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Date: Thu, 14 Apr 2016 12:10:01 +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 05:07:50PM +0800, Jason Wang wrote: > > > On 04/14/2016 05:05 PM, Michael S. Tsirkin wrote: > > 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? > > It means we don't need qdisc for this tuntap, so we can set IFF_NO_QUEUE > flag. But what does it mean for the user? When to set it and when not to set it? -- MST
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