Re: [PATCH v2] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/16/26 17:23, Waiman Long wrote:
> On 6/16/26 2:59 AM, David Hildenbrand (Arm) wrote:
>> On 6/16/26 05:43, Waiman Long wrote:
>>> BTW, I still prefer the v2 patch. If it is decided we should use the
>>> guarantee_online_mems() value instead, it will have to be a separate patch with
>>> changes in the relevant documentation like Documentation/admin-guide/cgroup-v1/
>>> cpuset.rst.
>> newmems is "obviously" correct, so I really don't see why we should add
>> something that needs half a page of text to explain why it is fine -- if newmems
>> just does the trick?
>>
>> Please enlighten me.
> 
> Yes, taking newmems is a reasonable choice and there are pros and cons with each
> options. My focus is more on not changing how v1 cpuset behaves as it is well
> defined in the v1 cpusets.rst file:
> 
>     Requests by a task, using the sched_setaffinity(2) system call to
>     include CPUs in its CPU affinity mask, and using the mbind(2) and
>     set_mempolicy(2) system calls to include Memory Nodes in its memory
>     policy, are both filtered through that task's cpuset, filtering out any
>     CPUs or Memory Nodes not in that cpuset.  The scheduler will not
>     schedule a task on a CPU that is not allowed in its cpus_allowed
>     vector, and the kernel page allocator will not allocate a page on a
>     node that is not allowed in the requesting task's mems_allowed vector.
> 
> v2, OTOH, is more vague as to what setting cpuset.mems will mean and we
> generally follow what v1 is doing, but we have more leeway of what we can do.
> 
> Using newmems will make the above text not totally correct. At least the offline
> memory nodes will be filtered out which will not be utilized by the task when
> the offline node becomes online. That is why I am saying that we will have to
> correct the documentation if we want to make this change.

So IIUC:

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1335e437098e..cdfc615f35a5 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2645,7 +2645,13 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
 
                migrate = is_memory_migrate(cs);
 
-               mpol_rebind_mm(mm, &cs->mems_allowed);
+               /*
+                * For v1 we can have empty effective_mems, but we cannot
+                * attach any tasks (see cpuset_can_attach_check()). For v2,
+                * it's guaranteed to not be empty.
+                */
+               VM_WARN_ON_ONCE(nodes_empty(cs->effective_mems));
+               mpol_rebind_mm(mm, &cs->effective_mems);
                if (migrate)
                        cpuset_migrate_mm(mm, &cs->old_mems_allowed, &newmems);
                else


-- 
Cheers,

David