netdev - Re: [PATCH v2 net-next] gro_cells: avoid using synchronize_rcu() in gro_cells

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Date:   Mon, 21 Feb 2022 09:23:57 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 net-next] gro_cells: avoid using synchronize_rcu() in
 gro_cells_destroy()

Hello,

On Sat, 2022-02-19 at 20:11 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Another thing making netns dismantles potentially very slow is located
> in gro_cells_destroy(),
> whenever cleanup_net() has to remove a device using gro_cells framework.
> 
> RTNL is not held at this stage, so synchronize_net()
> is calling synchronize_rcu():
> 
> netdev_run_todo()
>  ip_tunnel_dev_free()
>   gro_cells_destroy()
>    synchronize_net()
>     synchronize_rcu() // Ouch.
> 
> This patch uses call_rcu(), and gave me a 25x performance improvement
> in my tests.
> 
> cleanup_net() is no longer blocked ~10 ms per synchronize_rcu()
> call.
> 
> In the case we could not allocate the memory needed to queue the
> deferred free, use synchronize_rcu_expedited()
> 
> v2: made percpu_free_defer_callback() static
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

I'm sorry for the late feedback. I'm wondering if you considered
placing the 'defer' pointer inside 'gro_cells' and allocating it at
gro_cells_init() init time?

Thanks!

Paolo