linux-kernel - Re: [PATCH v3 04/10] sched/fair: Let low-priority cores help high-priority busy SMT cores

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Date:   Mon, 13 Feb 2023 14:40:24 +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 04/10] sched/fair: Let low-priority cores help
 high-priority busy SMT cores

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

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 80c86462c6f6..c9d0ddfd11f2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10436,11 +10436,20 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  		    nr_running == 1)
>  			continue;
>  
> -		/* Make sure we only pull tasks from a CPU of lower priority */
> +		/*
> +		 * Make sure we only pull tasks from a CPU of lower priority
> +		 * when balancing between SMT siblings.
> +		 *
> +		 * If balancing between cores, let lower priority CPUs help
> +		 * SMT cores with more than one busy sibling.
> +		 */
>  		if ((env->sd->flags & SD_ASYM_PACKING) &&
>  		    sched_asym_prefer(i, env->dst_cpu) &&
> -		    nr_running == 1)
> -			continue;
> +		    nr_running == 1) {
> +			if (env->sd->flags & SD_SHARE_CPUCAPACITY ||
> +			    (!(env->sd->flags & SD_SHARE_CPUCAPACITY) && is_core_idle(i)))
> +				continue;

is_core_idle(i) returns true for !CONFIG_SCHED_SMP. So far it was always
guarded by `flags & SD_SHARE_CPUCAPACITY` which is only set for
CONFIG_SCHED_SMP.

Here it's different but still depends on `flags & SD_ASYM_PACKING`.

Can we have SD_ASYM_PACKING w/o CONFIG_SCHED_SMP? The comment just says
`If balancing between cores (MC), let lower priority CPUs help SMT cores
with more than one busy sibling.`

So this only mentions your specific asymmetric e-cores w/o SMT and
p-cores w/ SMT case.

I'm asking since numa_idle_core(), the only user of is_core_idle() so
far has an extra `!static_branch_likely(&sched_smt_present)` condition
before calling it.

> +		}
>  
>  		switch (env->migration_type) {
>  		case migrate_load:
> @@ -10530,8 +10539,20 @@ asym_active_balance(struct lb_env *env)
>  	 * lower priority CPUs in order to pack all tasks in the
>  	 * highest priority CPUs.
>  	 */
> -	return env->idle != CPU_NOT_IDLE && (env->sd->flags & SD_ASYM_PACKING) &&
> -	       sched_asym_prefer(env->dst_cpu, env->src_cpu);
> +	if (env->idle != CPU_NOT_IDLE && (env->sd->flags & SD_ASYM_PACKING)) {
> +		/* Always obey priorities between SMT siblings. */
> +		if (env->sd->flags & SD_SHARE_CPUCAPACITY)
> +			return sched_asym_prefer(env->dst_cpu, env->src_cpu);
> +
> +		/*
> +		 * A lower priority CPU can help an SMT core with more than one
> +		 * busy sibling.
> +		 */
> +		return sched_asym_prefer(env->dst_cpu, env->src_cpu) ||
> +		       !is_core_idle(env->src_cpu);

Here it is similar.

> +	}
> +
> +	return false;