Re: [PATCH] net: Convert net_mutex into rw_semaphore and down read it on net->init/->exit

[Eric Dumazet]

On Tue, 2017-11-14 at 09:44 -0800, Andrei Vagin wrote:
> On Tue, Nov 14, 2017 at 04:53:33PM +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
> > 
> > The solution is to convert net_mutex to the rw_semaphore. 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, like you may see below. There
> > is measured sequential net namespace creation in a cycle, in single
> > thread, without other tasks (single user mode):
> > 
> > 1)int main(int argc, char *argv[])
> > {
> >         unsigned nr;
> >         if (argc < 2) {
> >                 fprintf(stderr, "Provide nr iterations arg\n");
> >                 return 1;
> >         }
> >         nr = atoi(argv[1]);
> >         while (nr-- > 0) {
> >                 if (unshare(CLONE_NEWNET)) {
> >                         perror("Can't unshare");
> >                         return 1;
> >                 }
> >         }
> >         return 0;
> > }
> > 
> > Origin, 100000 unshare():
> > 0.03user 23.14system 1:39.85elapsed 23%CPU
> > 
> > Patched, 100000 unshare():
> > 0.03user 67.49system 1:08.34elapsed 98%CPU
> > 
> > 2)for i in {1..10000}; do unshare -n bash -c exit; done
> 
> Hi Kirill,
> 
> This mutex has another role. You know that net namespaces are destroyed
> asynchronously, and the net mutex gurantees that a backlog will be not
> big. If we have something in backlog, we know that it will be handled
> before creating a new net ns.
> 
> As far as I remember net namespaces are created much faster than
> they are destroyed, so with this changes we can create a really big
> backlog, can't we?

Please take a look at the recent patches I did :

8ca712c373a462cfa1b62272870b6c2c74aa83f9 Merge branch 'net-speedup-netns-create-delete-time'
64bc17811b72758753e2b64cd8f2a63812c61fe1 ipv4: speedup ipv6 tunnels dismantle
bb401caefe9d2c65e0c0fa23b21deecfbfa473fe ipv6: speedup ipv6 tunnels dismantle
789e6ddb0b2fb5d5024b760b178a47876e4de7a6 tcp: batch tcp_net_metrics_exit
a90c9347e90ed1e9323d71402ed18023bc910cd8 ipv6: addrlabel: per netns list
d464e84eed02993d40ad55fdc19f4523e4deee5b kobject: factorize skb setup in kobject_uevent_net_broadcast()
4a336a23d619e96aef37d4d054cfadcdd1b581ba kobject: copy env blob in one go
16dff336b33d87c15d9cbe933cfd275aae2a8251 kobject: add kobject_uevent_net_broadcast()