Message-ID: <b112394d-7efa-c6f9-bbef-a73c501ff02c@redhat.com>
Date:   Fri, 27 Jan 2023 09:26:16 -0500
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Subject: Re: BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
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To:     Boqun Feng <boqun.feng@gmail.com>,
        Chris Murphy <lists@colorremedies.com>
Cc:     =?UTF-8?B?0JzQuNGF0LDQuNC7INCT0LDQstGA0LjQu9C+0LI=?= 
        <mikhail.v.gavrilov@gmail.com>, David Sterba <dsterba@suse.cz>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>
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 <Y9K6m5USnON/19GT@boqun-archlinux>
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From:   Waiman Long <longman@redhat.com>
In-Reply-To: <Y9NN9CFWc40oxmzP@boqun-archlinux>
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On 1/26/23 23:07, Boqun Feng wrote:
> On Thu, Jan 26, 2023 at 10:37:56PM -0500, Chris Murphy wrote:
>>
>> On Thu, Jan 26, 2023, at 7:20 PM, Waiman Long wrote:
>>> On 1/26/23 17:42, Mikhail Gavrilov wrote:
>>>>> I'm not sure whether these options are better than just increasing the
>>>>> number, maybe to unblock your ASAP, you can try make it 30 and make sure
>>>>> you have large enough memory to test.
>>>> About just to increase the LOCKDEP_CHAINS_BITS by 1. Where should this
>>>> be done? In vanilla kernel on kernel.org? In a specific distribution?
>>>> or the user must rebuild the kernel himself? Maybe increase
>>>> LOCKDEP_CHAINS_BITS by 1 is most reliable solution, but it difficult
>>>> to distribute to end users because the meaning of using packaged
>>>> distributions is lost (user should change LOCKDEP_CHAINS_BITS in
>>>> config and rebuild the kernel by yourself).
>>> Note that lockdep is typically only enabled in a debug kernel shipped by
>>> a distro because of the high performance overhead. The non-debug kernel
>>> doesn't have lockdep enabled. When LOCKDEP_CHAINS_BITS isn't big enough
>>> when testing on the debug kernel, you can file a ticket to the distro
>>> asking for an increase in CONFIG_LOCKDEP_CHAIN_BITS. Or you can build
>>> your own debug kernel with a bigger CONFIG_LOCKDEP_CHAIN_BITS.
>> Fedora bumped CONFIG_LOCKDEP_CHAINS_BITS=17 to 18 just 6 months ago for debug kernels.
>> https://gitlab.com/cki-project/kernel-ark/-/merge_requests/1921
>>
>> If 19 the recommended value I don't mind sending an MR for it. But if
>> the idea is we're going to be back here talking about bumping it to 20
>> in six months, I'd like to avoid that.
>>
> How about a boot parameter then?

A boot parameter doesn't work for a statically allocated array which is 
determined at compile time. Dynamic memory allocation isn't enabled yet 
at early boot when lockdep will be used.

Cheers,
Longman