netdev - Re: [PATCH v1 net-next] net: pkt

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Date:	Sat, 28 Sep 2013 10:03:42 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, shemminger@...tta.com,
	Mythili Prabhu <mysuryan@...co.com>,
	Dave Taht <dave.taht@...ferbloat.net>
Subject: Re: [PATCH v1 net-next] net: pkt_sched: PIE AQM scheme

On Fri, 2013-09-27 at 18:56 -0700, Vijay Subramanian wrote:

> +static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> +{
> +	struct pie_sched_data *q = qdisc_priv(sch);
> +
> +	if (unlikely(qdisc_qlen(sch) >= sch->limit))
> +		goto out;
> +

...

> +out:
> +	q->stats.overlimit++;
> +	qdisc_drop(skb, sch);
> +	return NET_XMIT_CN;  /*indicate congestion*/
> +}


If a Qdisc drops a packet because sch->limit is hit, you must :

return qdisc_drop(skb, sch);

So that NET_XMIT_DROP is returned, not NET_XMIT_CN.

This packet was dropped for sure.

vi +96 net/sched/sch_api.c

   ---enqueue

   enqueue returns 0, if packet was enqueued successfully.
   If packet (this one or another one) was dropped, it returns
   not zero error code.
   NET_XMIT_DROP        - this packet dropped
     Expected action: do not backoff, but wait until queue will clear.
   NET_XMIT_CN          - probably this packet enqueued, but another one dropped.
     Expected action: backoff or ignore
   NET_XMIT_POLICED     - dropped by police.
     Expected action: backoff or error to real-time apps.


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