netdev - Re: [PATCH] net_sched: blackhole: tell upper qdisc about dropped packets

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Date:   Fri, 15 Jun 2018 06:13:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH] net_sched: blackhole: tell upper qdisc about dropped
 packets



On 06/15/2018 03:27 AM, Konstantin Khlebnikov wrote:
> When blackhole is used on top of classful qdisc like hfsc it breaks
> qlen and backlog counters because packets are disappear without notice.
> 
> In HFSC non-zero qlen while all classes are inactive triggers warning:
> WARNING: ... at net/sched/sch_hfsc.c:1393 hfsc_dequeue+0xba4/0xe90 [sch_hfsc]
> and schedules watchdog work endlessly.
> 
> This patch return __NET_XMIT_BYPASS in addition to NET_XMIT_SUCCESS,
> this flag tells upper layer: this packet is gone and isn't queued.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> ---
>  net/sched/sch_blackhole.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_blackhole.c b/net/sched/sch_blackhole.c
> index c98a61e980ba..9c4c2bb547d7 100644
> --- a/net/sched/sch_blackhole.c
> +++ b/net/sched/sch_blackhole.c
> @@ -21,7 +21,7 @@ static int blackhole_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  			     struct sk_buff **to_free)
>  {
>  	qdisc_drop(skb, sch, to_free);
> -	return NET_XMIT_SUCCESS;
> +	return NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;

Why do not we use instead :

	return qdisc_drop(skb, sch, to_free);

Although noop_enqueue() seems to use :

	return NET_XMIT_CN;

Oh well.