Re: [PATCH] cpu: Fix online housekeeping CPU check in _cpu_down()

[Waiman Long <longman@redhat.com>]

Mathieu,

Sorry, I forgot to cc you on this patch.

Cheers,
Longman

On 11/8/26 1:08 PM, Waiman Long wrote:
> There is currently a check in _cpu_down() to make sure that there is
> at least one online housekeeping (HK) CPU left after the shutdown of
> the given @cpu parameter. However the cpu_online_mask used for the
> check may include the current @cpu to be shut down. So the fact that
> there is at least one online housekeeping CPU at the moment does not
> guarantee that there will be any online housekeeping CPU left for the
> sched domain after the shutdown.
>
> Fix that by making sure that there are at least 2 online housekeeping
> CPUs available if @cpu is currently a housekeeping CPU.
>
> Fixes: de715325cc47 ("cpu: Revert "cpu/hotplug: Prevent self deadlock on CPU hot-unplug"")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cpu.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index b3c8553d7bd6..ed641ee6b37c 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1415,11 +1415,14 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
>   	cpus_write_lock();
>   
>   	/*
> -	 * Keep at least one housekeeping cpu onlined to avoid generating
> -	 * an empty sched_domain span.
> +	 * Keep at least one housekeeping cpu online to avoid generating an
> +	 * empty sched_domain span. As cpu_online_mask includes the @cpu to be
> +	 * shut down, there should be at least 2 online HK CPUs at the moment
> +	 * if @cpu is currently a HK CPU.
>   	 */
> -	if (cpumask_any_and(cpu_online_mask,
> -			    housekeeping_cpumask(HK_TYPE_DOMAIN)) >= nr_cpu_ids) {
> +	if (cpumask_test_cpu(cpu, housekeeping_cpumask(HK_TYPE_DOMAIN)) &&
> +	   (cpumask_weight_and(cpu_online_mask,
> +			       housekeeping_cpumask(HK_TYPE_DOMAIN)) < 2)) {
>   		ret = -EBUSY;
>   		goto out;
>   	}