netdev - random thoughts on optmizing network namespace exit.

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Date:   Wed, 23 Nov 2016 17:52:48 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andrei Vagin <avagin@...nvz.org>
CC:     <netdev@...r.kernel.org>
Subject: random thoughts on optmizing network namespace exit.

Fundamentally if we want things to get better we have to remove
unnecessary serialization.  It is entirely too easy to sleep when
cleaning up a networking subsystem and create long hold times on
net_mutex for no particular reasons.

What probably makes sense to do is to add the concept of a
non-serialized pernet_operation.  And then work through the networking
stack converting all of the pernet_operations.  That should allow
network namespace exits to overlap while they clean up, and it should
allow the net_mutex to be dropped at the same point we drop rtnl_lock
in cleanup_net.

It might be a touch tricky during the transition period to take
advantage of an early drop of net_mutex, but that is where I would
start.

Once net_mutex is no longer used to serialize initialization/cleanup
methods for a network namespace.  We can look at other bottlenecks.

Eric