[QUESTION] sched/fair: EEVDF min_slice stalls in parent cgroup with a continuously running child task

[Chen Jinghuang]

Hi all,

I observed an unexpected behavior regarding the EEVDF min_slice update
mechanism in a hierarchical cgroup v1 setup. It appears that the parent
cgroup's min_slice can become stale when a child cgroup contains a
continuously running task (100% load, no sleep). The parent's min_slice
fails to update when global or entity slices change, until a dequeue/enqueue
event occurs in the child cgroup.

Here is the topology of the scenario:

    Root cfs_rq
        |
    cgroup A  (se A, contains cpu_sim_A)
        |
    cgroup A1 (se A1, contains cpu_sim_A1, 100% CPU load)

Steps to reproduce:
1. Create cgroup A, and a sub-cgroup A1 under A.
2. Move a task (cpu_sim_A) into cgroup A, and use the syscall
   __NR_sched_setattr to explicitly set its slice to 3ms.
3. Set the global base_slice_ns to 3ms:
   echo 3000000 > /sys/kernel/debug/sched/base_slice_ns
4. Move a 100% load task (cpu_sim_A1, which never voluntary sleeps) into
   cgroup A1.
   At this point, both cgroup A's min_slice and cgroup A1's min_slice are
   observed as 0.1ms (the initialized or previous low value).
5. Change the global base_slice_ns to 2.8ms:
   echo 2800000 > /sys/kernel/debug/sched/base_slice_ns

Observations:
- cgroup A1's min_slice correctly updates to 2.8ms.
- However, cgroup A's min_slice remains unchanged (stuck at 0.1ms).
- Even if we use syscall(__NR_sched_setattr) to explicitly set cpu_sim_A's
  slice in cgroup A to 2.8ms, cgroup A's min_slice still does not update.
- The only way to force cgroup A's min_slice to update to 2.8ms is to
  trigger a dequeue/enqueue cycle for the task in cgroup A1 (e.g., by
  renice it)

It appears that a parent cgroup's min_slice is only updated when its
children are enqueued or dequeued. If a child task runs continuously
without sleeping, the parent's min_slice gets stuck and ignores any
changes to base_slice_ns or individual entity slices.

Is this expected by design in EEVDF, or is there a missing update hook?

Any insights would be greatly appreciated.

Thanks,
Chen Jinghuang