netdev - Re: [PATCH net-next v2] net/sched: fix false lockdep warning on qdisc root lock

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Message-ID: <CANn89iKH9FQFjnkmSCX2qcjcvG2GZigT+hFgKEd6P4L5fvGmTA@mail.gmail.com>
Date: Tue, 23 Apr 2024 11:40:08 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Davide Caratti <dcaratti@...hat.com>, Saeed Mahameed <saeedm@...dia.com>, 
	Tariq Toukan <tariqt@...dia.com>, netdev@...r.kernel.org, renmingshuai@...wei.com, 
	jiri@...nulli.us, xiyou.wangcong@...il.com, xmu@...hat.com, 
	Christoph Paasch <cpaasch@...le.com>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Maxim Mikityanskiy <maxim@...valent.com>, Victor Nogueira <victor@...atatu.com>
Subject: Re: [PATCH net-next v2] net/sched: fix false lockdep warning on qdisc
 root lock

On Tue, Apr 23, 2024 at 11:21 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Thu, 2024-04-18 at 16:01 +0200, Davide Caratti wrote:
> > hello,
> >
> > On Thu, Apr 18, 2024 at 3:50 PM Davide Caratti <dcaratti@...hat.com> wrote:
> > >
> >
> > [...]
> >
> > > This happens when TC does a mirred egress redirect from the root qdisc of
> > > device A to the root qdisc of device B. As long as these two locks aren't
> > > protecting the same qdisc, they can be acquired in chain: add a per-qdisc
> > > lockdep key to silence false warnings.
> > > This dynamic key should safely replace the static key we have in sch_htb:
> > > it was added to allow enqueueing to the device "direct qdisc" while still
> > > holding the qdisc root lock.
> > >
> > > v2: don't use static keys anymore in HTB direct qdiscs (thanks Eric Dumazet)
> >
> > I didn't have the correct setup to test HTB offload, so any feedback
> > for the HTB part is appreciated. On a debug kernel the extra time
> > taken to register / de-register dynamic lockdep keys is very evident
> > (more when qdisc are created: the time needed for "tc qdisc add ..."
> > becomes an order of magnitude bigger, while the time for "tc qdisc del
> > ..." doubles).
>
> @Eric: why do you think the lockdep slowdown would be critical? We
> don't expect to see lockdep in production, right?

I think you missed one of my update, where I said this was absolutely ok.

https://lore.kernel.org/netdev/CANn89iJQZ5R=Cct494W0DbNXR3pxOj54zDY7bgtFFCiiC1abDg@mail.gmail.com/



>
> Enabling lockdep will defeat most/all cacheline optimization moving
> around all fields after a lock, performances should be significantly
> impacted anyway.
>
> WDYT?
>
> The HTB bits looks safe to me, but it would be great if someone @nvidia
> could actually test it (AFAICS mlx5 is the only user of such
> annotation).
>
> Thanks!
>
> Paolo
>