netdev - Re: Long delays creating a netns after deleting one (possibly RCU related)

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Date:   Mon, 14 Nov 2016 15:09:27 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Rolf Neugebauer <rolf.neugebauer@...ker.com>,
        LKML <linux-kernel@...r.kernel.org>,
        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: Long delays creating a netns after deleting one (possibly RCU
 related)

On Mon, 2016-11-14 at 14:46 -0800, Eric Dumazet wrote:
> On Mon, 2016-11-14 at 16:12 -0600, Eric W. Biederman wrote:
> 
> > synchronize_rcu_expidited is not enough if you have multiple network
> > devices in play.
> > 
> > Looking at the code it comes down to this commit, and it appears there
> > is a promise add rcu grace period combining by Eric Dumazet.
> > 
> > Eric since people are hitting noticable stalls because of the rcu grace
> > period taking a long time do you think you could look at this code path
> > a bit more?
> > 
> > commit 93d05d4a320cb16712bb3d57a9658f395d8cecb9
> > Author: Eric Dumazet <edumazet@...gle.com>
> > Date:   Wed Nov 18 06:31:03 2015 -0800
> 
> Absolutely, I will take a loop asap.

The worst offender should be fixed by the following patch.

busy poll needs to poll the physical device, not a virtual one...

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);
 	}