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.wangc...@gmail.com> wrote:
> > > On Tue, Jun 30, 2020 at 2:08 PM Josh Hunt <joh...@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
