linux-kernel - Re: [PATCH] sched/fair: Add SMT4 group_smt

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Message-ID: <20230905104136.GC28319@noisy.programming.kicks-ass.net>
Date:   Tue, 5 Sep 2023 12:41:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>, bristot@...hat.com,
        bsegall@...gle.com, dietmar.eggemann@....com, hdanton@...a.com,
        ionela.voinescu@....com, juri.lelli@...hat.com,
        len.brown@...el.com, linux-kernel@...r.kernel.org, mgorman@...e.de,
        naveen.n.rao@...ux.vnet.ibm.com, rafael.j.wysocki@...el.com,
        ravi.v.shankar@...el.com, ricardo.neri@...el.com,
        rostedt@...dmis.org, srikar@...ux.vnet.ibm.com,
        srinivas.pandruvada@...ux.intel.com, v-songbaohua@...o.com,
        vincent.guittot@...aro.org, vschneid@...hat.com, x86@...nel.org,
        yangyicong@...ilicon.com, yu.c.chen@...el.com
Subject: Re: [PATCH] sched/fair: Add SMT4 group_smt_balance handling

On Thu, Jul 27, 2023 at 06:32:44AM -0700, Tim Chen wrote:

>  kernel/sched/fair.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a87988327f88..566686c5f2bd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9563,7 +9563,7 @@ static inline long sibling_imbalance(struct lb_env *env,
>  	imbalance /= ncores_local + ncores_busiest;
>  
>  	/* Take advantage of resource in an empty sched group */
> -	if (imbalance == 0 && local->sum_nr_running == 0 &&
> +	if (imbalance <= 1 && local->sum_nr_running == 0 &&
>  	    busiest->sum_nr_running > 1)
>  		imbalance = 2;
>  
> @@ -9751,6 +9751,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		break;
>  
>  	case group_smt_balance:
> +		/* no idle cpus on both groups handled by group_fully_busy below */
> +		if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
> +			if (sgs->idle_cpus > busiest->idle_cpus)
> +				return false;
> +			if (sgs->idle_cpus < busiest->idle_cpus)
> +				return true;
> +			if (sgs->sum_nr_running <= busiest->sum_nr_running)
> +				return false;
> +			else
> +				return true;
> +		}
> +		goto fully_busy;
> +		break;

This is really daft; why can't this simply be: fallthrough; ? At the
very least that break must go.

> +
>  	case group_fully_busy:
>  		/*
>  		 * Select the fully busy group with highest avg_load. In
> @@ -9763,7 +9777,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		 * select the 1st one, except if @sg is composed of SMT
>  		 * siblings.
>  		 */
> -
> +fully_busy:
>  		if (sgs->avg_load < busiest->avg_load)
>  			return false;
>  
> -- 
> 2.32.0
> 
>