Re: [PATCH-next v5 1/6] cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()
From: Waiman Long
Date: Tue Jun 02 2026 - 14:44:03 EST
On 6/2/26 9:37 AM, Ridong Chen wrote:
Yes, it makes sense to put it directly into guarantee_online_mems().
On 2026/6/2 10:31, Waiman Long wrote:
Whenever memory node mask is changed, there are 4 places where the nodeI noticed this pattern in several places:
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.
```
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);
}
```
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.I agree, but the implementation is not as simple as what I mentioned above.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
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.
*/
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