linux-kernel - Scheduling tasks on idle cpu

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Date:   Mon, 11 Apr 2022 08:26:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Qais Yousef' <qais.yousef@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
CC:     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>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "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: Scheduling tasks on idle cpu

From: Qais Yousef
> Sent: 09 April 2022 18:09
...
> RT scheduler will push/pull tasks to ensure the task will get to run ASAP if
> there's another cpu at lower priority is available

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.
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.

	David

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