netdev - Re: [PATCH RFC 2/2] tun: don't set a default qdisc

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Message-ID: <20160414090532.GA2185@redhat.com>
Date:	Thu, 14 Apr 2016 12:05:32 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...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>
Subject: Re: [PATCH RFC 2/2] tun: don't set a default qdisc

On Thu, Apr 14, 2016 at 02:49:28PM +0800, Jason Wang wrote:
> 
> 
> On 04/13/2016 06:26 PM, Michael S. Tsirkin wrote:
> > On Wed, Apr 13, 2016 at 11:04:47AM +0200, Paolo Abeni wrote:
> >> This patch disables the default qdisc by explicitly setting the
> >> IFF_NO_QUEUE private flag so that now the tun xmit path do not
> >> require any lock by default.
> >>
> >> The default qdisc was first removed as a side effect of commit
> >> f84bb1eac027 ("net: fix IFF_NO_QUEUE for drivers using alloc_netdev")
> >> and recently restored with commit 016adb7260f4 ("tuntap: restore default qdisc")
> >>
> >> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > I wonder about this back and forth.
> > Jason, do you see a workload where the default qdisc
> > is preferable?
> 
> I don't know, but we used to behave like this so we'd better keep it.
> 
> An interesting thing is I vaguely remember that you have some concern
> when I propose IFF_NO_QUEUE for macvtap[1] :)
> 
> [1] https://lkml.org/lkml/2015/8/24/147

It's the same concern - that we aren't fully addressing
the problem, so if user configures a qdisc, we are back to square 1.
It's especially annoying that IIUC in this setup, if one
does configured a non default qdisc, there's no way to go back.
It doesn't necessarily mean we must not do it as an intermediate step,
though.

> 
> I think this could be done by management or more safe by introducing a
> new tun flag (TUN_NO_QUEUE).

What exactly does this flag do/mean?

> >
> >> ---
> >>  drivers/net/tun.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >> index 42992dc..0a0b63c 100644
> >> --- a/drivers/net/tun.c
> >> +++ b/drivers/net/tun.c
> >> @@ -1081,6 +1081,7 @@ static void tun_net_init(struct net_device *dev)
> >>  
> >>  		break;
> >>  	}
> >> +	dev->priv_flags |= IFF_NO_QUEUE;
> >>  }
> >>  
> >>  /* Character device part */
> >> -- 
> >> 1.8.3.1