linux-kernel - Re: [RFC v7 11/19] lockdep: Fix recursive read lock related safe->unsafe detection

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Date:   Wed, 16 Sep 2020 17:11:59 -0400
From:   Qian Cai <cai@...hat.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Waiman Long <longman@...hat.com>
Subject: Re: [RFC v7 11/19] lockdep: Fix recursive read lock related
 safe->unsafe detection

On Thu, 2020-09-17 at 00:14 +0800, Boqun Feng wrote:
> Found a way to resolve this while still keeping the BFS. Every time when
> we want to enqueue a lock_list, we basically enqueue a whole dep list of
> entries from the previous lock_list, so we can use a trick here: instead
> enqueue all the entries, we only enqueue the first entry and we can
> fetch other silbing entries with list_next_or_null_rcu(). Patch as
> below, I also took the chance to clear the code up and add more
> comments. I could see this number (in /proc/lockdep_stats):
> 
> 	max bfs queue depth:                   201
> 
> down to (after apply this patch)
> 
> 	max bfs queue depth:                   61
> 
> with x86_64_defconfig along with lockdep and selftest configs.
> 
> Qian, could you give it a try?

It works fine as the number went down from around 3000 to 500 on our workloads.