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cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()

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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 mostly be useful for v1
where mems_allowed is the same as effective_mems. With v2, the memory
nodes in effective_mems should be a subset of node_states[N_MEMORY]
except 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
the effective_mems's as cpuset_handle_hotplug() has not been called
from cpuset_track_online_nodes().

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 most of the time. For
v1, mpol_rebind_mm() uses mems_allowed which may differ from what
guarantee_online_mems() returns, but it conforms to what the cpuset v1
documentation says with respect to setting memory policy.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Gregory Price <gourry@gourry.net>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Waiman Long 2026-07-02 17:47:48 -04:00 committed by Tejun Heo
parent 95220e1f18
commit 4d73368514
1 changed files with 18 additions and 12 deletions
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30
kernel/cgroup/cpuset.c
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@ -489,7 +489,10 @@ static void guarantee_active_cpus(struct task_struct *tsk,
* Return in *pmask the portion of a cpusets's mems_allowed that
* are online, with memory. If none are online with memory, walk
* up the cpuset hierarchy until we find one that does have some
* online mems. The top cpuset always has some mems online.
* online mems. The top cpuset always has some mems online. With v2,
* effective_mems should always contain online memory nodes except
* during the transition period where a memory node hotunplug operation
* is in progress.
*
* One way or another, we guarantee to return some non-empty subset
* of node_states[N_MEMORY].
@ -2633,6 +2636,14 @@ 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 return a node mask that is the same
* as the effective_mems of current cpuset.
*/
void cpuset_update_tasks_nodemask(struct cpuset *cs)
{
@ -2641,7 +2652,6 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
struct task_struct *task;
cpuset_being_rebound = cs; /* causes mpol_dup() rebind */
guarantee_online_mems(cs, &newmems);
/*
@ -3159,19 +3169,16 @@ static void cpuset_attach(struct cgroup_taskset *tset)
cpus_updated = !cpumask_equal(cs->effective_cpus,
oldcs->effective_cpus);
mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
/*
* 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) {
cpuset_attach_nodemask_to = cs->effective_mems;
if (cpuset_v2() && !cpus_updated && !mems_updated)
goto out;
}
guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
cgroup_taskset_for_each(task, css, tset)
cpuset_attach_task(cs, task);
@ -3182,7 +3189,6 @@ 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;
@ -3190,7 +3196,7 @@ 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