Re: net: cleanup_net is slow

[Eric Dumazet]

On Fri, Apr 21, 2017 at 10:50 AM, Andrey Konovalov
<andreyknvl@xxxxxxxxxx> wrote:
> Hi!
>
> We're investigating some approaches to improve isolation of syzkaller
> programs. One of the ideas is run each program in it's own user/net
> namespace. However, while I was experimenting with this, I stumbled
> upon a problem.
>
> It seems that cleanup_net() might take a very long time to execute.
>
> I've attached the reproducer and kernel .config that I used. Run as
> "./a.out 1". The reproducer just forks and does unshare(CLONE_NEWNET)
> in a loop. Note, that I have a lot of network-related configs enabled,
> which causes a few interfaces to be set up by default.
>
> What I see with this reproducer is that at first a huge number
> (~200-300) net namespaces are created without any contention. But then
> (probably when one of these namespaces gets destroyed) the program
> hangs for a considerable amount of time (~100 seconds in my vm).
> Nothing locks up inside the kernel and the CPU is mostly idle.
>
> Adding debug printfs showed that the part that takes almost all of
> that time is the lines between synchronize_rcu() and
> mutex_unlock(&net_mutex) in cleanup_net. Running perf showed that the
> cause of this might be a lot of calls to synchronize_net that happen
> while executing those lines.
>
> Is there any change that can be done to speed up the
> creation/destruction of a huge number of net namespaces?
>

We have batches, but fundamentally this is a hard problem to solve.

Every time we try, we add bugs :/

RTNL is the new BKL (Big Kernel Lock of early linux) of today.

Even the synchronize_rcu_expedited() done from synchronize_net() is a
serious issue on some platforms.