linux-kernel - Re: [PATCH] locking/lockdep: Increase MAX_LOCKDEP

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Date:   Tue, 26 May 2020 23:27:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Qian Cai <cai@....pw>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/lockdep: Increase MAX_LOCKDEP_ENTRIES by half

On Tue, May 26, 2020 at 04:30:58PM -0400, Waiman Long wrote:
> On 5/26/20 3:56 PM, Peter Zijlstra wrote:
> > On Tue, May 26, 2020 at 02:58:50PM -0400, Qian Cai wrote:
> > 
> > > I still don't understand why reading all sysfs files on this system
> > > could increase that much, but here is the lockdep file after
> > > running sysfs read to see if you could spot anything obviously,
> > > 
> > > https://cailca.github.io/files/lockdep.txt
> > 00000000f011a2a5 OPS:      20 FD:   45 BD:    1 .+.+: kn->active#834
> > 
> > is that somewhere near the number of CPUs you have?
> > 
> > Anyway, there's very long "kn->active#..." chains in there, which seems
> > to suggest some annotation is all sorts of buggered.
> > 
> It is actually one active lock per instance of the kerfs_node structures.
> That means more than 800 sysfs files are accessed in some way. As we could
> have much more than 800 sysfs files in the system, we could easily overwhelm
> the lockdep tables if we really try to access all of them.

A lock per instance is crazy, that's not what lockdep is made for.
Fixing this seems like a far better idea than increasing the numbers.