From: John Fastabend <john.fastabend@gmail.com>
To: Cong Wang <xiyou.wangcong@gmail.com>, Paolo Abeni <pabeni@redhat.com>
Cc: Kehuan Feng <kehuan.feng@gmail.com>,
Hillf Danton <hdanton@sina.com>, Jike Song <albcamus@gmail.com>,
Josh Hunt <johunt@akamai.com>,
Jonas Bonn <jonas.bonn@netrounds.com>,
Michael Zhivich <mzhivich@akamai.com>,
David Miller <davem@davemloft.net>,
John Fastabend <john.fastabend@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Netdev <netdev@vger.kernel.org>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
Date: Thu, 03 Sep 2020 22:07:57 -0700 [thread overview]
Message-ID: <5f51cbad3cc2_3eceb208fc@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <CAM_iQpXZMeAGkq_=rG6KEabFNykszpRU_Hnv65Qk7yesvbRDrw@mail.gmail.com>
Cong Wang wrote:
> On Thu, Sep 3, 2020 at 1:40 AM Paolo Abeni <pabeni@redhat.com> wrote:
> >
> > On Wed, 2020-09-02 at 22:01 -0700, Cong Wang wrote:
> > > Can you test the attached one-line fix? I think we are overthinking,
> > > probably all
> > > we need here is a busy wait.
> >
> > I think that will solve, but I also think that will kill NOLOCK
> > performances due to really increased contention.
>
> Yeah, we somehow end up with more locks (seqlock, skb array lock)
> for lockless qdisc. What an irony... ;)
I went back to the original nolock implementation code to try and figure
out how this was working in the first place.
After initial patch series we have this in __dev_xmit_skb()
if (q->flags & TCQ_F_NOLOCK) {
if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
__qdisc_drop(skb, &to_free);
rc = NET_XMIT_DROP;
} else {
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
__qdisc_run(q);
}
if (unlikely(to_free))
kfree_skb_list(to_free);
return rc;
}
One important piece here is we used __qdisc_run(q) instead of
what we have there now qdisc_run(q). Here is the latest code,
if (q->flags & TCQ_F_NOLOCK) {
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
qdisc_run(q);
...
__qdisc_run is going to always go into a qdisc_restart loop and
dequeue packets. There is no check here to see if another CPU
is running or not. Compare that to qdisc_run()
static inline void qdisc_run(struct Qdisc *q)
{
if (qdisc_run_begin(q)) {
__qdisc_run(q);
qdisc_run_end(q);
}
}
Here we have all the racing around qdisc_is_running() that seems
unsolvable.
Seems we flipped __qdisc_run to qdisc_run here 32f7b44d0f566
("sched: manipulate __QDISC_STATE_RUNNING in qdisc_run_* helpers").
Its not clear to me from thatpatch though why it was even done
there?
Maybe this would unlock us,
diff --git a/net/core/dev.c b/net/core/dev.c
index 7df6c9617321..9b09429103f1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3749,7 +3749,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
if (q->flags & TCQ_F_NOLOCK) {
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
- qdisc_run(q);
+ __qdisc_run(q);
if (unlikely(to_free))
kfree_skb_list(to_free);
Per other thread we also need the state deactivated check added
back.
>
> >
> > At this point I fear we could consider reverting the NOLOCK stuff.
> > I personally would hate doing so, but it looks like NOLOCK benefits are
> > outweighed by its issues.
>
> I agree, NOLOCK brings more pains than gains. There are many race
> conditions hidden in generic qdisc layer, another one is enqueue vs.
> reset which is being discussed in another thread.
Sure. Seems they crept in over time. I had some plans to write a
lockless HTB implementation. But with fq+EDT with BPF it seems that
it is no longer needed, we have a more generic/better solution. So
I dropped it. Also most folks should really be using fq, fq_codel,
etc. by default anyways. Using pfifo_fast alone is not ideal IMO.
Thanks,
John
next prev parent reply other threads:[~2020-09-04 5:08 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 6:46 Packet gets stuck in NOLOCK pfifo_fast qdisc Jonas Bonn
2019-10-09 19:14 ` Paolo Abeni
2019-10-10 6:27 ` Jonas Bonn
2019-10-11 0:39 ` Jonas Bonn
2020-06-23 13:42 ` Michael Zhivich
2020-06-30 19:14 ` Josh Hunt
2020-07-01 7:53 ` Jonas Bonn
2020-07-01 16:05 ` Cong Wang
2020-07-01 19:58 ` Cong Wang
2020-07-01 22:02 ` Josh Hunt
2020-07-02 6:14 ` Jonas Bonn
2020-07-02 9:45 ` Paolo Abeni
2020-07-02 18:08 ` Josh Hunt
2020-07-07 14:18 ` Paolo Abeni
2020-07-08 20:16 ` Cong Wang
2020-07-09 9:20 ` Paolo Abeni
2020-07-08 20:33 ` Zhivich, Michael
2020-08-20 7:43 ` Jike Song
2020-08-20 18:13 ` Josh Hunt
[not found] ` <20200822032800.16296-1-hdanton@sina.com>
2020-08-25 2:18 ` Fengkehuan Feng
[not found] ` <20200825032312.11776-1-hdanton@sina.com>
2020-08-25 7:14 ` Fengkehuan Feng
[not found] ` <20200825162329.11292-1-hdanton@sina.com>
2020-08-26 2:38 ` Kehuan Feng
[not found] ` <CACS=qqKptAQQGiMoCs1Zgs9S4ZppHhasy1AK4df2NxnCDR+vCw@mail.gmail.com>
[not found] ` <5f46032e.1c69fb81.9880c.7a6cSMTPIN_ADDED_MISSING@mx.google.com>
2020-08-27 6:56 ` Kehuan Feng
[not found] ` <20200827125747.5816-1-hdanton@sina.com>
2020-08-28 1:45 ` Kehuan Feng
2020-09-03 5:01 ` Cong Wang
2020-09-03 8:39 ` Paolo Abeni
2020-09-03 17:43 ` Cong Wang
2020-09-04 5:07 ` John Fastabend [this message]
2020-09-10 20:15 ` Cong Wang
2020-09-10 21:07 ` John Fastabend
2020-09-10 21:40 ` Paolo Abeni
2021-04-02 19:25 ` Jiri Kosina
2021-04-02 19:33 ` Josh Hunt
[not found] ` <20210403003537.2032-1-hdanton@sina.com>
2021-04-03 12:23 ` Jiri Kosina
2021-04-06 0:55 ` Yunsheng Lin
2021-04-06 7:06 ` Michal Kubecek
2021-04-06 10:13 ` Juergen Gross
2021-04-06 12:17 ` Yunsheng Lin
2021-04-06 1:49 ` Cong Wang
2021-04-06 2:46 ` Yunsheng Lin
2021-04-06 7:31 ` Michal Kubecek
2021-04-06 12:24 ` Yunsheng Lin
[not found] ` <20200903101957.428-1-hdanton@sina.com>
2020-09-04 3:20 ` Kehuan Feng
2020-09-10 20:19 ` Cong Wang
2020-09-14 2:10 ` Yunsheng Lin
2020-09-17 19:52 ` Cong Wang
2020-09-18 2:06 ` Kehuan Feng
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