Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
From: Paolo Abeni <pabeni@redhat.com>
Date: 2020-07-02 09:46:07
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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:quoted
On Wed, Jul 1, 2020 at 9:05 AM Cong Wang [off-list ref] wrote:quoted
On Tue, Jun 30, 2020 at 2:08 PM Josh Hunt [off-list ref] wrote:quoted
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 ---
quoted hunk ↗ jump to hunk
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