linux-kernel - Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

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Date:   Thu, 19 Nov 2020 00:31:59 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

On 2020/11/19 0:10, Peter Zijlstra wrote:
> On Wed, Nov 18, 2020 at 11:30:05PM +0900, Tetsuo Handa wrote:
>> The problem is that we can't know what exactly is consuming these resources.
>> My question is do you have a plan to make it possible to know what exactly is
>> consuming these resources.
> 
> I'm pretty sure it's in /proc/lockdep* somewhere.

OK. Then...

Dmitry, can you update syzkaller to dump /proc/lockdep* before terminating as
a crash as soon as encountering one of

  BUG: MAX_LOCKDEP_ENTRIES too low!
  BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
  BUG: MAX_LOCKDEP_CHAINS too low!
  BUG: MAX_LOCKDEP_KEYS too low!
  WARNING in print_bfs_bug

messages?

On 2020/09/16 21:14, Dmitry Vyukov wrote:
> On Wed, Sep 16, 2020 at 1:51 PM <peterz@...radead.org> wrote:
>>
>> 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).
> 
> Maybe on "BUG: MAX_LOCKDEP_CHAINS too low!" we should then aggregate,
> sort and show existing chains so that it's possible to identify if
> there are any worst offenders and who they are.
> 
> Currently we only have a hypothesis that there are some worst
> offenders vs lots of normal load. And we can't point fingers which
> means that, say, sysfs, or other maintainers won't be too inclined to
> fix anything.
> 
> If we would know for sure that lock class X is guilty. That would make
> the situation much more actionable.
>