linux-kernel - Re: [Patch v2 1/6] sched/fair: Determine active load balance for SMT sched groups

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Message-ID: <f7dc49b5fb80b25ad1f3968a623f993da29c0526.camel@linux.intel.com>
Date:   Mon, 12 Jun 2023 13:12:10 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     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,
        Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>,
        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>
Subject: Re: [Patch v2 1/6] sched/fair: Determine active load balance for
 SMT sched groups

On Mon, 2023-06-12 at 13:13 +0200, Peter Zijlstra wrote:
> 
> 
> 
> > @@ -9537,6 +9581,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >  		break;
> >  
> >  	case group_has_spare:
> > +		/*
> > +		 * Do not pick sg with SMT CPUs over sg with pure CPUs,
> > +		 * as we do not want to pull task off half empty SMT core
> > +		 * and make the core idle.
> > +		 */
> > +		if (smt_vs_nonsmt_groups(sds->busiest, sg)) {
> > +			if (sg->flags & SD_SHARE_CPUCAPACITY)
> > +				return false;
> > +			else
> > +				return true;
> > +		}
> 
> However, here I'm not at all sure. Consider SMT-4 with 2 active CPUs, we
> still very much would like to pull one task off if we have an idle core
> somewhere, no?
> 

How about making this modification to take care of SMT-4 case?

Tim

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 33246dce10db..e2261c24e536 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9642,11 +9642,11 @@ static bool update_sd_pick_busiest(struct lb_env *env,
        case group_has_spare:
                /*
                 * Do not pick sg with SMT CPUs over sg with pure CPUs,
-                * as we do not want to pull task off half empty SMT core
+                * as we do not want to pull task off SMT core with one task
                 * and make the core idle.
                 */
                if (smt_vs_nonsmt_groups(sds->busiest, sg)) {
-                       if (sg->flags & SD_SHARE_CPUCAPACITY)
+                       if (sg->flags & SD_SHARE_CPUCAPACITY && sgs->sum_h_nr_running <= 1)
                                return false;
                        else
                                return true;