[PATCH v5 4/6] cgroup/cpuset: Allow non-top parent partition to distribute out all CPUs

Waiman Long
2021-08-14T17:41:53Z

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

To accommodate such use cases, a parent partition can now have all its
CPUs distributed to its child partitions with 0 effective cpu left as
long as it is not the top cpuset and it has no task at the time the
child partition is being created. A terminal partition with no child
partition underlying it, however, cannot have 0 effective cpu which
will make the partition invalid.

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

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/cgroup/cpuset.c | 96 ++++++++++++++++++++++++++++++------------
1 file changed, 69 insertions(+), 27 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index d7b3eb024b84..73eff47a38de 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -305,6 +305,21 @@ static inline void notify_partition_change(struct cpuset *cs,
WRITE_ONCE(cs->prs_err, PERR_NONE);
}

+static inline int cpuset_has_tasks(const struct cpuset *cs)
+{
+ return cs->css.cgroup->nr_populated_csets;
+}
+
+/*
+ * A empty partition (one with no effective cpu) is valid if it has no
+ * associated task and all its cpus have been distributed out to child
+ * partitions.
+ */
+static inline bool valid_empty_partition(const struct cpuset *cs)
+{
+ return !cpuset_has_tasks(cs) && cs->nr_subparts_cpus;
+}
+
static struct cpuset top_cpuset = {
.flags = ((1 << CS_ONLINE) | (1 << CS_CPU_EXCLUSIVE) |
(1 << CS_MEM_EXCLUSIVE)),
@@ -1211,22 +1226,32 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
if ((cmd == partcmd_enable) && 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;
old_prs = new_prs = cpuset->partition_root_state;
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 && cpuset_has_tasks(parent))
+ return -EINVAL;
+
cpumask_copy(tmp->addmask, cpuset->cpus_allowed);
adding = true;
} else if (cmd == partcmd_disable) {
@@ -1257,9 +1282,9 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
parent->subparts_cpus);
/*
* Make partition invalid if parent's effective_cpus could
- * become empty.
+ * become empty and there are tasks in the parent.
*/
- if (adding &&
+ if (adding && cpuset_has_tasks(parent) &&
cpumask_equal(parent->effective_cpus, tmp->addmask)) {
if (!deleting)
part_error = true;
@@ -1294,7 +1319,9 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
parent->effective_cpus);
part_error = (is_partition_root(cpuset) &&
!parent->nr_subparts_cpus) ||
- cpumask_equal(tmp->addmask, parent->effective_cpus);
+ (cpumask_equal(tmp->addmask, parent->effective_cpus) &&
+ cpuset_has_tasks(parent));
+
if (is_partition_root(cpuset) && part_error)
WRITE_ONCE(cpuset->prs_err, PERR_NOCPUS);
}
@@ -1397,9 +1424,15 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp)

/*
* If it becomes empty, inherit the effective mask of the
- * parent, which is guaranteed to have some CPUs.
+ * parent, which is guaranteed to have some CPUs unless
+ * it is a partition root that has explicitly distributed
+ * out all its CPUs.
*/
if (is_in_v2_mode() && cpumask_empty(tmp->new_cpus)) {
+ if (is_partition_root(cp) &&
+ cpumask_equal(cp->cpus_allowed, cp->subparts_cpus))
+ goto update_parent_subparts;
+
cpumask_copy(tmp->new_cpus, parent->effective_cpus);
if (!cp->use_parent_ecpus) {
cp->use_parent_ecpus = true;
@@ -1421,6 +1454,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp)
continue;
}

+update_parent_subparts:
/*
* update_parent_subparts_cpumask() should have been called
* for cs already in update_cpumask(). We should also call
@@ -1497,12 +1531,9 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp)
*/
cpumask_andnot(cp->effective_cpus, cp->effective_cpus,
cp->subparts_cpus);
- WARN_ON_ONCE(cpumask_empty(cp->effective_cpus));
}

- if (new_prs != old_prs)
- cp->partition_root_state = new_prs;
-
+ cp->partition_root_state = new_prs;
spin_unlock_irq(&callback_lock);
notify_partition_change(cp, old_prs, new_prs);

@@ -2254,6 +2285,13 @@ 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)
@@ -3130,7 +3168,8 @@ hotplug_update_tasks(struct cpuset *cs,
struct cpumask *new_cpus, nodemask_t *new_mems,
bool cpus_updated, bool mems_updated)
{
- if (cpumask_empty(new_cpus))
+ /* A partition root is allowed to have empty effective cpus */
+ if (cpumask_empty(new_cpus) && !is_partition_root(cs))
cpumask_copy(new_cpus, parent_cs(cs)->effective_cpus);
if (nodes_empty(*new_mems))
*new_mems = parent_cs(cs)->effective_mems;
@@ -3199,22 +3238,25 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)

/*
* In the unlikely event that a partition root has empty
- * effective_cpus, we will have to force any child partitions,
- * if present, to become invalid by setting nr_subparts_cpus to 0
- * without causing itself to become invalid.
+ * effective_cpus with tasks, we will have to force any child
+ * partitions, if present, to become invalid by setting
+ * nr_subparts_cpus to 0 without causing itself to become invalid.
*/
- if (is_partition_root(cs) && cs->nr_subparts_cpus &&
- cpumask_empty(&new_cpus)) {
+ if (is_partition_root(cs) && cpumask_empty(&new_cpus) &&
+ !valid_empty_partition(cs)) {
cs->nr_subparts_cpus = 0;
cpumask_clear(cs->subparts_cpus);
compute_effective_cpumask(&new_cpus, cs, parent);
}

/*
- * If empty effective_cpus or zero nr_subparts_cpus or its parent
- * becomes erroneous, we have to transition it to the erroneous state.
+ * Force the partition to become invalid if either one of
+ * the following conditions hold:
+ * 1) empty effective cpus but not valid empty partition.
+ * 2) parent is invalid or doesn't grant any cpus to child partitions.
*/
- if (is_partition_root(cs) && (cpumask_empty(&new_cpus) ||
+ if (is_partition_root(cs) &&
+ ((cpumask_empty(&new_cpus) && !valid_empty_partition(cs)) ||
(parent->partition_root_state == PRS_ERROR) ||
!parent->nr_subparts_cpus)) {
int old_prs;
--
2.18.1