[PATCH] cpuset: Make cpuset hotplug synchronous

[Prateek Sood <prsood-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

Hi Tejun & Peter,

It seems that after following patch we can make cpuset_hotplug_workfn
synchronous:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/cgroup/cpuset.c?h=v5.5-rc7&id=d74b27d63a8bebe2fe634944e4ebdc7b10db7a39


Could you please share your opinion on the same and below patch. 

Thanks,
Prateek

>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8

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-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
---
 include/linux/cpuset.h |  3 ---
 kernel/cgroup/cpuset.c | 31 +++++++++++++++++++------------
 kernel/power/process.c |  2 --
 3 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 04c20de66..cede4cb 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -54,7 +54,6 @@ static inline void cpuset_dec(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_read_lock(void);
 extern void cpuset_read_unlock(void);
 extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
@@ -176,8 +175,6 @@ 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_read_lock(void) { }
 static inline void cpuset_read_unlock(void) { }
 
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 58f5073..cafd4d2 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3101,7 +3101,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 }
 
 /**
- * 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
@@ -3116,7 +3116,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
  * 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;
@@ -3201,25 +3201,32 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 	/* rebuild sched domains if cpus_allowed has changed */
 	if (cpus_updated || force_rebuild) {
 		force_rebuild = false;
-		rebuild_sched_domains();
+		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.
+			*/
+			percpu_down_write(&cpuset_rwsem);
+			rebuild_sched_domains_locked();
+			percpu_up_write(&cpuset_rwsem);
+		}
 	}
 
 	free_cpumasks(NULL, ptmp);
 }
 
-void cpuset_update_active_cpus(void)
+static void cpuset_hotplug_workfn(struct work_struct *work)
 {
-	/*
-	 * 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);
+	cpuset_hotplug(true);
 }
 
-void cpuset_wait_for_hotplug(void)
+void cpuset_update_active_cpus(void)
 {
-	flush_work(&cpuset_hotplug_work);
+	cpuset_hotplug(false);
 }
 
 /*
diff --git a/kernel/power/process.c b/kernel/power/process.c
index 4b6a54d..08f7019 100644
--- a/kernel/power/process.c
+++ b/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 */
-- 
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