Re: [QUESTION/REGRESSION] Unbound kthreads scheduled on nohz_full CPUs after commit 041ee6f3727a

[sheviks]

Frederic Weisbecker <frederic@xxxxxxxxxx> 於 2026年3月23日週一 上午6:33寫道：
> Yes. The affinity is applied right after the first wake-up of the kthread.
> And this wake-up is supposed to happen right after the kthread creation
> because the only purpose of this first wake-up is to allow for calling
> kthread_park(), kthread_bind() or kthread_affine_preferred() between
> kthread_create() and wake_up_process().
>
> So I'm wondering why you're facing such an issue. Because by the time
> you create cgroup isolated partitions, all kthreads should have performed
> their first wake-up already.
>
> Which kthread did you observe after cgroup setting that didn't perform its
> first wake-up?
>

Thank you for the response. I have performed a more detailed
observation to track the migration behavior of these kthreads over
time.

Setup and Procedure:
Booted with: nohz_full=1-7 rcu_nocbs=1-7 irqaffinity=0 (No isolcpus).
Monitored kthreads on CPUs 1-7 using: ps -eLo cpuid,comm | grep -e
COMM -e "^ *[1-7] " | grep -ve "/[1-7]$" -e "kworker/[1-7]:" -e nvme0q

Initially, there were 30 kthreads residing on CPUs 1-7 right after boot.
Manually created /sys/fs/cgroup/isolated1.slice and configured
cpuset.cpus.exclusive and cpuset.cpus.partition=isolated.

Migration Timeline:
Within 1 minute: rcuog/0 and rcuog/4 migrated back to CPU 0.
At 2 minutes: khungtaskd and jbd2/zram0-8 migrated.
At 8 minutes: kthreadd migrated.
At 17 minutes: pr/legacy migrated. The count dropped to 24 kthreads.
After 9 hours: 24 kthreads remain on CPUs 1-7.

The 24 kthreads remaining on CPUs 1-7 after 9 hours:
CPUID COMMAND
    1 card0-crtc3
    1 ksmd
    1 scsi_eh_4
    1 scsi_eh_9
    3 pool_workqueue_release
    4 card0-crtc0
    4 kdevtmpfs
    4 rcu_exp_gp_kthread_worker
    4 scsi_eh_5
    5 oom_reaper
    5 rcub/0
    5 scsi_eh_0
    5 scsi_eh_1
    5 scsi_eh_2
    5 scsi_eh_8
    6 kswapd0
    6 psimon
    6 scsi_eh_6
    6 watchdogd
    7 card0-crtc1
    7 card0-crtc2
    7 psimon
    7 scsi_eh_3
    7 scsi_eh_7

> Well, domain isolation wants unbound kthreads to move away. And nohz_full
> doesn't make sense without domain isolation. So we should focus on making
> things work for HK_TYPE_DOMAIN.

I understand your point. Then let’s focus on why these kthreads are
not being re-migrated when the housekeeping mask for HK_TYPE_DOMAIN
effectively changes at runtime via cgroups.

Thanks,
Sheviks