netdev - Re: tc: network egress frozen during qdisc update with debug kernel

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Message-ID: <CAM0EoM=NJEeCcDdJ5kp0e8iyRG1LmvfzvBVpb2Mq5zP+QcvmMg@mail.gmail.com>
Date: Wed, 19 Mar 2025 17:05:08 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Breno Leitao <leitao@...ian.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, longman@...hat.com, bvanassche@....org, 
	Eric Dumazet <edumazet@...gle.com>, kuba@...nel.org, xiyou.wangcong@...il.com, 
	jiri@...nulli.us, kuniyu@...zon.com, rcu@...r.kernel.org, 
	kasan-dev@...glegroups.com, netdev@...r.kernel.org
Subject: Re: tc: network egress frozen during qdisc update with debug kernel

On Wed, Mar 19, 2025 at 2:12 PM Breno Leitao <leitao@...ian.org> wrote:
>
> On Wed, Mar 19, 2025 at 09:05:07AM -0700, Paul E. McKenney wrote:
>
> > > I think we should redesign lockdep_unregister_key() to work on a separately
> > > allocated piece of memory,
> > > then use kfree_rcu() in it.
> > >
> > > Ie not embed a "struct lock_class_key" in the struct Qdisc, but a pointer to
> > >
> > > struct ... {
> > >      struct lock_class_key;
> > >      struct rcu_head  rcu;
> > > }
> >
> > Works for me!
>
> I've tested a different approach, using synchronize_rcu_expedited()
> instead of synchronize_rcu(), given how critical this function is
> called, and the command performance improves dramatically.
>
> This approach has some IPI penalties, but, it might be quicker to review
> and get merged, mitigating the network issue.
>
> Does it sound a bad approach?
>
> Date:   Wed Mar 19 10:23:56 2025 -0700
>
>     lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization
>
>     lockdep_unregister_key() is called from critical code paths, including
>     sections where rtnl_lock() is held. When replacing a qdisc in a network
>     device, network egress traffic is disabled while __qdisc_destroy() is
>     called for every queue. This function calls lockdep_unregister_key(),
>     which was blocked waiting for synchronize_rcu() to complete.
>
>     For example, a simple tc command to replace a qdisc could take 13
>     seconds:
>
>       # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1234: mq
>         real    0m13.195s
>         user    0m0.001s
>         sys     0m2.746s
>

Could you please add the "after your change"  output as well?

cheers,
jamal