On Wed, Oct 23, 2019 at 04:37:44PM +0100, Valentin Schneider <valentin.schneider@arm.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