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

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Date:   Mon, 14 Nov 2016 16:28:42 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        David Miller <davem@...emloft.net>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Cong Wang <xiyou.wangcong@...il.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: [PATCH net] gro_cells: mark napi struct as not busy poll candidates

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>
---
 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);
 	}