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

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Date:	Wed, 13 Apr 2016 09:42:45 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Paolo Abeni <pabeni@...hat.com>, 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, 2016-04-13 at 06:43 -0700, Eric Dumazet wrote:
> On Wed, 2016-04-13 at 16:17 +0300, Michael S. Tsirkin wrote:
> > On Wed, Apr 13, 2016 at 06:09:26AM -0700, Eric Dumazet wrote:
> > > You really need to convince John Fastabend to work full time on the real
> > > thing
> > 
> > Meaning making all qdiscs themselves lockless? With complex policies
> > like codel I can see how that might be challenging ...
> 
> Codel is a fifo, plus some droping capabilities at dequeue time.
> 
> It totally can be made lockless.

By lockless, I really meant decouple the enqueue() and dequeue() phases.

Both sides could use a separate exclusion mechanism.

So when qdisc_run() is dequeuing a bunch of packets (owning
__QDISC___STATE_RUNNING), other cpus would still be able to queue
additional packets.