Re: sched/deadline: total_bw leak when a task leaves SCHED_DEADLINE while not queued

[Tejun Heo <tj@kernel.org>]

Hello, Juri.

On Fri, Oct 30, 2026 at 10:22:43AM +0100, Juri Lelli wrote:
> Does the below help?

Yes, it fixes the leak. On abf8c33470f7 with the patch, the reproducer leaks
0 in all three cases, and 60s of random policy churn ends with total_bw back
at its starting value. Two problems though.

sched_setattr(SCHED_OTHER) with SCHED_FLAG_KEEP_PARAMS on a sleeping
DEADLINE task leaves it DEADLINE, but each call releases its bandwidth, and
its exit releases it again. total_bw relative to before the task became
DEADLINE, with a 2ms/10ms task (0.2 CPU = 209715):

  after the DEADLINE set        +209715
  after the 1st KEEP_PARAMS call      0
  after the 2nd                 -209715
  after the 3rd                 -419430
  after the task exits          -629145

After that, sched_setattr(SCHED_DEADLINE) for a new 2ms/10ms task fails with
-EBUSY while a 3ms/10ms one is admitted, and offlining a CPU fails with
-EBUSY.

Switching a sleeping DEADLINE task to SCHED_OTHER after its CPU was offlined
crashes in the __dl_sub() the patch adds, dividing by
dl_bw_cpus(task_cpu(p)), which is 0 for that root domain:

  Oops: divide error: 0000 [#1] SMP NOPTI
  RIP: 0010:switched_from_dl+0x13a/0x3e0

Unpatched mainline only leaks in this case.

Thanks.

--
tejun