Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

[Tejun Heo <tj@kernel.org>]

On Wed, Jul 13, 2016 at 02:21:02PM -0400, Tejun Heo wrote:
> One interesting thing to try would be replacing it with a regular
> non-percpu rwsem and see how it behaves.  That should easily tell us
> whether this is from actual contention or artifacts from percpu_rwsem
> implementation.

So, something like the following.  Can you please see whether this
makes any difference?

Thanks.

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 5b17de6..bc1e4d8 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -14,7 +14,7 @@
 #include <linux/mutex.h>
 #include <linux/rcupdate.h>
 #include <linux/percpu-refcount.h>
-#include <linux/percpu-rwsem.h>
+#include <linux/rwsem.h>
 #include <linux/workqueue.h>
 
 #ifdef CONFIG_CGROUPS
@@ -518,7 +518,7 @@ struct cgroup_subsys {
 	unsigned int depends_on;
 };
 
-extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
+extern struct rw_semaphore cgroup_threadgroup_rwsem;
 
 /**
  * cgroup_threadgroup_change_begin - threadgroup exclusion for cgroups
@@ -529,7 +529,7 @@ extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
  */
 static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
 {
-	percpu_down_read(&cgroup_threadgroup_rwsem);
+	down_read(&cgroup_threadgroup_rwsem);
 }
 
 /**
@@ -541,7 +541,7 @@ static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
  */
 static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
 {
-	percpu_up_read(&cgroup_threadgroup_rwsem);
+	up_read(&cgroup_threadgroup_rwsem);
 }
 
 #else	/* CONFIG_CGROUPS */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 86cb5c6..ed9c142 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -113,7 +113,7 @@ static DEFINE_SPINLOCK(cgroup_file_kn_lock);
  */
 static DEFINE_SPINLOCK(release_agent_path_lock);
 
-struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
+DECLARE_RWSEM(cgroup_threadgroup_rwsem);
 
 #define cgroup_assert_mutex_or_rcu_locked()				\
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&			\
@@ -2899,7 +2899,7 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
 	if (!cgrp)
 		return -ENODEV;
 
-	percpu_down_write(&cgroup_threadgroup_rwsem);
+	down_write(&cgroup_threadgroup_rwsem);
 	rcu_read_lock();
 	if (pid) {
 		tsk = find_task_by_vpid(pid);
@@ -2937,7 +2937,7 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
 out_unlock_rcu:
 	rcu_read_unlock();
 out_unlock_threadgroup:
-	percpu_up_write(&cgroup_threadgroup_rwsem);
+	up_write(&cgroup_threadgroup_rwsem);
 	for_each_subsys(ss, ssid)
 		if (ss->post_attach)
 			ss->post_attach();
@@ -3077,7 +3077,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 
 	lockdep_assert_held(&cgroup_mutex);
 
-	percpu_down_write(&cgroup_threadgroup_rwsem);
+	down_write(&cgroup_threadgroup_rwsem);
 
 	/* look up all csses currently attached to @cgrp's subtree */
 	spin_lock_bh(&css_set_lock);
@@ -3112,7 +3112,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	ret = cgroup_taskset_migrate(&tset, cgrp->root);
 out_finish:
 	cgroup_migrate_finish(&preloaded_csets);
-	percpu_up_write(&cgroup_threadgroup_rwsem);
+	up_write(&cgroup_threadgroup_rwsem);
 	return ret;
 }
 
@@ -5601,7 +5601,7 @@ int __init cgroup_init(void)
 	int ssid;
 
 	BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
-	BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
+	//BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));