linux-kernel - Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance calculation between asymmetric groups

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Message-ID: <04c369a9087353270a17ab474dd6ba88cf3a3df9.camel@linux.intel.com>
Date:   Fri, 14 Jul 2023 17:11:42 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        Yicong Yang <yangyicong@...ilicon.com>,
        Barry Song <v-songbaohua@...o.com>,
        Chen Yu <yu.c.chen@...el.com>, Hillf Danton <hdanton@...a.com>,
        Tobias Huschle <huschle@...ux.ibm.com>
Subject: Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance
 calculation between asymmetric groups

On Fri, 2023-07-14 at 18:44 +0530, Shrikanth Hegde wrote:
> 
> > +	/* Take advantage of resource in an empty sched group */
> > +	if (imbalance == 0 && local->sum_nr_running == 0 &&
> > +	    busiest->sum_nr_running > 1)
> > +		imbalance = 2;
> > +
> 
> I don't see how this case would be true. When there are unequal number of cores and local->sum_nr_ruuning 
> is 0, and busiest->sum_nr_running is atleast 2, imbalance will be atleast 1. 

I think you are correct.  With at least 2 task in the busiest group,
imbalance will be at least 1.  This is the effect of doing rounding
when adding the (ncores_local + ncores_busy) rounding factor.  

Returning an imbalance value of 1 will not be correct as we
will be dividing imbalance by 2 and we will still not move task
to the empty group as intended. 

So this code should be updated as below:


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3fc8d3a3bd22..16bf75e6a775 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9400,7 +9400,7 @@ static inline long sibling_imbalance(struct lb_env *env,
        imbalance /= ncores_local + ncores_busiest;
 
        /* Take advantage of resource in an empty sched group */
-       if (imbalance == 0 && local->sum_nr_running == 0 &&
+       if (imbalance <= 1 && local->sum_nr_running == 0 &&
            busiest->sum_nr_running > 1)
                imbalance = 2;
 

Tim