linux-kernel - Re: [RFC PATCH v3 00/16] Core scheduling v3

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Date:   Sat, 10 Aug 2019 22:18:09 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Julien Desfossez <jdesfossez@...italocean.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3

On Thu, Aug 08, 2019 at 09:39:45AM -0700, Tim Chen wrote:
> On 8/8/19 5:55 AM, Aaron Lu wrote:
> > On Mon, Aug 05, 2019 at 08:55:28AM -0700, Tim Chen wrote:
> >> On 8/2/19 8:37 AM, Julien Desfossez wrote:
> >>> We tested both Aaron's and Tim's patches and here are our results.
> 
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 26fea68f7f54..542974a8da18 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3888,7 +3888,7 @@ next_class:;
> >  		WARN_ON_ONCE(!rq_i->core_pick);
> >  
> >  		if (is_idle_task(rq_i->core_pick) && rq_i->nr_running)
> > -			rq->core_forceidle = true;
> > +			rq_i->core_forceidle = true;
> 
> Good catch!
> 
> >  
> >  		rq_i->core_pick->core_occupation = occ;
> > 
> > With this fixed and together with the patch to let schedule always
> > happen, your latest 2 patches work well for the 10s cpuhog test I
> > described previously:
> > https://lore.kernel.org/lkml/20190725143003.GA992@aaronlu/
> 
> That's encouraging.  You are talking about my patches
> that try to keep the force idle time between sibling threads
> balanced, right?

Yes.

> > 
> > overloaded workload without any cpu binding doesn't work well though, I
> > haven't taken a closer look yet.
> > 
> 
> I think we need a load balancing scheme among the cores that will try
> to minimize force idle.

Agree.

> 
> One possible metric to measure load compatibility imbalance that leads to
> force idle is 
> 
> Say i, j are sibling threads of a cpu core
> imbalanace = \sum_tagged_cgroup  abs(Load_cgroup_cpui - Load_cgroup_cpuj)
> 
> This gives us a metric to decide if migrating a task will improve
> load compatability imbalance.  As we already track cgroup load on a CPU,
> it should be doable without adding too much overhead.