[PATCH 3/5] cgroup/cpuset: Allow non-top parent partition root to distribute out all CPUs

Waiman Long
2021-06-03T21:25:08Z

Currently, a parent partition root cannot distribute all its CPUs to
child partition roots with no CPUs left. However in some use cases,
a management application may want to create a parent partition root as
a management unit with no task associated with it and has all its CPUs
distributed to various child partition roots dynamically according to
their needs. Leaving a cpu in the parent partition root in such a case is
now a waste.

To accommodate such use cases, a parent partition root can now have
all its CPUs distributed to its child partition roots as long as:
1) it is not the top cpuset; and
2) there is no task directly associated with the parent.

Once an empty parent partition root is formed, no new task can be moved
into it.

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/cgroup/cpuset.c | 44 +++++++++++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 78dd6c91dcd6..ef19eb317fef 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1117,7 +1117,7 @@ enum subparts_cmd {
* cpus_allowed can be granted or an error code will be returned.
*
* For partcmd_disable, the cpuset is being transofrmed from a partition
- * root back to a non-partition root. any CPUs in cpus_allowed that are in
+ * root back to a non-partition root. Any CPUs in cpus_allowed that are in
* parent's subparts_cpus will be taken away from that cpumask and put back
* into parent's effective_cpus. 0 should always be returned.
*
@@ -1172,21 +1172,31 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
if ((cmd != partcmd_update) && css_has_online_children(&cpuset->css))
return -EBUSY;

- /*
- * Enabling partition root is not allowed if not all the CPUs
- * can be granted from parent's effective_cpus or at least one
- * CPU will be left after that.
- */
- if ((cmd == partcmd_enable) &&
- (!cpumask_subset(cpuset->cpus_allowed, parent->effective_cpus) ||
- cpumask_equal(cpuset->cpus_allowed, parent->effective_cpus)))
- return -EINVAL;
-
/*
* A cpumask update cannot make parent's effective_cpus become empty.
*/
adding = deleting = false;
if (cmd == partcmd_enable) {
+ bool parent_is_top_cpuset = !parent_cs(parent);
+ bool no_cpu_in_parent = cpumask_equal(cpuset->cpus_allowed,
+ parent->effective_cpus);
+ /*
+ * Enabling partition root is not allowed if not all the CPUs
+ * can be granted from parent's effective_cpus. If the parent
+ * is the top cpuset, at least one CPU must be left after that.
+ */
+ if (!cpumask_subset(cpuset->cpus_allowed, parent->effective_cpus) ||
+ (parent_is_top_cpuset && no_cpu_in_parent))
+ return -EINVAL;
+
+ /*
+ * A non-top parent can be left with no CPU as long as there
+ * is no task directly associated with the parent. For such
+ * a parent, no new task can be moved into it.
+ */
+ if (no_cpu_in_parent && parent->css.cgroup->nr_populated_csets)
+ return -EINVAL;
+
cpumask_copy(tmp->addmask, cpuset->cpus_allowed);
adding = true;
} else if (cmd == partcmd_disable) {
@@ -1208,9 +1218,10 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
adding = cpumask_andnot(tmp->addmask, tmp->addmask,
parent->subparts_cpus);
/*
- * Return error if the new effective_cpus could become empty.
+ * Return error if the new effective_cpus could become empty
+ * and there are tasks in the parent.
*/
- if (adding &&
+ if (adding && parent->css.cgroup->nr_populated_csets &&
cpumask_equal(parent->effective_cpus, tmp->addmask)) {
if (!deleting)
return -EINVAL;
@@ -2181,6 +2192,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
(cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
goto out_unlock;

+ /*
+ * On default hierarchy, task cannot be moved to a cpuset with empty
+ * effective cpus.
+ */
+ if (is_in_v2_mode() && cpumask_empty(cs->effective_cpus))
+ goto out_unlock;
+
cgroup_taskset_for_each(task, css, tset) {
ret = task_can_attach(task, cs->cpus_allowed);
if (ret)
--
2.18.1