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

[Waiman Long <longman@redhat.com>]

On 5/20/26 4:29 AM, Ridong Chen wrote:
>
>
> 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.
DL task accounting doesn't use the new oldcs stored in the task 
structure which is only used for mm migration. BTW, I believe 
task_cs(task) doesn't return the old cs in cpuset_attach().
>
> 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?

Yes, that makes sense. I will rework the patch series.

Thanks,
Longman

>
>
>
>> 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(-)
>>
>