From: Eric Dumazet <eric.dumazet@gmail.com>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: John Fastabend <john.fastabend@gmail.com>,
Networking <netdev@vger.kernel.org>,
Jamal Hadi Salim <jhs@mojatatu.com>,
Jiri Pirko <jiri@resnulli.us>
Subject: Re: [BUG] net/sched : qlen can not really be per cpu ?
Date: Wed, 27 Feb 2019 08:59:15 -0800 [thread overview]
Message-ID: <e312625a-7946-42a4-88f0-95536768d5c8@gmail.com> (raw)
In-Reply-To: <d075c0f5-c2b5-6ea8-f09a-7af5ba4ce05b@gmail.com>
On 02/26/2019 04:56 PM, Eric Dumazet wrote:
>
>
> On 02/26/2019 03:51 PM, Cong Wang wrote:
>> On Tue, Feb 26, 2019 at 3:19 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>>
>>>
>>>
>>> On 02/25/2019 10:42 PM, Eric Dumazet wrote:
>>>> HTB + pfifo_fast as a leaf qdisc hits badly the following warning in htb_activate() :
>>>>
>>>> WARN_ON(cl->level || !cl->leaf.q || !cl->leaf.q->q.qlen);
>>>>
>>>> This is because pfifo_fast does not update sch->q.qlen, but per cpu counters.
>>>> So cl->leaf.q->q.qlen is zero.
>>>>
>>>> HFSC, CBQ, DRR, QFQ have the same problem.
>>>>
>>>> Any ideas how we can fix this ?
>>>
>>> What about something simple for stable ?
>>> ( I yet have to boot/test this )
>>
>> Is merely updating qlen sufficient for fixing it?
>>
>> I thought it is because of the lack of qdisc_tree_reduce_backlog()
>> in pfifo_fast.
>
> It does not seem to be the qdisc_tree_reduce_backlog() thing.
>
> HTB, HFSC, CBQ, DRR, QFQ only peek at their children 'qlen' to decide if there
> is at least one packet in them.
>
> The backlog is only reported for dumps, but the actual backlog value is not used in data path.
>
>
Another way to fix this would be to have a shadow version of pfifo_fast, which
basically would be the old version of it, that would be automatically selected
when used as a child of another qdisc (except mq/mqprio of course)
This seems not a stable candidate though.
next prev parent reply other threads:[~2019-02-27 16:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-26 6:42 [BUG] net/sched : qlen can not really be per cpu ? Eric Dumazet
2019-02-26 23:19 ` Eric Dumazet
2019-02-26 23:51 ` Cong Wang
2019-02-27 0:56 ` Eric Dumazet
2019-02-27 16:59 ` Eric Dumazet [this message]
2019-02-28 2:46 ` Cong Wang
2019-02-28 5:15 ` Eric Dumazet
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