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

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Message-ID: <20160414125822-mutt-send-email-mst@redhat.com>
Date:	Thu, 14 Apr 2016 13:01:39 +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 05:21:40PM +0800, Jason Wang wrote:
> 
> 
> 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 <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?
> 
> It was used for user that don't want qdisc (e.g for the user that only
> cares about performance).

However
- for tun, how is a fifo qdisc functionally different?
- it doesn't prevent configuring a qdisc, does it?

-- 
MST