linux-kernel - Re: [PATCH v3 07/10] sched/fair: Do not even the number of busy CPUs via asym

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Message-ID: <f8a75f47-0f7e-14cc-adf4-2854e235b26e@arm.com>
Date:   Mon, 13 Feb 2023 13:44:20 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     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>,
        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>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 07/10] sched/fair: Do not even the number of busy CPUs
 via asym_packing

On 07/02/2023 05:58, Ricardo Neri wrote:

[...]

> @@ -9269,33 +9264,11 @@ static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
>  				    struct sched_group *sg)
>  {
>  #ifdef CONFIG_SCHED_SMT
> -	bool local_is_smt;
>  	int sg_busy_cpus;
>  
> -	local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
>  	sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
>  
> -	if (!local_is_smt) {
> -		/*
> -		 * If we are here, @dst_cpu is idle and does not have SMT
> -		 * siblings. Pull tasks if candidate group has two or more
> -		 * busy CPUs.
> -		 */
> -		if (sg_busy_cpus >= 2) /* implies sg_is_smt */
> -			return true;
> -
> -		/*
> -		 * @dst_cpu does not have SMT siblings. @sg may have SMT
> -		 * siblings and only one is busy. In such case, @dst_cpu
> -		 * can help if it has higher priority and is idle (i.e.,
> -		 * it has no running tasks).
> -		 */
> -		return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> -	}
> -
>  	/*
> -	 * @dst_cpu has SMT siblings and are also idle.
> -	 *
>  	 * If the difference in the number of busy CPUs is two or more, let
>  	 * find_busiest_group() take care of it. We only care if @sg has
>  	 * exactly one busy CPU. This covers SMT and non-SMT sched groups.

Can't this be made lighter by removing asym_smt_can_pull_tasks() and
putting the logic to exclude the call to sched_asym_prefer() into
sched_asym() directly?
Not sure if we need the CONFIG_SCHED_SMT since it's all guarded by
`flags & SD_SHARE_CPUCAPACITY` already, which is only set under.
CONFIG_SCHED_SMT.

static inline bool
sched_asym(struct lb_env *env, struct sd_lb_stats *sds,
           struct sg_lb_stats *sgs, struct sched_group *group)
{
        bool local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;

        if (local_is_smt && !is_core_idle(env->dst_cpu))
                return false;

        if ((local_is_smt || group->flags & SD_SHARE_CPUCAPACITY)) {
                int sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;

                if (sg_busy_cpus != 1)
                        return false;
        }

        return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
}

[...]