netdev - Re: [PATCH -next] cbq: remove only caller of qdisc->drop()

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Date:	Mon, 06 Jun 2016 15:13:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH -next] cbq: remove only caller of qdisc->drop()

On Mon, 2016-06-06 at 23:20 +0200, Florian Westphal wrote:
> since initial revision of cbq in 2004 iproute2 never implemented
> support for TCA_CBQ_OVL_STRATEGY, which is what needs to be set to
> activate the class->drop() call (TC_CBQ_OVL_DROP strategy must be
> set by userspace).
> 
> So lets remove this.  We can even do this in a backwards compatible
> way by switching ovl_drop to perform a dequeue+drop on the leaf.
> 
> A followup commit can then remove all .drop qdisc methods since this
> was the only caller.
> 
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  On a related note, iproute2 doesn't support the TCA_CBQ_POLICE
>  attribute either.  If we'd remove that too we could then get rid
>  of __parent and reshape_fail in struct Qdisc.
> 
>  However, in TCA_CBQ_POLICE case there is no way to replace
>  the functionality, i.e. we'd have to -EOPNOTSUPP or ignore
>  TCA_CBQ_POLICE if some other non-iproute2 tool presents it to us.
> 
>  AFAICS TCA_CBQ_POLICE doesn't work all that well even if userspace
>  would set it, one needs to:
> 
>   add a class
>   attach a qdisc to the class (default pfifo doesn't work as
>       q->handle is 0 and cbq_set_police() is a no-op in this case)
>   re-'add' the same class (tc class change ...) again
> 
>   user must also specifiy a defmap (e.g. 'split 1:0 defmap 3f'), since
>   this 'police' feature relies on its presence
>   the added qdisc (or the leaves) must be one of bfifo, pfifo, tbf or
>   netem as only these implement qdisc_reshape_fail() call.
> 
>   If all of these conditions are met, then instead of drop the leaf
>   qdiscs mentioned above would attempt to re-enqueue the skb in the
>   cbq TC_PRIO_BESTEFFORT class if their limit is reached.
> 
>   I think it would be safe to just ignore TCA_CBQ_POLICE and change the
>   qdiscs to drop right away.
> 
>   Does anyone have a reason to leave this in the tree?  Thanks!
> 
> diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
> index baafddf..df79791 100644
> --- a/net/sched/sch_cbq.c
> +++ b/net/sched/sch_cbq.c
> @@ -542,9 +542,14 @@ static void cbq_ovl_lowprio(struct cbq_class *cl)
>  
>  static void cbq_ovl_drop(struct cbq_class *cl)
>  {
> -	if (cl->q->ops->drop)
> -		if (cl->q->ops->drop(cl->q))
> -			cl->qdisc->q.qlen--;
> +	struct sk_buff *skb = cl->q->ops->dequeue(cl->q);
> +
> +	if (skb) {
> +		cl->deficit -= qdisc_pkt_len(skb);
> +		cl->qdisc->q.qlen--;
> +		qdisc_drop(skb, cl->qdisc);
> +	}
> +
>  	cl->xstats.overactions++;
>  	cbq_ovl_classic(cl);
>  }

A drop() is not equivalent to a dequeue() followed by qdisc_drop() for
statistics.

dequeue() will update stats of _sent_ packets/bytes, while drop() should
not.