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

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Message-Id: <5697DF47-5CCB-41F3-9BD1-62DE42AD61B8@gmail.com>
Date:   Thu, 17 Nov 2016 16:31:19 -0800
From:   Jarno Rajahalme <jarno.rajahalme@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "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 Nov 14, 2016, at 3:09 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> 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);
> 	}
> 

This solved a ~20 second slowdown between OVS datapath unit tests for me.

  Jarno