linux-kernel - Re: [RFC, PATCH] CLONE_NEWIPC and exit

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Date:	Thu, 12 Jul 2012 18:07:02 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Doug Ledford <dledford@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-kernel@...r.kernel.org, "Dmitry V. Levin" <ldv@...linux.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [RFC, PATCH] CLONE_NEWIPC and exit_group()

On Wed, Jul 11, 2012 at 03:24:22PM -0700, Andrew Morton wrote:
> Am I reading that right?  1000 forks take 33 seconds, with basically
> all of it just sitting there asleep?  This look quite terrible - what
> causes this?

It seems free_nsproxy() + synchronize_rcu() are too heavy to be in
exit_group() path. Patch below helps: 8s -> ~0.5s for me.
I'll prepare cleaner patch later if the approach is good enough.

Also, I noticed that put_nsproxy() calls free_nsproxy() without
synchronize_rcu(). It looks wrong.

diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index cc37a55..1d26be7 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -24,6 +24,7 @@ struct fs_struct;
  */
 struct nsproxy {
 	atomic_t count;
+	struct work_struct free_nsproxy_work;
 	struct uts_namespace *uts_ns;
 	struct ipc_namespace *ipc_ns;
 	struct mnt_namespace *mnt_ns;
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index b576f7f..63618cc 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -41,13 +41,17 @@ struct nsproxy init_nsproxy = {
 #endif
 };
 
+static void free_nsproxy_synced(struct work_struct *work);
+
 static inline struct nsproxy *create_nsproxy(void)
 {
 	struct nsproxy *nsproxy;
 
 	nsproxy = kmem_cache_alloc(nsproxy_cachep, GFP_KERNEL);
-	if (nsproxy)
+	if (nsproxy) {
 		atomic_set(&nsproxy->count, 1);
+		INIT_WORK(&nsproxy->free_nsproxy_work, free_nsproxy_synced);
+	}
 	return nsproxy;
 }
 
@@ -178,6 +182,23 @@ void free_nsproxy(struct nsproxy *ns)
 	kmem_cache_free(nsproxy_cachep, ns);
 }
 
+static void free_nsproxy_synced(struct work_struct *work)
+{
+	struct nsproxy *ns = container_of(work, struct nsproxy,
+			free_nsproxy_work);
+
+	/*
+	 * wait for others to get what they want from this nsproxy.
+	 *
+	 * cannot release this nsproxy via the call_rcu() since
+	 * put_mnt_ns() will want to sleep
+	 */
+	synchronize_rcu();
+
+	free_nsproxy(ns);
+}
+
+
 /*
  * Called from unshare. Unshare all the namespaces part of nsproxy.
  * On success, returns the new nsproxy.
@@ -215,16 +236,8 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 
 	rcu_assign_pointer(p->nsproxy, new);
 
-	if (ns && atomic_dec_and_test(&ns->count)) {
-		/*
-		 * wait for others to get what they want from this nsproxy.
-		 *
-		 * cannot release this nsproxy via the call_rcu() since
-		 * put_mnt_ns() will want to sleep
-		 */
-		synchronize_rcu();
-		free_nsproxy(ns);
-	}
+	if (ns && atomic_dec_and_test(&ns->count))
+		schedule_work(&ns->free_nsproxy_work);
 }
 
 void exit_task_namespaces(struct task_struct *p)
-- 
 Kirill A. Shutemov

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