Re: [PATCH cgroup/for-next v2 0/5] cgroup/cpuset: Support multiple source/destination cpusets for cpuset_*attach()

[Ridong Chen <ridong.chen@linux.dev>]

On 2026/5/16 12:24, Waiman Long wrote:
> Sashiko AI review of another cpuset patch had found that cpuset_attach()
> and cpuset_can_attach() can be passed a cgroup_taskset with tasks
> migrating from one source cpuset to multiple destination cpusets and
> vice versa.  Further testing of the cpuset code indicates that this is
> indeed the case when the v2 cpuset controller is enabled or disabled.
> 
> Unfortunately, cpuset_attach() and cpuset_can_attach() still assume that
> there will be one source and one destinaton cpuset which may result in
> inocrrect behavior.
> 

Hi Longman,

I am thinking whether we can use the pids subsystem's approach to solve 
this issue, which I think could be much simpler.

For the DL task accounting, we can handle it the same way 
pids_can_attach() does - just call task_cs(task) for each task 
individually inside the can_attach() loop and do the nr_deadline_tasks 
adjustment right there. This eliminates the need to pass per-task source 
cpuset information to the attach() callback entirely for DL accounting 
purposes.

For cpuset_migrate_mm(), I don't think we need per-task oldcs storage in 
task_struct either. The scenarios where multiple source cpusets are 
involved are:

enable cpuset controller: child cpusets inherit parent's effective_mems, 
so attach_mems_updated is false and cpuset_migrate_mm() is never called.

disable cpuset controller: tasks move from children to parent. Since 
children's effective_mems is always a subset of parent's effective_mems, 
even if cpuset_migrate_mm() is triggered, it's effectively a noop (no 
pages need to move from a subset to its superset).

cgroup.procs write with threads in different cpusets: this is a 
many-to-one migration with a single process, so there is only one 
group_leader and one mm. We only need to record the leader's oldcs, 
which a single static variable can handle.

So in all cases, the migration path only needs one oldcs for the leader. 
We don't need to add a field to task_struct.

What do you think?



> This patch series is created to fix this issue. The first 2 patches are
> just preparatory patches to make the remaining patches easier to review.
> 
> Patch 3 adds a new attach_old_cs field into task_struct to track the
> old cpuset to be used in case when cpuset_migrate_mm() needs to be
> called in cpuset_attach().
> 
> Patch 4 moves mpol_rebind_mm() and cpuset_migrate_mm() inside
> cpuset_attach_task() to make CLONE_INTO_CGROUP flag of clone(2) works
> more like moving task from one cpuset to another one, while also make
> supporting multiple source and destination cpusets easier.
> 
> Patch 5 makes the necessary changes to enable the support of multiple
> source and destination cpusets by keeping all the source and destination
> cpusets found during task iterations in two singly linked lists for
> source and destination cpusets respectively.
> 
> Waiman Long (5):
>    cgroup/cpuset: Add a cpuset_reserve_dl_bw() helper
>    cgroup/cpuset: Expand the scope of cpuset_can_attach_check()
>    cgroup/cpuset: Replace cpuset_attach_old_cs by a new attach_old_cs
>      field in task_struct
>    cgroup/cpuset: Move mpol_rebind_mm/cpuset_migrate_mm() calls inside
>      cpuset_attach_task()
>    cgroup/cpuset: Support multiple source/destination cpusets for
>      cpuset_*attach()
> 
>   include/linux/sched.h           |   3 +
>   kernel/cgroup/cpuset-internal.h |   6 +
>   kernel/cgroup/cpuset.c          | 358 +++++++++++++++++++++-----------
>   3 files changed, 249 insertions(+), 118 deletions(-)
> 

-- 
Best regards,
Ridong