Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

[Breno Leitao <leitao@debian.org>]

Hello Waiman, Boqun,

On Fri, Mar 21, 2025 at 02:30:49AM -0700, Breno Leitao wrote:
> lockdep_unregister_key() is called from critical code paths, including
> sections where rtnl_lock() is held. For example, when replacing a qdisc
> in a network device, network egress traffic is disabled while
> __qdisc_destroy() is called for every network queue.
> 
> If lockdep is enabled, __qdisc_destroy() calls lockdep_unregister_key(),
> which gets 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 0x1: mq
>     real    0m13.195s
>     user    0m0.001s
>     sys     0m2.746s
> 
> During this time, network egress is completely frozen while waiting for
> RCU synchronization.
> 
> Use synchronize_rcu_expedited() instead to minimize the impact on
> critical operations like network connectivity changes.
> 
> This improves 10x the function call to tc, when replacing the qdisc for
> a network card.
> 
>    # time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
>      real     0m1.789s
>      user     0m0.000s
>      sys      0m1.613s

Can I have this landed as a workaround for the problem above, while
hazard pointers doesn't get merged?

This is affecting some systems that runs the Linus' upstream kernel with
some debug flags enabled, and I would like to have they unblocked.

Once hazard pointer lands, this will be reverted. Is this a fair
approach?

Thanks for your help,
--breno