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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:     Tobias Huschle <huschle@linux.ibm.com>,
        Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>
Date:   Fri, 14 Jul 2023 16:35:12 -0700
In-Reply-To: <b119d88384584e603056cec942c47e14@linux.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>
         <b119d88384584e603056cec942c47e14@linux.ibm.com>
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On Fri, 2023-07-14 at 16:22 +0200, Tobias Huschle wrote:
>=20
> >=20
> > Could this work for case where number of CPU/cores would differ
> > between two sched groups in a sched domain? Such as problem pointed
> > by tobias on S390. It would be nice if this patch can work for that=20
> > case
> > as well. Ran numbers for a few cases. It looks to work.
> > https://lore.kernel.org/lkml/20230704134024.GV4253@hirez.programming.ki=
cks-ass.net/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901
> >=20
>=20
>=20
> Just stumbled upon this patch series as well. In this version it looks
> similar to the prototypes I played around with, but more complete.
> So I'm happy that my understanding of the load balancer was kinda=20
> correct :)
>=20
>  From a functional perspective this appears to address the issues we saw=
=20
> on s390.

Glad that this patch addresses this common issue across architectures.
I did aim to address the asymmetric groups balancing in general.
Peter pointed out this problem that's inherent when he reviewed the first
version of my patchset.=20

Tim

>=20
> >=20
> >=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
> > is 0, and busiest->sum_nr_running is atleast 2, imbalance will be=20
> > atleast 1.
> >=20
> >=20
> > Reviewed-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
> >=20
> > > +	return imbalance;
> > > +}
> > > +
> > >  static inline bool
> > >  sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
> > >  {
> > > @@ -10230,14 +10265,12 @@ static inline void=20
> > > calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> > >  		}
> > >=20
> > >  		if (busiest->group_weight =3D=3D 1 || sds->prefer_sibling) {
> > > -			unsigned int nr_diff =3D busiest->sum_nr_running;
> > >  			/*
> > >  			 * When prefer sibling, evenly spread running tasks on
> > >  			 * groups.
> > >  			 */
> > >  			env->migration_type =3D migrate_task;
> > > -			lsub_positive(&nr_diff, local->sum_nr_running);
> > > -			env->imbalance =3D nr_diff;
> > > +			env->imbalance =3D sibling_imbalance(env, sds, busiest, local);
> > >  		} else {
> > >=20
> > >  			/*
> > > @@ -10424,7 +10457,7 @@ static struct sched_group=20
> > > *find_busiest_group(struct lb_env *env)
> > >  	 * group's child domain.
> > >  	 */
> > >  	if (sds.prefer_sibling && local->group_type =3D=3D group_has_spare =
&&
> > > -	    busiest->sum_nr_running > local->sum_nr_running + 1)
> > > +	    sibling_imbalance(env, &sds, busiest, local) > 1)
> > >  		goto force_balance;
> > >=20
> > >  	if (busiest->group_type !=3D group_overloaded) {