netdev - Re: [RFC PATCH 06/12] net: sched: support qdisc

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Date:	Wed, 13 Jan 2016 11:20:16 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	john.fastabend@...il.com
Cc:	daniel@...earbox.net, eric.dumazet@...il.com, jhs@...atatu.com,
	aduyck@...antis.com, brouer@...hat.com, john.r.fastabend@...el.com,
	netdev@...r.kernel.org
Subject: Re: [RFC PATCH 06/12] net: sched: support qdisc_reset on NOLOCK
 qdisc

From: John Fastabend <john.fastabend@...il.com>
Date: Wed, 30 Dec 2015 09:53:13 -0800

>  case 2: dev_deactivate sequence. This can come from a user bringing
> 	 the interface down which causes the gso_skb list to be flushed
> 	 and the qlen zero'd. At the moment this is protected by the
> 	 qdisc lock so while we clear the qlen/gso_skb fields we are
> 	 guaranteed no new skbs are added. For the lockless case
> 	 though this is not true. To resolve this move the qdisc_reset
> 	 call after the new qdisc is assigned and a grace period is
> 	 exercised to ensure no new skbs can be enqueued. Further
> 	 the RTNL lock is held so we can not get another call to
> 	 activate the qdisc while the skb lists are being free'd.
> 
> 	 Finally, fix qdisc_reset to handle the per cpu stats and
> 	 skb lists.

Just wanted to note that some setups are sensitive to device
register/deregister costs.  This is why we batch register and
unregister operations in the core, so that the RCU grace period
is consolidated into one when we register/unregister a lot of
net devices.

If we now will incur a new per-device unregister RCU grace period
when the qdisc is destroyed, it could cause a regression.