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

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Date:   Fri, 15 Jun 2018 21:00:59 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH] net_sched: blackhole: tell upper qdisc about dropped packets

On Fri, Jun 15, 2018 at 6:21 AM, Konstantin Khlebnikov
<khlebnikov@...dex-team.ru> wrote:
> 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.

But anything !NET_XMIT_SUCCESS is basically same for upper
layer:

        err = qdisc_enqueue(skb, cl->qdisc, to_free);
        if (unlikely(err != NET_XMIT_SUCCESS)) {
                if (net_xmit_drop_count(err)) {
                        cl->qstats.drops++;
                        qdisc_qstats_drop(sch);
                }
                return err;
        }

So using NET_XMIT_DROP is same in this case?