linux-kernel - Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM

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Message-ID: <YGxDSMorjMADJGIT@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Apr 2021 13:17:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.comi>, Mel Gorman <mgorman@...e.de>,
        Len Brown <len.brown@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Quentin Perret <qperret@...gle.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance

On Mon, Apr 05, 2021 at 09:11:07PM -0700, Ricardo Neri wrote:
> @@ -8507,6 +8619,10 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  	if (!sgs->sum_h_nr_running)
>  		return false;
>  
> +	if (sgs->group_type == group_asym_packing &&
> +	    !asym_can_pull_tasks(env->dst_cpu, sds, sgs, sg))
> +		return false;

All of this makes my head hurt; but afaict this isn't right.

Your update_sg_lb_stats() change makes that we unconditionally set
sgs->group_asym_packing, and then this is to undo that. But it's not
clear this covers all cases right.

Even if !sched_asym_prefer(), we could end up selecting this sg as
busiest, but you're just bailing out here.