netdev - Re: Way to Know When cleanup

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Message-ID: <CAM_iQpXH8Ubs9eQdyhr-5qW8gmymyaBdVTFqWqZ4zwvO8Oo2tQ@mail.gmail.com>
Date:   Wed, 5 Jul 2017 16:14:54 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Leon Yang <leon.gh.yang@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: Way to Know When cleanup_net() Calls Are Done

On Sat, Jul 1, 2017 at 10:50 PM, Leon Yang <leon.gh.yang@...il.com> wrote:
> Hi,
>
> I am measuring the time cost of parallel creation and destruction of
> net namespaces.
>
> As cleanup_net() is invoked from workqueue, the exit of a net
> namespace does not mean the cleanup is completed. Therefore I use
> ftrace to count the number of net_drop_ns() are called. But I found
> that the number of namespaces created does not match with the number
> of net_drop_ns() calls even after a long wait.
>
> Can you give some hints on why this is happening? and what would be
> the correct way to know when the last cleanup_net() is done?

Probably no way to know this in user-space, netns is refcount'ed,
we don't know which is the last one releasing it.

There is a per-userns counter for netns, but I don't see it is ever
exposed to user-space.