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

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Message-Id: <20160413.112202.341079594713169936.davem@davemloft.net>
Date:	Wed, 13 Apr 2016 11:22:02 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mst@...hat.com
Cc:	pabeni@...hat.com, netdev@...r.kernel.org,
	hannes@...essinduktion.org, ebiederm@...ssion.com,
	gkurz@...ux.vnet.ibm.com, jasowang@...hat.com
Subject: Re: [PATCH RFC 2/2] tun: don't set a default qdisc

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Wed, 13 Apr 2016 13:26:51 +0300

> 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 think you should change this, putting the default back was a
bug fix.

I think this series is taking way too many liberties in order to
achieve locklessness, in my opinion.

If you have proper multi-queue, the qdisc lock and the netdev transmit
lock are almost completely free.