cpuset: inherit ancestor's masks if effective_{cpus, mems} becomes empty

We're going to have separate user-configured masks and effective ones.

Eventually configured masks can only be changed by writing cpuset.cpus
and cpuset.mems, and they won't be restricted by parent cpuset. While
effective masks reflect cpu/memory hotplug and hierachical restriction,
and these are the real masks that apply to the tasks in the cpuset.

We calculate effective mask this way:
  - top cpuset's effective_mask == online_mask, otherwise
  - cpuset's effective_mask == configured_mask & parent effective_mask,
    if the result is empty, it inherits parent effective mask.

Those behavior changes are for default hierarchy only. For legacy
hierarchy, effective_mask and configured_mask are the same, so we won't
break old interfaces.

To make cs->effective_{cpus,mems} to be effective masks, we need to
  - update the effective masks at hotplug
  - update the effective masks at config change
  - take on ancestor's mask when the effective mask is empty

The last item is done here.

This won't introduce behavior change.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index da766c3736c4..f8340026d01c 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -877,6 +877,13 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
 
                 cpumask_and(new_cpus, cp->cpus_allowed, parent->effective_cpus);
 
+                /*
+                 * If it becomes empty, inherit the effective mask of the
+                 * parent, which is guaranteed to have some CPUs.
+                 */
+                if (cpumask_empty(new_cpus))
+                        cpumask_copy(new_cpus, parent->effective_cpus);
+
                 /* Skip the whole subtree if the cpumask remains the same. */
                 if (cpumask_equal(new_cpus, cp->effective_cpus)) {
                         pos_css = css_rightmost_descendant(pos_css);
@@ -1123,6 +1130,13 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
 
                 nodes_and(*new_mems, cp->mems_allowed, parent->effective_mems);
 
+                /*
+                 * If it becomes empty, inherit the effective mask of the
+                 * parent, which is guaranteed to have some MEMs.
+                 */
+                if (nodes_empty(*new_mems))
+                        *new_mems = parent->effective_mems;
+
                 /* Skip the whole subtree if the nodemask remains the same. */
                 if (nodes_equal(*new_mems, cp->effective_mems)) {
                         pos_css = css_rightmost_descendant(pos_css);
@@ -2102,7 +2116,11 @@ retry:
 
         mutex_lock(&callback_mutex);
         cpumask_andnot(cs->cpus_allowed, cs->cpus_allowed, &off_cpus);
+
+        /* Inherit the effective mask of the parent, if it becomes empty. */
         cpumask_andnot(cs->effective_cpus, cs->effective_cpus, &off_cpus);
+        if (on_dfl && cpumask_empty(cs->effective_cpus))
+                cpumask_copy(cs->effective_cpus, parent_cs(cs)->effective_cpus);
         mutex_unlock(&callback_mutex);
 
         /*
@@ -2117,7 +2135,11 @@ retry:
 
         mutex_lock(&callback_mutex);
         nodes_andnot(cs->mems_allowed, cs->mems_allowed, off_mems);
+
+        /* Inherit the effective mask of the parent, if it becomes empty */
         nodes_andnot(cs->effective_mems, cs->effective_mems, off_mems);
+        if (on_dfl && nodes_empty(cs->effective_mems))
+                cs->effective_mems = parent_cs(cs)->effective_mems;
         mutex_unlock(&callback_mutex);
 
         /*