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SubjectRe: [PATCH] net: Convert net_mutex into rw_semaphore and down read it on net->init/->exit
On 14.11.2017 20:44, 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?

I don't think limitation is a good goal or a gool for the mutex,
because it's very easy to create many net namespaces in case of
the mutex exists. You may open /proc/[pid]/ns/net like a file,
and net_ns counter will increment. Then, do unshare(), and
the mutex has no a way to protect against that. Anyway, mutex
can't limit a number of something in general, I've never seen
a (good) example in kernel.

As I see, the real limitation happen in inc_net_namespaces(),
which is decremented after RCU grace period in cleanup_net(),
and it has not changed.

> There was a discussion a few month ago:
>> 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.

You're right, and I predict, the sum time, spent on cpu, will remain the same,
but the think is that now creation and destroying may be executed in parallel.

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