refrain from
migrating tasks to idle CPUs that are on other NUMA nodes as much as
possible. Were those other 5 idle CPUs on another node?

-- Steve


> cpu 14 is busy running a RT thread, but migrating to cpu 17 seems wrong.
> 
> This is on a RHEL7 kernel, I've not replicated it on anything recent.
> But I've very much like a RT thread to be able to schedule a non-RT
> thread to run on an idle cpu.