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

There was a discussion a few month ago:
https://lists.onap.org/pipermail/containers/2016-October/037509.html


>
> Origin:
> real 1m24,190s
> user 0m6,225s
> sys 0m15,132s

Here you measure time of creating and destroying net namespaces.

>
> Patched:
> real 0m18,235s (4.6 times faster)
> user 0m4,544s
> sys 0m13,796s

But here you measure time of crearing namespaces and you know nothing
when they will be destroyed.

Thanks,
Andrei

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