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

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Date: Tue, 23 Apr 2024 11:21:23 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Davide Caratti <dcaratti@...hat.com>, Eric Dumazet
 <edumazet@...gle.com>,  Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan
 <tariqt@...dia.com>
Cc: 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 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? 

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