Re: [PATCH 1/3] cgroup/cpuset: Fix deadline bandwidth leak in cpuset_can_attach()

[Aaron Tomlin]

On Mon, May 11, 2026 at 01:10:02AM -0400, Waiman Long wrote:
> 
> On 5/9/26 12:48 PM, Aaron Tomlin wrote:
> > During a cgroup migration, cpuset_can_attach() iterates over the
> > provided taskset. If a task within the batch is a deadline (DL) task,
> > the destination cpuset's DL metrics (i.e., nr_migrate_dl_tasks and
> > sum_migrate_dl_bw) are appropriately incremented.
> > 
> > However, if a subsequent task in the same migration batch fails the
> > task_can_attach() check, the loop aborts and jumps directly to
> > out_unlock. Consequently, any DL metrics accumulated from previously
> > processed tasks in the batch remain permanently inflated in the
> > destination cpuset. Because the migration is subsequently aborted by the
> > cgroup core, cpuset_cancel_attach() is never invoked to unwind these
> > specific increments.
> > 
> > This behaviour results in a permanent leak of deadline bandwidth, which
> > incorrectly restricts the admission control capacity of the destination
> > cpuset.
> > 
> > To resolve this, introduce an out_unlock_reset failure path that
> > conditionally invokes reset_migrate_dl_data(). This guarantees that if a
> > batch migration is aborted for any reason, the pending DL metrics are
> > safely reset before returning the error.
> > 
> > Fixes: 0a67b847e1f06 ("cpuset: Allow setscheduler regardless of manipulated task")
> 
> That is not the commit that introduced the bug. Anyway, there is already
> another patch sent recently to fix this bug. See
> 
> https://lore.kernel.org/lkml/20260509102031.97608-2-zhangguopeng@xxxxxxxxxx/
> 
Hi Waiman,

Thank you for the follow up.

Acknowledged. I will drop this patch in the next iteration due to [1].

Please note, the sashiko AI Review bot reported: cpuset_can_attach()
incorrectly assumes all migrating tasks originate from the same source
cpuset. At first glance, this feedback is valid. I plan to submit a patch,
if no solution was already proposed.

[1]: https://lore.kernel.org/lkml/20260509102031.97608-2-zhangguopeng@xxxxxxxxxx/

Kind regards,
-- 
Aaron Tomlin

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