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

[Jason Wang <jasowang@redhat.com>]

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@redhat.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).