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SubjectRe: [PATCH v2] ns: do not block exit_task_namespaces() for a long time
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On Mon, 2012-07-16 at 18:09 +0300, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> On exiting of the last task in a namespace we need to trigger freeing of
> the namespace. Currently, we call synchronize_rcu() and free_nsproxy()
> directly on do_exit() path.
>
> On my machine synchronize_rcu() blocks for about 0.01 seconds. For
> comparing: normal exit_group() syscall takes less than 0.0003 seconds.
>
> Let's offload synchronize_rcu() and free_nsproxy() to a workqueue.
>
> I also move synchronize_rcu() inside free_nsproxy(). It fixes racy
> put_nsproxy() which calls free_nsproxy() without synchronize_rcu().
> I guess it was missed during switch to RCU (see cf7b708).
>
> Microbenchmark:
>
> : #define _GNU_SOURCE
> : #include <unistd.h>
> : #include <sched.h>
> : #include <stdlib.h>
> : #include <sys/wait.h>
> :
> : int
> : main(void)
> : {
> : int i;
> : for (i = 0; i < 1024; i++) {
> : if (fork()) {
> : wait(NULL);
> : continue;
> : }
> : unshare(CLONE_NEWIPC);
> : exit(0);
> : }
> : return 0;
> : }
>
> Before the patch:
>
> real 0m8.335s
> user 0m0.000s
> sys 0m0.265s
>
> After:
>
> real 0m0.569s
> user 0m0.001s
> sys 0m0.154s
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
> ---
>
> v2:
> - Updated description.
>
> ---
> include/linux/nsproxy.h | 1 +
> kernel/nsproxy.c | 34 +++++++++++++++++++++++-----------
> 2 files changed, 24 insertions(+), 11 deletions(-)
>
> 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..ebc7d40 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -41,13 +41,17 @@ struct nsproxy init_nsproxy = {
> #endif
> };
>
> +static void free_nsproxy_work(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_work);
> + }
> return nsproxy;
> }
>
> @@ -166,6 +170,14 @@ out:
>
> void free_nsproxy(struct nsproxy *ns)
> {
> + /*
> + * 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();
> +
> if (ns->mnt_ns)
> put_mnt_ns(ns->mnt_ns);
> if (ns->uts_ns)
> @@ -178,6 +190,14 @@ void free_nsproxy(struct nsproxy *ns)
> kmem_cache_free(nsproxy_cachep, ns);
> }
>
> +static void free_nsproxy_work(struct work_struct *work)
> +{
> + struct nsproxy *ns = container_of(work, struct nsproxy,
> + free_nsproxy_work);
> +
> + free_nsproxy(ns);
> +}
> +
> /*
> * Called from unshare. Unshare all the namespaces part of nsproxy.
> * On success, returns the new nsproxy.
> @@ -215,16 +235,8 @@ 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);

What's wrong with using call_rcu()? The above will cause a workqueue
thread to block for no good reason.

> }
>
> void exit_task_namespaces(struct task_struct *p)

--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com

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