Re: [cgroup/for-6.19 PATCH 2/3] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Chen Ridong]

On 2025/11/3 9:34, Waiman Long wrote:
> From: Gabriele Monaco <gmonaco@xxxxxxxxxx>
> 
> Currently the user can set up isolated cpus via cpuset and nohz_full in
> such a way that leaves no housekeeping CPU (i.e. no CPU that is neither
> domain isolated nor nohz full). This can be a problem for other
> subsystems (e.g. the timer wheel imgration).
> 
> Prevent this configuration by blocking any assignation that would cause
> the union of domain isolated cpus and nohz_full to covers all CPUs.
> 
> Acked-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/cgroup/cpuset.c | 67 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 66 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index da770dac955e..d6d459c95d82 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1329,6 +1329,19 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
>  		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
>  }
>  
> +/*
> + * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
> + * @prs: new or old partition_root_state
> + * @parent: parent cpuset
> + * Return: true if isolated_cpus needs modification, false otherwise
> + */
> +static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
> +{
> +	if (!parent)
> +		parent = &top_cpuset;
> +	return prs != parent->partition_root_state;
> +}
> +

Hi Longman,

I am confused about this function.

Why do we need to compare the partition_root_state (prs) with the parent's partition_root_state?

For example, when a local partition is assigned to a member, I don't think the isolated cpumasks
should be updated in this case.

In my understanding, the isolated CPUs should only be updated when an isolated partition is being
disabled or enabled. I was thinking of something like this:

bool isolated_cpus_should_update(int new_prs, int old_prs)
{
    if (new_prs == old_prs)
        return false;
    if (old_prs == 2 || new_prs == 2)
        return true;
    return false;
}

I would really appreciate it if you could provide some further explanation on this.

-- 
Best regards,
Ridong