netdev - Re: [PATCH net] gro_cells: mark napi struct as not busy poll candidates

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Date:   Mon, 14 Nov 2016 21:21:28 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        David Miller <davem@...emloft.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Rolf Neugebauer <rolf.neugebauer@...ker.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Justin Cormack <justin.cormack@...ker.com>,
        Ian Campbell <ian.campbell@...ker.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] gro_cells: mark napi struct as not busy poll candidates

On Mon, Nov 14, 2016 at 4:28 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Rolf Neugebauer reported very long delays at netns dismantle.
>
> Eric W. Biederman was kind enough to look at this problem
> and noticed synchronize_net() occurring from netif_napi_del() that was
> added in linux-4.5
>
> Busy polling makes no sense for tunnels NAPI.
> If busy poll is used for sessions over tunnels, the poller will need to
> poll the physical device queue anyway.
>
> netif_tx_napi_add() could be used here, but function name is misleading,
> and renaming it is not stable material, so set NAPI_STATE_NO_BUSY_POLL
> bit directly.
>
> This will avoid inserting gro_cells napi structures in napi_hash[]
> and avoid the problematic synchronize_net() (per possible cpu) that
> Rolf reported.
>
> Fixes: 93d05d4a320c ("net: provide generic busy polling to all NAPI drivers")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Rolf Neugebauer <rolf.neugebauer@...ker.com>
> Reported-by: Eric W. Biederman <ebiederm@...ssion.com>


Acked-by: Cong Wang <xiyou.wangcong@...il.com>