Re: [PATCH-next v5 1/6] cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()

[Waiman Long <longman@redhat.com>]

On 6/2/26 9:37 AM, Ridong Chen wrote:
>
> On 2026/6/2 10:31, Waiman Long wrote:
>> Whenever memory node mask is changed, there are 4 places where the node
>> mask has to be updated or used.
>>   1) task's node mask via cpuset_change_task_nodemask()
>>   2) memory policy binding via mpol_rebind_mm()
>>   3) if memory migration is enabled, migrate from old_mems_allowed to
>>      the new node mask via cpuset_migrate_mm().
>>   4) setting old_mems_allowed
>>
>> These memory actions are done in cpuset_update_tasks_nodemask() and
>> cpuset_attach(). However there are inconsistencies in what node masks
>> are being used in these 2 functions.
>>
>> In cpuset_update_tasks_nodemask(),
>>   - cpuset_change_task_nodemask(): guarantee_online_mems()
>>   - mpol_rebind_mm(): mems_allowed
>>   - cpuset_migrate_mm(): guarantee_online_mems()
>>   - old_mems_allowed: guarantee_online_mems()
>>
>> In cpuset_attach(),
>>   - cpuset_change_task_nodemask(): guarantee_online_mems()
>>   - mpol_rebind_mm(): effective_mems
>>   - cpuset_migrate_mm(): effective_mems
>>   - old_mems_allowed: effective_mems
>>
>> These inconsistencies dates back to quite a long time ago and it is
>> hard to say what should be the correct values.
>>
>> The guarantee_online_mems() function returns a node mask from current or
>> an ancestor cpuset that is a subset of node_states[N_MEMORY]. Nodes in
>> node_states[N_MEMORY] are all online, i.e. in node_states[N_ONLINE].
>> However, node in node_states[N_ONLINE] may not have memory. So
>> node_states[N_MEMORY] should be a subset of node_states[N_ONLINE].
>>
>> The guarantee_online_mems() function should only be useful for v1 where
>> mems_allowed is the same as effective_mems. With v2, the memory nodes
>> in effective_mems should always be a subset of node_states[N_MEMORY].
>> The only time that may not be true is when a memory hot-unplug operation
>> is in progress and a memory node is removed from node_states[N_MEMORY]
>> but not yet reflected in effective_mems as cpuset_handle_hotplug()
>> has not yet been called from cpuset_track_online_nodes(). When
>> cpuset_handle_hotplug() is called later, the memory node setting
>> of the relevant cpusets and tasks will be updated. So replacing the
>> guarantee_online_mems() call by just using cs->effective_mems should
>> be fine.
>>
> I noticed this pattern in several places:
>
> ```
> if (cpuset_v2())
> 	newmems = cs->effective_mems;
> else
> 	guarantee_online_mems(cs, &newmems);
> ```
>
> Would it be simpler to move the v2 logic into guarantee_online_mems?
>
> ```
> static void guarantee_online_mems(struct cpuset *cs, nodemask_t *pmask)
> {
> 	if (cpuset_v2()) {
> 		*pmask = cs->effective_mems;
> 		return;
> 	}
> 	while (!nodes_and(*pmask, cs->effective_mems, node_states[N_MEMORY]))
> 		cs = parent_cs(cs);
> }
> ```
Yes, it makes sense to put it directly into guarantee_online_mems().
>> Let use the following setup for both of them and make them consistent.
>>   - cpuset_change_task_nodemask(): guarantee_online_mems()
>>   - mpol_rebind_mm(): effective_mems
>>   - cpuset_migrate_mm(): guarantee_online_mems()
>>   - old_mems_allowed: guarantee_online_mems()
>>
>> So for v2, it is effectively all effective_mems. For v1, mpol_rebind_mm()
>> uses cpus_allowed which may differ from what guarantee_online_mems()
> 	^
> mems_allowed?

Thanks for catching it. Will fix that and update your email address in 
the next version.

Cheers,
Longman

>> returns.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 32 +++++++++++++++++++++-----------
>>   1 file changed, 21 insertions(+), 11 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 6bdb68689c24..987456b6d879 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -2616,6 +2616,13 @@ static void *cpuset_being_rebound;
>>    * Iterate through each task of @cs updating its mems_allowed to the
>>    * effective cpuset's.  As this function is called with cpuset_mutex held,
>>    * cpuset membership stays stable.
>> + *
>> + * - cpuset_change_task_nodemask(): guarantee_online_mems()
>> + * - mpol_rebind_mm(): effective_mems
>> + * - cpuset_migrate_mm(): guarantee_online_mems()
>> + * - old_mems_allowed: guarantee_online_mems()
>> + *
>> + * For v2, guarantee_online_mems() should just return effective_mems.
> I agree, but the implementation is not as simple as what I mentioned above.
>
>>    */
>>   void cpuset_update_tasks_nodemask(struct cpuset *cs)
>>   {
>> @@ -2625,7 +2632,10 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
>>   
>>   	cpuset_being_rebound = cs;		/* causes mpol_dup() rebind */
>>   
>> -	guarantee_online_mems(cs, &newmems);
>> +	if (cpuset_v2())
>> +		newmems = cs->effective_mems;
>> +	else
>> +		guarantee_online_mems(cs, &newmems);
>>   
>>   	/*
>>   	 * The mpol_rebind_mm() call takes mmap_lock, which we couldn't
>> @@ -2650,7 +2660,7 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
>>   
>>   		migrate = is_memory_migrate(cs);
>>   
>> -		mpol_rebind_mm(mm, &cs->mems_allowed);
>> +		mpol_rebind_mm(mm, &cs->effective_mems);
>>   		if (migrate)
>>   			cpuset_migrate_mm(mm, &cs->old_mems_allowed, &newmems);
>>   		else
>> @@ -3148,17 +3158,18 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>>   
>>   	/*
>>   	 * In the default hierarchy, enabling cpuset in the child cgroups
>> -	 * will trigger a number of cpuset_attach() calls with no change
>> -	 * in effective cpus and mems. In that case, we can optimize out
>> -	 * by skipping the task iteration and update.
>> +	 * will trigger a cpuset_attach() call with no change in effective cpus
>> +	 * and mems. In that case, we can optimize out by skipping the task
>> +	 * iteration and update.
>>   	 */
>> -	if (cpuset_v2() && !cpus_updated && !mems_updated) {
>> +	if (cpuset_v2()) {
>>   		cpuset_attach_nodemask_to = cs->effective_mems;
>> -		goto out;
>> +		if (!cpus_updated && !mems_updated)
>> +			goto out;
>> +	} else {
>> +		guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
>>   	}
>>   
>> -	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
>> -
>>   	cgroup_taskset_for_each(task, css, tset)
>>   		cpuset_attach_task(cs, task);
>>   
>> @@ -3168,7 +3179,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>>   	 * if there is no change in effective_mems and CS_MEMORY_MIGRATE is
>>   	 * not set.
>>   	 */
>> -	cpuset_attach_nodemask_to = cs->effective_mems;
>>   	if (!is_memory_migrate(cs) && !mems_updated)
>>   		goto out;
>>   
>> @@ -3176,7 +3186,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>>   		struct mm_struct *mm = get_task_mm(leader);
>>   
>>   		if (mm) {
>> -			mpol_rebind_mm(mm, &cpuset_attach_nodemask_to);
>> +			mpol_rebind_mm(mm, &cs->effective_mems);
>>   
>>   			/*
>>   			 * old_mems_allowed is the same with mems_allowed