Re: [PATCH-next 1/6] cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition

[Guopeng Zhang <guopeng.zhang@linux.dev>]

在 2026/10/22 11:16, Waiman Long 写道:
> Commit 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus
> update in update_cpumasks_hier()") searches for the closet partition
> root ancestor to determine if the isolated CPUs in a to be invalidated
> isolation partition should be removed from isolated_cpus or not.
> 
> As reported by sashiko, this method may not work when there is a deeply
> nested isolated partition hierarchy. For instance, Consider a nested
> hierarchy A0 -> A1 -> A2 -> A3 where all four are PRS_ISOLATED:
> 
>   A0: effective_xcpus = 0-4
>   A1: effective_xcpus = 1-4
>   A2: effective_xcpus = 2-4
>   A3: effective_xcpus = 3-4
> 
> When A1 shrinks its exclusive_cpus from 1-4 to 1-2, A2 will update it
> effective_xcpus to 2. A3 will find its closet partition root ancestor
> A2 and decides that its effective_xcpus 3-4 should be removed from
> isolated_cpus which is incorrect.
> 
> Address this corner case by searching for the furthest isolated partition
> root ancestor instead to decide if its isolated CPUs should be returned
> to isolated_cpus.
> 

This looks good to me. Would it be worth adding a test case for the
scenario described above, something like:

    # A middle isolated parent shrunk below coverage: the released
    # CPUs of the invalidated child stay isolated as a higher
    # ancestor still covers them.
    " CX0-4:P2 CX2-4:P2 CX3-4:P2 .      .      X2     .      .     0 A1:0-1,3-4|A2:2|A3:2 A1:P2|A2:P2|A3:P-2 0-4"

Reviewed-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Tested-by: Guopeng Zhang <zhangguopeng@kylinos.cn>

Thanks,
Guopeng

> Fixes: 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus update in update_cpumasks_hier()")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 36 +++++++++++++++++-------------------
>  1 file changed, 17 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 585a982249ad..7ead8c3b7974 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2156,22 +2156,20 @@ static void compute_partition_effective_cpumask(struct cpuset *cs,
>  }
>  
>  /*
> - * ancestor_partition - Find and return the closest valid partition root ancestor
> + * iancestor_partition - Return furthest isolated partition root ancestor available
>   * @cs: the current cpuset
> - *
> - * Return: The closet partition root ancestor cpuset
> + * Return: The furthest isolated partition root ancestor available or NULL if
> + *	   not found.
>   */
> -static struct cpuset *ancestor_partition(struct cpuset *cs)
> +static struct cpuset *iancestor_partition(struct cpuset *cs)
>  {
>  	struct cpuset *ancestor = parent_cs(cs);
> +	struct cpuset *iancestor = NULL;
>  
> -	/*
> -	 * The top_cpuset is always a valid partition root which will stop
> -	 * the loop if it is reached.
> -	 */
> -	while (!is_partition_valid(ancestor))
> -		ancestor = parent_cs(ancestor);
> -	return ancestor;
> +	for (; ancestor; ancestor = parent_cs(ancestor))
> +		if (ancestor->partition_root_state == PRS_ISOLATED)
> +			iancestor = ancestor;
> +	return iancestor;
>  }
>  
>  /*
> @@ -2283,23 +2281,23 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>  				 * For a to be invalidated isolated partition,
>  				 * its isolated CPUs may or may not need to be
>  				 * removed from isolated_cpus depending on if
> -				 * its closest partition root ancestor is a root
> -				 * or isolated partition. With a PRS_ISOLATED
> -				 * ancestor, the CPUs are checked against its
> -				 * effective xcpus to see if any of them needed
> -				 * to be removed.
> +				 * there is a furthest isolated partition root
> +				 * ancestor available. When such a PRS_ISOLATED
> +				 * ancestor is available, the CPUs are checked
> +				 * against its effective xcpus to see if any of
> +				 * them needed to be removed.
>  				 */
>  				if (is_partition_valid(cp))
>  					new_prs = -old_prs;
>  
>  				if (old_prs == PRS_ISOLATED) {
> -					struct cpuset *ancestor = ancestor_partition(cp);
> +					struct cpuset *iancestor = iancestor_partition(cp);
>  
> -					if (ancestor->partition_root_state == PRS_ROOT)
> +					if (!iancestor)
>  						del_cpus = cp->effective_xcpus;
>  					else if (cpumask_andnot(tmp->delmask,
>  								cp->effective_xcpus,
> -								ancestor->effective_xcpus))
> +								iancestor->effective_xcpus))
>  						del_cpus = tmp->delmask;
>  
>  					if (del_cpus) {