linux-kernel - Re: [PATCH] lockdep: Speed up lockdep_unregister

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Message-ID: <798d0707-8f05-4ffd-9ee5-7d3945276ee8@redhat.com>
Date: Wed, 9 Jul 2025 09:57:44 -0400
From: Waiman Long <llong@...hat.com>
To: Breno Leitao <leitao@...ian.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>
Cc: aeh@...a.com, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 edumazet@...gle.com, jhs@...atatu.com, kernel-team@...a.com,
 Erik Lundgren <elundgren@...a.com>, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited
 RCU synchronization

On 7/9/25 6:00 AM, Breno Leitao wrote:
> 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

I am fine with this patch going in as a temporary workaround. Boqun, 
what do you think?

Cheers,
Longman