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

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Date:	Thu, 14 Apr 2016 14:49:28 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Paolo Abeni <pabeni@...hat.com>
Cc:	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 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] :)

I think this could be done by management or more safe by introducing a
new tun flag (TUN_NO_QUEUE).

[1] https://lkml.org/lkml/2015/8/24/147

>
>> ---
>>  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