linux-kernel - Re: [PATCH v4 1/2] sched/topology: Don't try to build empty sched domains

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Message-ID: <20191031162334.GA18570@blackbody.suse.cz>
Date:   Thu, 31 Oct 2019 17:23:34 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        lizefan@...wei.com, tj@...nel.org, hannes@...xchg.org,
        mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        Dietmar.Eggemann@....com, morten.rasmussen@....com,
        qperret@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v4 1/2] sched/topology: Don't try to build empty sched
 domains

On Wed, Oct 23, 2019 at 04:37:44PM +0100, Valentin Schneider <valentin.schneider@....com> wrote:
> Prevent generate_sched_domains() from returning empty cpumasks, and add
> some assertion in build_sched_domains() to scream bloody murder if it
> happens again.
Good catch. It makes sense to prune the empty domains in
generate_sched_domains already.

> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index c52bc91f882b..c87ee6412b36 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -798,7 +798,8 @@ static int generate_sched_domains(cpumask_var_t **domains,
>  		    cpumask_subset(cp->cpus_allowed, top_cpuset.effective_cpus))
>  			continue;
>  
> -		if (is_sched_load_balance(cp))
> +		if (is_sched_load_balance(cp) &&
> +		    !cpumask_empty(cp->effective_cpus))
>  			csa[csn++] = cp;
If I didn't overlook anything, cp->effective_cpus can contain CPUs
exluded by housekeeping_cpumask(HK_FLAG_DOMAIN) later, i.e. possibly
still returning domains with empty cpusets.

I'd suggest moving the emptiness check down into the loop where domain
cpumasks are ultimately constructed.

Michal

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