Messages in this thread Patch in this message | | | Subject | Re: Packet gets stuck in NOLOCK pfifo_fast qdisc | From | Paolo Abeni <> | Date | Thu, 02 Jul 2020 11:45:52 +0200 |
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Hi all,
On Thu, 2020-07-02 at 08:14 +0200, Jonas Bonn wrote: > Hi Cong, > > On 01/07/2020 21:58, Cong Wang wrote: > > On Wed, Jul 1, 2020 at 9:05 AM Cong Wang <xiyou.wangcong@gmail.com> wrote: > > > On Tue, Jun 30, 2020 at 2:08 PM Josh Hunt <johunt@akamai.com> wrote: > > > > Do either of you know if there's been any development on a fix for this > > > > issue? If not we can propose something. > > > > > > If you have a reproducer, I can look into this. > > > > Does the attached patch fix this bug completely? > > It's easier to comment if you inline the patch, but after taking a quick > look it seems too simplistic. > > i) Are you sure you haven't got the return values on qdisc_run reversed?
qdisc_run() returns true if it was able to acquire the seq lock. We need to take special action in the opposite case, so Cong's patch LGTM from a functional PoV.
> ii) There's a "bypass" path that skips the enqueue/dequeue operation if > the queue is empty; that needs a similar treatment: after releasing > seqlock it needs to ensure that another packet hasn't been enqueued > since it last checked.
That has been reverted with commit 379349e9bc3b42b8b2f8f7a03f64a97623fff323
--- > diff --git a/net/core/dev.c b/net/core/dev.c > index 90b59fc50dc9..c7e48356132a 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -3744,7 +3744,8 @@ 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); > + if (!qdisc_run(q) && rc == NET_XMIT_SUCCESS) > + __netif_schedule(q);
I fear the __netif_schedule() call may cause performance regression to the point of making a revert of TCQ_F_NOLOCK preferable. I'll try to collect some data.
Thanks!
Paolo
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