linux-kernel - Re: [PATCHv4 11/12] sched/core: Disable SD_ASYM_CPUCAPACITY for root

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Message-ID: <20180705133142.GD32579@e108498-lin.cambridge.arm.com>
Date:   Thu, 5 Jul 2018 14:31:43 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     peterz@...radead.org, mingo@...hat.com, valentin.schneider@....com,
        dietmar.eggemann@....com, vincent.guittot@...aro.org,
        gaku.inami.xh@...esas.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 11/12] sched/core: Disable SD_ASYM_CPUCAPACITY for
 root_domains without asymmetry

Hi Morten,

On Wednesday 04 Jul 2018 at 11:17:49 (+0100), Morten Rasmussen wrote:
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 71330e0e41db..29c186961345 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1160,6 +1160,26 @@ sd_init(struct sched_domain_topology_level *tl,
>  	sd_id = cpumask_first(sched_domain_span(sd));
>  
>  	/*
> +	 * Check if cpu_map eclipses cpu capacity asymmetry.
> +	 */
> +
> +	if (sd->flags & SD_ASYM_CPUCAPACITY) {
> +		int i;
> +		bool disable = true;
> +		long capacity = arch_scale_cpu_capacity(NULL, sd_id);
> +
> +		for_each_cpu(i, sched_domain_span(sd)) {
> +			if (capacity != arch_scale_cpu_capacity(NULL, i)) {
> +				disable = false;
> +				break;
> +			}
> +		}
> +
> +		if (disable)
> +			sd->flags &= ~SD_ASYM_CPUCAPACITY;
> +	}
> +
> +	/*
>  	 * Convert topological properties into behaviour.
>  	 */

If SD_ASYM_CPUCAPACITY means that some CPUs have different
arch_scale_cpu_capacity() values, we could also automatically _set_
the flag in sd_init() no ? Why should we let the arch set it and just
correct it later ?

I understand the moment at which we know the capacities of CPUs varies
from arch to arch, but the arch code could just call
rebuild_sched_domain when the capacities of CPUs change and let the
scheduler detect things automatically. I mean, even if the arch code
sets the flag in its topology level table, it will have to rebuild
the sched domains anyway ...

What do you think ?

Thanks,
Quentin