Additional issue with cpuset isolated partitions?

[Juri Lelli <juri.lelli@redhat.com>]

Hello,

While working on the recent cpuset/deadline fixes [1], I encountered
what looks like an issue to me. What I'm doing is (based on one of the
tests of test_cpuset_prs.sh):

# echo Y >/sys/kernel/debug/sched/verbose
# echo +cpuset >cgroup/cgroup.subtree_control
# mkdir cgroup/A1
# echo 0-3 >cgroup/A1/cpuset.cpus
# echo +cpuset >cgroup/A1/cgroup.subtree_control
# mkdir cgroup/A1/A2
# echo 1-3 >cgroup/A1/A2/cpuset.cpus
# echo +cpuset >cgroup/A1/A2/cgroup.subtree_control
# mkdir cgroup/A1/A2/A3
# echo 2-3 >cgroup/A1/A2/A3/cpuset.cpus
# echo 2-3 >cgroup/A1/cpuset.cpus.exclusive
# echo 2-3 >cgroup/A1/A2/cpuset.cpus.exclusive
# echo 2-3 >cgroup/A1/A2/A3/cpuset.cpus.exclusive
# echo isolated >cgroup/A1/A2/A3/cpuset.cpus.partition

and with this, on my 8 CPUs system, I correctly get a root domain for
0-1,4-7 and 2,3 are left isolated (attached to default root domain).

I now put the shell into the A1/A2/A3 cpuset

# echo $$ >cgroup/A1/A2/A3/cgroup.procs

and hotplug CPU 2,3

# echo 0 >/sys/devices/system/cpu/cpu2/online
# echo 0 >/sys/devices/system/cpu/cpu3/online

guess the shell is moved to the non-isolated domain. So far so good
then, only that if I turn CPUs 2,3 back on they are attached to the root
domain containing the non-isolated cpus

# echo 1 >/sys/devices/system/cpu/cpu2/online
...
[  990.133593] root domain span: 0-2,4-7
[  990.134480] rd 0-2,4-7

# echo 1 >/sys/devices/system/cpu/cpu3/online
...
[ 1082.858992] root domain span: 0-7
[ 1082.859530] rd 0-7

And now the A1/A2/A3 partition is not valid anymore

# cat cgroup/A1/A2/A3/cpuset.cpus.partition
isolated invalid (Invalid cpu list in cpuset.cpus.exclusive)

Is this expected? It looks like one need to put at least one process in
the partition before hotplugging its cpus for the above to reproduce
(hotpluging w/o processes involved leaves CPUs 2,3 in the default domain
and isolated).

Thanks,
Juri

1 - https://lore.kernel.org/lkml/20241114142810.794657-1-juri.lelli@redhat.com/
    https://lore.kernel.org/lkml/20241110025023.664487-1-longman@redhat.com/