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

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Message-ID: <CANn89iKqXjxPdFf5PKDZO1HTXXmbsHFFTDWGV9vAJgLVBdn_ag@mail.gmail.com>
Date: Mon, 10 Nov 2025 10:22:14 -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 10:05 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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...

 pkt_len was set in sch_handle_ingress(), I will add in V2 :

diff --git a/net/core/dev.c b/net/core/dev.c
index ac994974e2a81889fcc0a2e664edcdb7cfd0496d..10042139dbb054b9a93dfb019477a80263feb029
100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4435,7 +4435,7 @@ sch_handle_ingress(struct sk_buff *skb, struct
packet_type **pt_prev, int *ret,
                *pt_prev = NULL;
        }

-       qdisc_skb_cb(skb)->pkt_len = skb->len;
+       qdisc_pkt_len_segs_init(skb);
        tcx_set_ingress(skb, true);

        if (static_branch_unlikely(&tcx_needed_key)) {