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

[Waiman Long]

On 10/23/26 2:24 AM, Ridong Chen wrote:
> On 10/22/2026 11:16 AM, Waiman Long wrote:
> > 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.
> >
> > Fixes: 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus 
> > update in update_cpumasks_hier()")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   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)
>
> Nit.
>
> iancestor is confusing.
> Can we renmae it furthest_isolated_ancestor?

Yes, I can make the name change. Thanks for the suggestion.

Cheers,
Longman