Messages in this thread | | | Subject | Re: [PATCH v2 03/31] net: Introduce net_sem for protection of pernet_list | From | Kirill Tkhai <> | Date | Thu, 18 Jan 2018 13:14:26 +0300 |
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On 17.01.2018 23:04, Andrei Vagin wrote: > On Mon, Nov 20, 2017 at 09:32:34PM +0300, Kirill Tkhai wrote: >> Curently mutex is used to protect pernet operations list. It makes >> cleanup_net() to execute ->exit methods of the same operations set, >> which was used on the time of ->init, even after net namespace is >> unlinked from net_namespace_list. >> >> But the problem is it's need to synchronize_rcu() after net is removed >> from net_namespace_list(): >> >> Destroy net_ns: >> cleanup_net() >> mutex_lock(&net_mutex) >> list_del_rcu(&net->list) >> synchronize_rcu() <--- Sleep there for ages >> list_for_each_entry_reverse(ops, &pernet_list, list) >> ops_exit_list(ops, &net_exit_list) >> list_for_each_entry_reverse(ops, &pernet_list, list) >> ops_free_list(ops, &net_exit_list) >> mutex_unlock(&net_mutex) >> >> This primitive is not fast, especially on the systems with many processors >> and/or when preemptible RCU is enabled in config. So, all the time, while >> cleanup_net() is waiting for RCU grace period, creation of new net namespaces >> is not possible, the tasks, who makes it, are sleeping on the same mutex: >> >> Create net_ns: >> copy_net_ns() >> mutex_lock_killable(&net_mutex) <--- Sleep there for ages >> >> I observed 20-30 seconds hangs of "unshare -n" on ordinary 8-cpu laptop >> with preemptible RCU enabled. >> >> The solution is to convert net_mutex to the rw_semaphore and add small locks >> to really small number of pernet_operations, what really need them. Then, >> pernet_operations::init/::exit methods, modifying the net-related data, >> will require down_read() locking only, while down_write() will be used >> for changing pernet_list. >> >> This gives signify performance increase, after all patch set is applied, >> like you may see here: >> >> %for i in {1..10000}; do unshare -n bash -c exit; done >> >> *before* >> real 1m40,377s >> user 0m9,672s >> sys 0m19,928s >> >> *after* >> real 0m17,007s >> user 0m5,311s >> sys 0m11,779 >> >> (5.8 times faster) >> >> This patch starts replacing net_mutex to net_sem. It adds rw_semaphore, >> describes the variables it protects, and makes to use where appropriate. >> net_mutex is still present, and next patches will kick it out step-by-step. >> >> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com> >> --- >> include/linux/rtnetlink.h | 1 + >> net/core/net_namespace.c | 39 ++++++++++++++++++++++++++------------- >> net/core/rtnetlink.c | 4 ++-- >> 3 files changed, 29 insertions(+), 15 deletions(-) >> >> diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h >> index 2032ce2eb20b..f640fc87fe1d 100644 >> --- a/include/linux/rtnetlink.h >> +++ b/include/linux/rtnetlink.h >> @@ -35,6 +35,7 @@ extern int rtnl_is_locked(void); >> >> extern wait_queue_head_t netdev_unregistering_wq; >> extern struct mutex net_mutex; >> +extern struct rw_semaphore net_sem; >> >> #ifdef CONFIG_PROVE_LOCKING >> extern bool lockdep_rtnl_is_held(void); >> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c >> index 2e512965bf42..859dce31e37e 100644 >> --- a/net/core/net_namespace.c >> +++ b/net/core/net_namespace.c >> @@ -41,6 +41,11 @@ struct net init_net = { > >> static LIST_HEAD(pernet_list); >> static struct list_head *first_device = &pernet_list; >> DEFINE_MUTEX(net_mutex); > > With all patches, we still have the net_mutex, I think we need to add a > comment, which explains why we need it. Are "sync" pernet operations > depricated after this series? Or is it ok to have them?
