cpuset: Make cpuset hotplug synchronous

Convert cpuset_hotplug_workfn() into synchronous call for cpu hotplug
path. For memory hotplug path it still gets queued as a work item.

Since cpuset_hotplug_workfn() can be made synchronous for cpu hotplug
path, it is not required to wait for cpuset hotplug while thawing
processes.

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cpuset.h

@@ -52,9 +52,7 @@ static inline void cpuset_dec(void)
 
 extern int cpuset_init(void);
 extern void cpuset_init_smp(void);
-extern void cpuset_force_rebuild(void);
 extern void cpuset_update_active_cpus(void);
-extern void cpuset_wait_for_hotplug(void);
 extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
 extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
 extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
@@ -167,15 +165,11 @@ static inline bool cpusets_enabled(void) { return false; }
 static inline int cpuset_init(void) { return 0; }
 static inline void cpuset_init_smp(void) {}
 
-static inline void cpuset_force_rebuild(void) { }
-
 static inline void cpuset_update_active_cpus(void)
 {
 	partition_sched_domains(1, NULL, NULL);
 }
 
-static inline void cpuset_wait_for_hotplug(void) { }
-
 static inline void cpuset_cpus_allowed(struct task_struct *p,
 				       struct cpumask *mask)
 {

kernel/cgroup/cpuset.c

@@ -2277,15 +2277,8 @@ retry:
 	mutex_unlock(&cpuset_mutex);
 }
 
-static bool force_rebuild;
-
-void cpuset_force_rebuild(void)
-{
-	force_rebuild = true;
-}
-
 /**
- * cpuset_hotplug_workfn - handle CPU/memory hotunplug for a cpuset
+ * cpuset_hotplug - handle CPU/memory hotunplug for a cpuset
  *
  * This function is called after either CPU or memory configuration has
  * changed and updates cpuset accordingly.  The top_cpuset is always
@@ -2300,7 +2293,7 @@ void cpuset_force_rebuild(void)
  * Note that CPU offlining during suspend is ignored.  We don't modify
  * cpusets across suspend/resume cycles at all.
  */
-static void cpuset_hotplug_workfn(struct work_struct *work)
+static void cpuset_hotplug(bool use_cpu_hp_lock)
 {
 	static cpumask_t new_cpus;
 	static nodemask_t new_mems;
@@ -2358,25 +2351,31 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 	}
 
 	/* rebuild sched domains if cpus_allowed has changed */
-	if (cpus_updated || force_rebuild) {
+	if (cpus_updated) {
-		force_rebuild = false;
+		if (use_cpu_hp_lock)
 			rebuild_sched_domains();
+		else {
+			/* Acquiring cpu_hotplug_lock is not required.
+			 * When cpuset_hotplug() is called in hotplug path,
+			 * cpu_hotplug_lock is held by the hotplug context
+			 * which is waiting for cpuhp_thread_fun to indicate
+			 * completion of callback.
+			 */
+			mutex_lock(&cpuset_mutex);
+			rebuild_sched_domains_cpuslocked();
+			mutex_unlock(&cpuset_mutex);
 		}
 	}
+}
+
+static void cpuset_hotplug_workfn(struct work_struct *work)
+{
+	cpuset_hotplug(true);
+}
 
 void cpuset_update_active_cpus(void)
 {
-	/*
+	cpuset_hotplug(false);
-	 * We're inside cpu hotplug critical region which usually nests
-	 * inside cgroup synchronization.  Bounce actual hotplug processing
-	 * to a work item to avoid reverse locking order.
-	 */
-	schedule_work(&cpuset_hotplug_work);
-}
-
-void cpuset_wait_for_hotplug(void)
-{
-	flush_work(&cpuset_hotplug_work);
 }
 
 /*

kernel/power/process.c

@@ -204,8 +204,6 @@ void thaw_processes(void)
 	__usermodehelper_set_disable_depth(UMH_FREEZING);
 	thaw_workqueues();
 
-	cpuset_wait_for_hotplug();
-
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, p) {
 		/* No other threads should have PF_SUSPEND_TASK set */

kernel/sched/core.c

@@ -5624,7 +5624,6 @@ static void cpuset_cpu_active(void)
 		 * restore the original sched domains by considering the
 		 * cpuset configurations.
 		 */
-		cpuset_force_rebuild();
 	}
 	cpuset_update_active_cpus();
 }