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Message-ID: <04c369a9087353270a17ab474dd6ba88cf3a3df9.camel@linux.intel.com>
Subject: Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance
 calculation between asymmetric groups
From:   Tim Chen <tim.c.chen@linux.intel.com>
To:     Shrikanth Hegde <sshegde@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Valentin Schneider <vschneid@redhat.com>,
        Ionela Voinescu <ionela.voinescu@arm.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        naveen.n.rao@linux.vnet.ibm.com,
        Yicong Yang <yangyicong@hisilicon.com>,
        Barry Song <v-songbaohua@oppo.com>,
        Chen Yu <yu.c.chen@intel.com>, Hillf Danton <hdanton@sina.com>,
        Tobias Huschle <huschle@linux.ibm.com>
Date:   Fri, 14 Jul 2023 17:11:42 -0700
In-Reply-To: <c5a49136-3549-badd-ec8f-3de4e7bb7b7d@linux.vnet.ibm.com>
References: <cover.1688770494.git.tim.c.chen@linux.intel.com>
         <4eacbaa236e680687dae2958378a6173654113df.1688770494.git.tim.c.chen@linux.intel.com>
         <c5a49136-3549-badd-ec8f-3de4e7bb7b7d@linux.vnet.ibm.com>
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On Fri, 2023-07-14 at 18:44 +0530, Shrikanth Hegde wrote:
>=20
> > +	/* Take advantage of resource in an empty sched group */
> > +	if (imbalance =3D=3D 0 && local->sum_nr_running =3D=3D 0 &&
> > +	    busiest->sum_nr_running > 1)
> > +		imbalance =3D 2;
> > +
>=20
> I don't see how this case would be true. When there are unequal number of=
 cores and local->sum_nr_ruuning=20
> is 0, and busiest->sum_nr_running is atleast 2, imbalance will be atleast=
 1.=20

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. =20

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.=20

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 *e=
nv,
        imbalance /=3D ncores_local + ncores_busiest;
=20
        /* Take advantage of resource in an empty sched group */
-       if (imbalance =3D=3D 0 && local->sum_nr_running =3D=3D 0 &&
+       if (imbalance <=3D 1 && local->sum_nr_running =3D=3D 0 &&
            busiest->sum_nr_running > 1)
                imbalance =3D 2;
=20

Tim