net_mutex will leave till the time all pernet_operations are converted. But people, who don't use unconverted operations, will have performance profit already. Comment is not a problem :)
Thanks, Kirill
>> EXPORT_SYMBOL(init_net); >> >> static bool init_net_initialized; >> +/* >> + * net_sem: protects: pernet_list, net_generic_ids, >> + * init_net_initialized and first_device pointer. >> + */ >> +DECLARE_RWSEM(net_sem); >> >> #define MIN_PERNET_OPS_ID \ >> ((sizeof(struct net_generic) + sizeof(void *) - 1) / sizeof(void *)) >> @@ -279,7 +284,7 @@ struct net *get_net_ns_by_id(struct net *net, int id) >> */ >> static __net_init int setup_net(struct net *net, struct user_namespace *user_ns) >> { >> - /* Must be called with net_mutex held */ >> + /* Must be called with net_sem held */ >> const struct pernet_operations *ops, *saved_ops; >> int error = 0; >> LIST_HEAD(net_exit_list); >> @@ -411,12 +416,16 @@ struct net *copy_net_ns(unsigned long flags, >> net->ucounts = ucounts; >> get_user_ns(user_ns); >> >> - rv = mutex_lock_killable(&net_mutex); >> + rv = down_read_killable(&net_sem); >> if (rv < 0) >> goto put_userns; >> - >> + rv = mutex_lock_killable(&net_mutex); >> + if (rv < 0) >> + goto up_read; >> rv = setup_net(net, user_ns); >> mutex_unlock(&net_mutex); >> +up_read: >> + up_read(&net_sem); >> if (rv < 0) { >> put_userns: >> put_user_ns(user_ns); >> @@ -443,6 +452,7 @@ static void cleanup_net(struct work_struct *work) >> list_replace_init(&cleanup_list, &net_kill_list); >> spin_unlock_irq(&cleanup_list_lock); >> >> + down_read(&net_sem); >> mutex_lock(&net_mutex); >> >> /* Don't let anyone else find us. */ >> @@ -484,6 +494,7 @@ static void cleanup_net(struct work_struct *work) >> ops_free_list(ops, &net_exit_list); >> >> mutex_unlock(&net_mutex); >> + up_read(&net_sem); >> >> /* Ensure there are no outstanding rcu callbacks using this >> * network namespace. >> @@ -510,8 +521,10 @@ static void cleanup_net(struct work_struct *work) >> */ >> void net_ns_barrier(void) >> { >> + down_write(&net_sem); >> mutex_lock(&net_mutex); >> mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> } >> EXPORT_SYMBOL(net_ns_barrier); >> >> @@ -838,12 +851,12 @@ static int __init net_ns_init(void) >> >> rcu_assign_pointer(init_net.gen, ng); >> >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> if (setup_net(&init_net, &init_user_ns)) >> panic("Could not setup the initial network namespace"); >> >> init_net_initialized = true; >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> >> register_pernet_subsys(&net_ns_ops); >> >> @@ -983,9 +996,9 @@ static void unregister_pernet_operations(struct pernet_operations *ops) >> int register_pernet_subsys(struct pernet_operations *ops) >> { >> int error; >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> error = register_pernet_operations(first_device, ops); >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> return error; >> } >> EXPORT_SYMBOL_GPL(register_pernet_subsys); >> @@ -1001,9 +1014,9 @@ EXPORT_SYMBOL_GPL(register_pernet_subsys); >> */ >> void unregister_pernet_subsys(struct pernet_operations *ops) >> { >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> unregister_pernet_operations(ops); >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> } >> EXPORT_SYMBOL_GPL(unregister_pernet_subsys); >> >> @@ -1029,11 +1042,11 @@ EXPORT_SYMBOL_GPL(unregister_pernet_subsys); >> int register_pernet_device(struct pernet_operations *ops) >> { >> int error; >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> error = register_pernet_operations(&pernet_list, ops); >> if (!error && (first_device == &pernet_list)) >> first_device = &ops->list; >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> return error; >> } >> EXPORT_SYMBOL_GPL(register_pernet_device); >> @@ -1049,11 +1062,11 @@ EXPORT_SYMBOL_GPL(register_pernet_device); >> */ >> void unregister_pernet_device(struct pernet_operations *ops) >> { >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> if (&ops->list == first_device) >> first_device = first_device->next; >> unregister_pernet_operations(ops); >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> } >> EXPORT_SYMBOL_GPL(unregister_pernet_device); >> >> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c >> index dabba2a91fc8..cb06d43c4230 100644 >> --- a/net/core/rtnetlink.c >> +++ b/net/core/rtnetlink.c >> @@ -390,11 +390,11 @@ static void rtnl_lock_unregistering_all(void) >> void rtnl_link_unregister(struct rtnl_link_ops *ops) >> { >> /* Close the race with cleanup_net() */ >> - mutex_lock(&net_mutex); >> + down_write(&net_sem); >> rtnl_lock_unregistering_all(); >> __rtnl_link_unregister(ops); >> rtnl_unlock(); >> - mutex_unlock(&net_mutex); >> + up_write(&net_sem); >> } >> EXPORT_SYMBOL_GPL(rtnl_link_unregister); >> >>
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