linux-kernel - Re: [PATCH v2] lockdep: Allow tuning tracing capacity constants.

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Date:   Wed, 16 Sep 2020 13:50:57 +0200
From:   peterz@...radead.org
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2] lockdep: Allow tuning tracing capacity constants.

On Wed, Sep 16, 2020 at 01:28:19PM +0200, Dmitry Vyukov wrote:
> On Fri, Sep 4, 2020 at 6:05 PM Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
> >
> > Hello. Can we apply this patch?
> >
> > This patch addresses top crashers for syzbot, and applying this patch
> > will help utilizing syzbot's resource for finding other bugs.
> 
> Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
> 
> Peter, do you still have concerns with this?

Yeah, I still hate it with a passion; it discourages thinking. A bad
annotation that blows up the lockdep storage, no worries, we'll just
increase this :/

IIRC the issue with syzbot is that the current sysfs annotation is
pretty terrible and generates a gazillion classes, and syzbot likes
poking at /sys a lot and thus floods the system.

I don't know enough about sysfs to suggest an alternative, and haven't
exactly had spare time to look into it either :/

Examples of bad annotations is getting every CPU a separate class, that
leads to nr_cpus! chains if CPUs arbitrarily nest (nr_cpus^2 if there's
only a single nesting level).