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

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Date:   Fri, 15 Jun 2018 16:21:55 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Eric Dumazet <eric.dumazet@...il.com>, 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 15.06.2018 16:13, Eric Dumazet wrote:
> 
> 
> 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.
> 
> 

I suppose "blackhole" should work like "successful" xmit, but counted as drop.