netdev - Re: [PATCH] net_sched/sfq: update hierarchical backlog when drop packet

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Date:   Tue, 15 Aug 2017 17:33:06 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH] net_sched/sfq: update hierarchical backlog when drop
 packet

On 15.08.2017 17:09, Eric Dumazet wrote:
> On Tue, 2017-08-15 at 16:37 +0300, Konstantin Khlebnikov wrote:
>> When sfq_enqueue() drops head packet or packet from another queue it
>> have to update backlog at upper qdiscs too.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>> Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
>> ---
>>   net/sched/sch_sfq.c |    5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
>> index f80ea2cc5f1f..82469ef9655e 100644
>> --- a/net/sched/sch_sfq.c
>> +++ b/net/sched/sch_sfq.c
>> @@ -437,6 +437,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
>>   		qdisc_drop(head, sch, to_free);
>>   
>>   		slot_queue_add(slot, skb);
>> +		qdisc_tree_reduce_backlog(sch, 0, delta);
>>   		return NET_XMIT_CN;
>>   	}
>>   
>> @@ -468,8 +469,10 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
>>   	/* Return Congestion Notification only if we dropped a packet
>>   	 * from this flow.
>>   	 */
>> -	if (qlen != slot->qlen)
>> +	if (qlen != slot->qlen) {
>> +		qdisc_tree_reduce_backlog(sch, 0, dropped - qdisc_pkt_len(skb));
>>   		return NET_XMIT_CN;
>> +	}
>>   
>>   	/* As we dropped a packet, better let upper stack know this */
>>   	qdisc_tree_reduce_backlog(sch, 1, dropped);
>>
> 
> Are you sure you have tested this patch ?
> 

Nope. I'm not sure. But we have something similar in our 4.4 kernel for a while.

Also fq_codel and pfifo_head_drop do something similar tho this.

Probably this might crash without "[PATCH 1/2] net_sched: call qlen_notify only if child qdisc is empty".
I hadn't tested them separately.