Re: [RFC, PATCH] CLONE_NEWIPC and exit_group()

[Kirill A. Shutemov]

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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