linux-kernel - Re: [Patch v3 1/6] sched/fair: Determine active load balance for SMT sched groups

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Date:   Fri, 14 Jul 2023 16:29:00 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Tobias Huschle <huschle@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 v3 1/6] sched/fair: Determine active load balance for
 SMT sched groups


> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 87317634fab2..f636d6c09dc6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8279,6 +8279,11 @@ enum group_type {
> >  	 * more powerful CPU.
> >  	 */
> >  	group_misfit_task,
> > +	/*
> > +	 * Balance SMT group that's fully busy. Can benefit from migration
> > +	 * a task on SMT with busy sibling to another CPU on idle core.
> > +	 */
> > +	group_smt_balance,
> 
> Would it make sense to move smt_balance?, s.t. we get:
> 
> group_has_spare < group_smt_balance < group_fully_busy
> 
> Conceptually I would be more intuitive to me like this as the
> smt_balance groups are more busy than has_spare ones, but less busy
> then fully_busy ones.
> 
>  From a functional perspective I could also see some impact when
> update_sd_pick_busiest compares the group types. In that case we
> would remove tasks from fully busy groups before moving them
> from smt_balance groups. Not sure which way would be to prefer
> to increase overall throughput.
> 
> Since smt_balance is only selected if the group has SMT, this
> should still not pull the last task off of a non-SMT CPU.
> 
> 

I think you have similar concerns as Shrikanth on this patch.
Can you see if my fix to update_sd_pick_busiest() in my reply
to Shrikanth addresses what you have in mind.

Tim