netdev - Re: [PATCH RFC 0/2] tun: lockless xmit

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Date:	Wed, 13 Apr 2016 14:08:50 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kurz <gkurz@...ux.vnet.ibm.com>,
	Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFC 0/2] tun: lockless xmit

On Wed, Apr 13, 2016 at 11:04:45AM +0200, Paolo Abeni wrote:
> This patch series try to remove the need for any lock in the tun device
> xmit path, significantly improving the forwarding performance when multiple
> processes are accessing the tun device (i.e. in a nic->bridge->tun->vm scenario).
> 
> The lockless xmit is obtained explicitly setting the NETIF_F_LLTX feature bit
> and removing the default qdisc.
> 
> Unlikely most virtual devices, the tun driver has featured a default qdisc
> for a long period, but it already lost such feature in linux 4.3.

Thanks -  I think it's a good idea to reduce the
lock contention there.

But I think it's unfortunate that it requires
bypassing the qdisc completely: this means
that anyone trying to do traffic shaping will
get back the contention.

Can we solve the lock contention for qdisc?
E.g. add a small lockless queue in front of it,
whoever has the qdisc lock would be
responsible for moving things from there to qdisc
proper.

Thoughts? Is there a chance this might work reasonably well?


> Paolo Abeni (2):
>   tun: don't require serialization lock on tx
>   tun: don't set a default qdisc
> 
>  drivers/net/tun.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> -- 
> 1.8.3.1