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

[Waiman Long]

On 10/22/26 9:25 AM, Guopeng Zhang wrote:
> 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@xxxxxxxxxx>
> Tested-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>
> Thanks,
> Guopeng

Thanks for the review. This patch fixes a specific problem reported by 
sashiko. However, there are other pre-existing problem that will still 
fail test case that you have proposed above. Patches 2 & 3 of this 
series are there to address the other existing problems so that test 
cases like what you have proposed above will work. The first test case 
added in patch 2 was to test this though it is somewhat different from 
yours. I don't add any test case to patch 1 because I don't want to 
break bisection. I can add your test case in patch 2 though.

Cheers,
Longman