cpuset: make CPU / memory hotplug propagation asynchronous

cpuset_hotplug_workfn() has been invoking cpuset_propagate_hotplug()
directly to propagate hotplug updates to !root cpusets; however, this
has the following problems.

* cpuset locking is scheduled to be decoupled from cgroup_mutex,
  cgroup_mutex will be unexported, and cgroup_attach_task() will do
  cgroup locking internally, so propagation can't synchronously move
  tasks to a parent cgroup while walking the hierarchy.

* We can't use cgroup generic tree iterator because propagation to
  each cpuset may sleep.  With propagation done asynchronously, we can
  lose the rather ugly cpuset specific iteration.

Convert cpuset_propagate_hotplug() to
cpuset_propagate_hotplug_workfn() and execute it from newly added
cpuset->hotplug_work.  The work items are run on an ordered workqueue,
so the propagation order is preserved.  cpuset_hotplug_workfn()
schedules all propagations while holding cgroup_mutex and waits for
completion without cgroup_mutex.  Each in-flight propagation holds a
reference to the cpuset->css.

This patch doesn't cause any functional difference.

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

1 files changed, 48 insertions, 6 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 74e412f908db..a7bb547786d7 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -99,6 +99,8 @@ struct cpuset {
 
         /* used for walking a cpuset hierarchy */
         struct list_head stack_list;
+
+        struct work_struct hotplug_work;
 };
 
 /* Retrieve the cpuset for a cgroup */
@@ -254,7 +256,10 @@ static DEFINE_SPINLOCK(cpuset_buffer_lock);
 /*
  * CPU / memory hotplug is handled asynchronously.
  */
+static struct workqueue_struct *cpuset_propagate_hotplug_wq;
+
 static void cpuset_hotplug_workfn(struct work_struct *work);
+static void cpuset_propagate_hotplug_workfn(struct work_struct *work);
 
 static DECLARE_WORK(cpuset_hotplug_work, cpuset_hotplug_workfn);
 
@@ -1808,6 +1813,7 @@ static struct cgroup_subsys_state *cpuset_css_alloc(struct cgroup *cont)
         cpumask_clear(cs->cpus_allowed);
         nodes_clear(cs->mems_allowed);
         fmeter_init(&cs->fmeter);
+        INIT_WORK(&cs->hotplug_work, cpuset_propagate_hotplug_workfn);
         cs->relax_domain_level = -1;
         cs->parent = cgroup_cs(cont->parent);
 
@@ -2033,21 +2039,20 @@ static struct cpuset *cpuset_next(struct list_head *queue)
 }
 
 /**
- * cpuset_propagate_hotplug - propagate CPU/memory hotplug to a cpuset
+ * cpuset_propagate_hotplug_workfn - propagate CPU/memory hotplug to a cpuset
  * @cs: cpuset in interest
  *
  * Compare @cs's cpu and mem masks against top_cpuset and if some have gone
  * offline, update @cs accordingly.  If @cs ends up with no CPU or memory,
  * all its tasks are moved to the nearest ancestor with both resources.
- *
- * Should be called with cgroup_mutex held.
  */
-static void cpuset_propagate_hotplug(struct cpuset *cs)
+static void cpuset_propagate_hotplug_workfn(struct work_struct *work)
 {
         static cpumask_t off_cpus;
         static nodemask_t off_mems, tmp_mems;
+        struct cpuset *cs = container_of(work, struct cpuset, hotplug_work);
 
-        WARN_ON_ONCE(!cgroup_lock_is_held());
+        cgroup_lock();
 
         cpumask_andnot(&off_cpus, cs->cpus_allowed, top_cpuset.cpus_allowed);
         nodes_andnot(off_mems, cs->mems_allowed, top_cpuset.mems_allowed);
@@ -2071,6 +2076,36 @@ static void cpuset_propagate_hotplug(struct cpuset *cs)
 
         if (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed))
                 remove_tasks_in_empty_cpuset(cs);
+
+        cgroup_unlock();
+
+        /* the following may free @cs, should be the last operation */
+        css_put(&cs->css);
+}
+
+/**
+ * schedule_cpuset_propagate_hotplug - schedule hotplug propagation to a cpuset
+ * @cs: cpuset of interest
+ *
+ * Schedule cpuset_propagate_hotplug_workfn() which will update CPU and
+ * memory masks according to top_cpuset.
+ */
+static void schedule_cpuset_propagate_hotplug(struct cpuset *cs)
+{
+        /*
+         * Pin @cs.  The refcnt will be released when the work item
+         * finishes executing.
+         */
+        if (!css_tryget(&cs->css))
+                return;
+
+        /*
+         * Queue @cs->hotplug_work.  If already pending, lose the css ref.
+         * cpuset_propagate_hotplug_wq is ordered and propagation will
+         * happen in the order this function is called.
+         */
+        if (!queue_work(cpuset_propagate_hotplug_wq, &cs->hotplug_work))
+                css_put(&cs->css);
 }
 
 /**
@@ -2135,11 +2170,14 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
                 list_add_tail(&top_cpuset.stack_list, &queue);
                 while ((cs = cpuset_next(&queue)))
                         if (cs != &top_cpuset)
-                                cpuset_propagate_hotplug(cs);
+                                schedule_cpuset_propagate_hotplug(cs);
         }
 
         cgroup_unlock();
 
+        /* wait for propagations to finish */
+        flush_workqueue(cpuset_propagate_hotplug_wq);
+
         /* rebuild sched domains if cpus_allowed has changed */
         if (cpus_updated) {
                 struct sched_domain_attr *attr;
@@ -2196,6 +2234,10 @@ void __init cpuset_init_smp(void)
         top_cpuset.mems_allowed = node_states[N_MEMORY];
 
         hotplug_memory_notifier(cpuset_track_online_nodes, 10);
+
+        cpuset_propagate_hotplug_wq =
+                alloc_ordered_workqueue("cpuset_hotplug", 0);
+        BUG_ON(!cpuset_propagate_hotplug_wq);
 }
 
 /**