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

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Date:   Tue, 15 Nov 2016 17:08:28 +0000
From:   Rolf Neugebauer <rolf.neugebauer@...ker.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>,
        Cong Wang <xiyou.wangcong@...il.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>,
        Rolf Neugebauer <rolf.neugebauer@...ker.com>
Subject: Re: [PATCH net] gro_cells: mark napi struct as not busy poll candidates

On Tue, Nov 15, 2016 at 12:28 AM, 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.

Thanks for the quick turn-around. I've tested this on both 4.9-rc5 and
4.8.7  with different CPU configs and it dramatically reduced the
delay. I still see a small (1s) delay for creating a new namespace
after deleting one in one config but will follow up with the details
on the original email thread.

>
> 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>

Tested-by: Rolf Neugebauer <rolf.neugebauer@...ker.com>

> ---
>  include/net/gro_cells.h |    3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/net/gro_cells.h b/include/net/gro_cells.h
> index d15214d673b2e8e08fd6437b572278fb1359f10d..2a1abbf8da74368cd01adc40cef6c0644e059ef2 100644
> --- a/include/net/gro_cells.h
> +++ b/include/net/gro_cells.h
> @@ -68,6 +68,9 @@ static inline int gro_cells_init(struct gro_cells *gcells, struct net_device *de
>                 struct gro_cell *cell = per_cpu_ptr(gcells->cells, i);
>
>                 __skb_queue_head_init(&cell->napi_skbs);
> +
> +               set_bit(NAPI_STATE_NO_BUSY_POLL, &cell->napi.state);
> +
>                 netif_napi_add(dev, &cell->napi, gro_cell_poll, 64);
>                 napi_enable(&cell->napi);
>         }
>
>