From: Yunsheng Lin <linyunsheng@huawei.com>
To: Hillf Danton <hdanton@sina.com>
Cc: Juergen Gross <jgross@suse.com>, <netdev@vger.kernel.org>,
<linux-kernel@vger.kernel.org>, Jiri Kosina <JKosina@suse.com>
Subject: Re: [PATCH net v3] net: sched: fix packet stuck problem for lockless qdisc
Date: Mon, 12 Apr 2021 11:37:24 +0800 [thread overview]
Message-ID: <caadc833-ee47-8243-a238-57595e7fc446@huawei.com> (raw)
In-Reply-To: <20210412032111.1887-1-hdanton@sina.com>
On 2021/4/12 11:21, Hillf Danton wrote:
> On Mon, 12 Apr 2021 09:24:30 Yunsheng Lin wrote:
>> On 2021/4/9 17:09, Hillf Danton wrote:
>>> On Fri, 9 Apr 2021 07:31:03 Juergen Gross wrote:
>>>> On 25.03.21 04:13, Yunsheng Lin wrote:
>>>> I have a setup which is able to reproduce the issue quite reliably:
>>>>
>>>> In a Xen guest I'm mounting 8 NFS shares and run sysbench fileio on
>>>> each of them. The average latency reported by sysbench is well below
>>>> 1 msec, but at least once per hour I get latencies in the minute
>>>> range.
>>>>
>>>> With this patch I don't see these high latencies any longer (test
>>>> is running for more than 20 hours now).
>>>>
>>>> So you can add my:
>>>>
>>>> Tested-by: Juergen Gross <jgross@suse.com>
>>>>
>>>
>>> If retry is allowed in the dequeue method then a simple seqcount can do the
>>> work of serializing enqueuer and dequeuer. IIUC it was not attempted last year.
>>
>> At the first glance, I do not think the below patch fix the data race
>
> Thanks for taking a look.
>
>> described in the commit log, as it does not handle the time window
>> between dequeuing and q->seqlock releasing, as below:
>>
> Yes the time window does exist.
>
>> The cpu1 may not see the qdisc->pad changed after pfifo_fast_dequeue(),
>> and cpu2 is not able to take the q->seqlock yet because cpu1 do not
>> release the q->seqlock.
>>
> It's now covered by extending the seqcount aperture a bit.
>
> --- x/net/sched/sch_generic.c
> +++ y/net/sched/sch_generic.c
> @@ -380,14 +380,23 @@ void __qdisc_run(struct Qdisc *q)
> {
> int quota = dev_tx_weight;
> int packets;
> + int seq;
> +
> +again:
> + seq = READ_ONCE(q->pad);
> + smp_rmb();
>
> while (qdisc_restart(q, &packets)) {
> quota -= packets;
> if (quota <= 0) {
> __netif_schedule(q);
> - break;
> + return;
> }
> }
> +
> + smp_rmb();
> + if (seq != READ_ONCE(q->pad))
> + goto again;
As my understanding, there is still time window between q->pad checking
above and q->seqlock releasing in qdisc_run_end().
> }
>
> unsigned long dev_trans_start(struct net_device *dev)
> @@ -632,6 +641,9 @@ static int pfifo_fast_enqueue(struct sk_
> return qdisc_drop(skb, qdisc, to_free);
> }
>
> + qdisc->pad++;
> + smp_wmb();
> +
> qdisc_update_stats_at_enqueue(qdisc, pkt_len);
> return NET_XMIT_SUCCESS;
> }
>
> .
>
next prev parent reply other threads:[~2021-04-12 3:37 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-25 3:13 [PATCH net v3] net: sched: fix packet stuck problem for lockless qdisc Yunsheng Lin
2021-04-09 5:31 ` Juergen Gross
2021-04-12 1:04 ` Yunsheng Lin
2021-04-12 8:21 ` Juergen Gross
[not found] ` <20210409090909.1767-1-hdanton@sina.com>
2021-04-12 1:24 ` Yunsheng Lin
[not found] ` <20210412032111.1887-1-hdanton@sina.com>
2021-04-12 3:37 ` Yunsheng Lin [this message]
[not found] ` <20210412072856.2046-1-hdanton@sina.com>
2021-04-12 12:00 ` Yunsheng Lin
[not found] ` <20210413022129.2203-1-hdanton@sina.com>
2021-04-13 2:56 ` Yunsheng Lin
[not found] ` <20210413032620.2259-1-hdanton@sina.com>
2021-04-13 3:34 ` Yunsheng Lin
[not found] ` <20210413071241.2325-1-hdanton@sina.com>
2021-04-13 7:57 ` Yunsheng Lin
[not found] ` <20210413083352.2424-1-hdanton@sina.com>
2021-04-13 9:03 ` Yunsheng Lin
2021-04-13 9:15 ` Juergen Gross
2021-04-16 11:46 ` Jiri Kosina
2021-04-18 22:59 ` Michal Kubecek
2021-04-19 2:04 ` Yunsheng Lin
2021-04-19 12:21 ` [Linuxarm] " Yunsheng Lin
2021-04-19 15:25 ` Michal Kubecek
2021-04-19 14:57 ` Michal Kubecek
2021-04-20 1:45 ` Yunsheng Lin
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