Re: [PATCH v2] sched/topology: Allow sched_asym_cpucapacity to be disabled

[Quentin Perret <qperret@google.com>]

On Tuesday 15 Oct 2019 at 11:29:56 (+0100), Valentin Schneider wrote:
> While the static key is correctly initialized as being disabled, it will
> remain forever enabled once turned on. This means that if we start with an
> asymmetric system and hotplug out enough CPUs to end up with an SMP system,
> the static key will remain set - which is obviously wrong. We should detect
> this and turn off things like misfit migration and capacity aware wakeups.
> 
> As Quentin pointed out, having separate root domains makes this slightly
> trickier. We could have exclusive cpusets that create an SMP island - IOW,
> the domains within this root domain will not see any asymmetry. This means
> we need to count how many asymmetric root domains we have.
> 
> Change the simple key enablement to an increment, and decrement the key
> counter when destroying domains that cover asymmetric CPUs.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: df054e8445a4 ("sched/topology: Add static_key for asymmetric CPU capacity optimizations")
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> Changes since v1:
> 
> Use static_branch_{inc,dec} rather than enable/disable
> ---
>  kernel/sched/topology.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index b5667a273bf6..79944e969bcf 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2026,7 +2026,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  	rcu_read_unlock();
>  
>  	if (has_asym)
> -		static_branch_enable_cpuslocked(&sched_asym_cpucapacity);
> +		static_branch_inc_cpuslocked(&sched_asym_cpucapacity);
>  
>  	if (rq && sched_debug_enabled) {
>  		pr_info("root domain span: %*pbl (max cpu_capacity = %lu)\n",
> @@ -2124,8 +2124,17 @@ static void detach_destroy_domains(const struct cpumask *cpu_map)
>  	int i;
>  
>  	rcu_read_lock();
> +
> +	if (static_key_enabled(&sched_asym_cpucapacity)) {
> +		unsigned int cpu = cpumask_any(cpu_map);
> +
> +		if (rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu)))
> +			static_branch_dec_cpuslocked(&sched_asym_cpucapacity);

Lockdep should scream for this :)
> +	}
> +
>  	for_each_cpu(i, cpu_map)
>  		cpu_attach_domain(NULL, &def_root_domain, i);
> +
>  	rcu_read_unlock();
>  }
>  
> -- 
> 2.22.0
>