From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH RFC 2/2] tun: don't set a default qdisc Date: Thu, 14 Apr 2016 17:21:40 +0800 Message-ID: <570F6124.6060107@redhat.com> References: <1a216fb6cade7dcd7d2c1d4ce113a1ee6e3fdeb6.1460393493.git.pabeni@redhat.com> <20160413132411-mutt-send-email-mst@redhat.com> <570F3D78.3070203@redhat.com> <20160414090532.GA2185@redhat.com> <570F5DE6.9000305@redhat.com> <20160414120826-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: Paolo Abeni , netdev@vger.kernel.org, "David S. Miller" , Hannes Frederic Sowa , "Eric W. Biederman" , Greg Kurz To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:54260 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751742AbcDNJVn (ORCPT ); Thu, 14 Apr 2016 05:21:43 -0400 In-Reply-To: <20160414120826-mutt-send-email-mst@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On 04/14/2016 05:10 PM, Michael S. Tsirkin wrote: > 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 >>>>>>> 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? It was used for user that don't want qdisc (e.g for the user that only cares about performance).