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

[Dietmar Eggemann]

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;