linux-kernel - RE: Scheduling tasks on idle cpu

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Message-ID: <5fc162f60ba84ab3a61992084d76c726@AcuMS.aculab.com>
Date:   Thu, 14 Apr 2022 14:14:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vincent Guittot' <vincent.guittot@...aro.org>
CC:     Qais Yousef <qais.yousef@....com>,
        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: RE: Scheduling tasks on idle cpu

From: Vincent Guittot
> Sent: 14 April 2022 11:17
...
> > > David can rerun is use case after disabling sched_feat(SIS_PROP)
> >
> > How would I do that?
> 
> echo NO_SIS_PROP > /sys/kernel/debug/sched/features

That may not be in the kernel I'm using.

# cat /sys/kernel/debug/sched_features
GENTLE_FAIR_SLEEPERS START_DEBIT NO_NEXT_BUDDY LAST_BUDDY CACHE_HOT_BUDDY WAKEUP_PREEMPTION ARCH_POWER NO_HRTICK NO_DOUBLE_TICK LB_BIAS NONTASK_POWER TTWU_QUEUE RT_RUNTIME_SHARE NO_LB_MIN NUMA NUMA_FAVOUR_HIGHER NO_NUMA_RESIST_LOWER

I've been looking at another ftrace output.
The scheduler does like migrating the process to the current cpu.
I have seen it migrate from one idle cpu to another idle cpu.
I've not seen it migrate from an idle cpu to the current cpu.
  (But I've not looked hard.)

These are all the migrates:
    TiNG task:12-1005  [026] d... 1111081.796560: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=11 dest_cpu=16
    TiNG task:31-1026  [005] d... 1111081.836556: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=16 dest_cpu=11
    TiNG task:28-1023  [033] d... 1111081.856589: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=11 dest_cpu=33
    TiNG task:11-1004  [013] d... 1111081.856606: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=33 dest_cpu=13
    TiNG task:19-1012  [002] d... 1111081.896564: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=13 dest_cpu=10
    TiNG task:26-1019  [008] d... 1111081.956551: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=10 dest_cpu=18
    TiNG task:34-1029  [001] d... 1111082.016527: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=18 dest_cpu=1
    TiNG task:20-1013  [021] d... 1111082.016589: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=1 dest_cpu=21
    TiNG task:32-1027  [000] d... 1111082.036455: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=21 dest_cpu=2
    TiNG task:15-1008  [006] d... 1111082.056539: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=2 dest_cpu=14
    TiNG task:34-1029  [001] d... 1111082.076536: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=14 dest_cpu=1
    TiNG task:21-1014  [004] d... 1111082.076589: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=1 dest_cpu=4
    TiNG task:11-1004  [013] d... 1111082.096526: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=4 dest_cpu=13
    TiNG task:28-1023  [033] d... 1111082.096584: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=13 dest_cpu=33
    TiNG task:25-1018  [029] d... 1111082.116549: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=33 dest_cpu=11
    TiNG task:27-1020  [032] d... 1111082.176519: sched_migrate_task: comm=RTP sockets pid=990 prio=120 orig_cpu=11 dest_cpu=16
There are a couple of places where there 2 wakeups before the schedule.
The scheduler definitely doesn't like waking up a process on an even cpu from on odd one.
But there are also the 13->33 and 1->21 ones.

	David

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