From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH RFC 2/2] tun: don't set a default qdisc Date: Thu, 14 Apr 2016 12:05:32 +0300 Message-ID: <20160414090532.GA2185@redhat.com> References: <1a216fb6cade7dcd7d2c1d4ce113a1ee6e3fdeb6.1460393493.git.pabeni@redhat.com> <20160413132411-mutt-send-email-mst@redhat.com> <570F3D78.3070203@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Paolo Abeni , netdev@vger.kernel.org, "David S. Miller" , Hannes Frederic Sowa , "Eric W. Biederman" , Greg Kurz To: Jason Wang Return-path: Received: from mx1.redhat.com ([209.132.183.28]:58318 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753549AbcDNJFg (ORCPT ); Thu, 14 Apr 2016 05:05:36 -0400 Content-Disposition: inline In-Reply-To: <570F3D78.3070203@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: 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 > > 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