linux-kernel - Re: [PATCHv3 1/9] sched: Add static_key for asymmetric cpu capacity optimizations

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Message-ID: <20180622082222.GD23168@e108498-lin.cambridge.arm.com>
Date:   Fri, 22 Jun 2018 09:22:22 +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: [PATCHv3 1/9] sched: Add static_key for asymmetric cpu capacity
 optimizations

Hi Morten,

On Wednesday 20 Jun 2018 at 10:05:41 (+0100), Morten Rasmussen wrote:
> +static void update_asym_cpucapacity(int cpu)
> +{
> +	int enable = false;
> +
> +	rcu_read_lock();
> +	if (lowest_flag_domain(cpu, SD_ASYM_CPUCAPACITY))
> +		enable = true;
> +	rcu_read_unlock();
> +
> +	if (enable) {
> +		/* This expects to be hotplug-safe */
> +		static_branch_enable_cpuslocked(&sched_asym_cpucapacity);
> +	}
> +}

What would happen if you hotplugged an entire cluster ? You'd loose the
SD_ASYM_CPUCAPACITY flag but keep the static key is that right ? Should
we care ?

And also, Peter mentioned an issue with the EAS patches with multiple
root_domains. Does that apply here as well ? What if you had a
configuration with big and little CPUs in different root_domains for ex?

Should we disable the static key in the above cases ?

Thanks,
Quentin