Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance

From: Peter Zijlstra <peterz@infradead.org>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.comi>, Mel Gorman <mgorman@suse.de>,
	Len Brown <len.brown@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Aubrey Li <aubrey.li@linux.intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Ricardo Neri <ricardo.neri@intel.com>,
	Quentin Perret <qperret@google.com>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org, Aubrey Li <aubrey.li@intel.com>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance
Date: Tue, 6 Apr 2021 13:17:28 +0200	[thread overview]
Message-ID: <YGxDSMorjMADJGIT@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210406041108.7416-4-ricardo.neri-calderon@linux.intel.com>

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.