netdev - Re: [PATCH net-next 00/10] net_sched: speedup qdisc dequeue

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Message-ID: <CANn89i+szfpkVD8y-71RTHCmZn5KoHV5X33HzTCf-M1Xq8LJ3g@mail.gmail.com>
Date: Mon, 10 Nov 2025 10:05:06 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, 
	Toke Høiland-Jørgensen <toke@...hat.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next 00/10] net_sched: speedup qdisc dequeue

On Mon, Nov 10, 2025 at 9:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 10 Nov 2025 09:15:46 -0800 Eric Dumazet wrote:
> > On Mon, Nov 10, 2025 at 8:44 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Mon, 10 Nov 2025 09:44:55 +0000 Eric Dumazet wrote:
> > > > Avoid up to two cache line misses in qdisc dequeue() to fetch
> > > > skb_shinfo(skb)->gso_segs/gso_size while qdisc spinlock is held.
> > > >
> > > > Idea is to cache gso_segs at enqueue time before spinlock is
> > > > acquired, in the first skb cache line, where we already
> > > > have qdisc_skb_cb(skb)->pkt_len.
> > > >
> > > > This series gives a 8 % improvement in a TX intensive workload.
> > > >
> > > > (120 Mpps -> 130 Mpps on a Turin host, IDPF with 32 TX queues)
> > >
> > > According to CI this breaks a bunch of tests.
> > >
> > > https://netdev.bots.linux.dev/contest.html?branch=net-next-2025-11-10--12-00
> > >
> > > I think they all hit:
> > >
> > > [   20.682474][  T231] WARNING: CPU: 3 PID: 231 at ./include/net/sch_generic.h:843 __dev_xmit_skb+0x786/0x1550
> >
> > Oh well, I will add this in V2, thank you !
> >
> > diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> > index b76436ec3f4aa412bac1be3371f5c7c6245cc362..79501499dafba56271b9ebd97a8f379ffdc83cac
> > 100644
> > --- a/include/net/sch_generic.h
> > +++ b/include/net/sch_generic.h
> > @@ -841,7 +841,7 @@ static inline unsigned int qdisc_pkt_segs(const
> > struct sk_buff *skb)
> >         u32 pkt_segs = qdisc_skb_cb(skb)->pkt_segs;
> >
> >         DEBUG_NET_WARN_ON_ONCE(pkt_segs !=
> > -                              skb_is_gso(skb) ? skb_shinfo(skb)->gso_segs : 1);
> > +                       (skb_is_gso(skb) ? skb_shinfo(skb)->gso_segs : 1));
> >         return pkt_segs;
> >  }
>
> Hm, I think we need more..
>
> The non-debug workers are also failing and they have DEBUG_NET=n
>
> Looks like most of the non-debug tests are tunnel and bridge related.
> VxLAN, GRE etc.
>
> https://netdev.bots.linux.dev/contest.html?pass=0&branch=net-next-2025-11-10--12-00&executor=vmksft-forwarding

Nice !

tc_run()
   mini_qdisc_bstats_cpu_update()  //

I am not sure this path was setting qdisc_pkt_len() either...