linux-kernel - Re: [PATCH 4/5] locking/lockdep: Reuse free chain

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Message-ID: <2763959e-b0e9-a8cd-3468-232d128c8260@redhat.com>
Date:   Fri, 13 Dec 2019 15:08:51 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH 4/5] locking/lockdep: Reuse free chain_hlocks entries

On 12/13/19 1:47 PM, Peter Zijlstra wrote:
> On Fri, Dec 13, 2019 at 01:35:05PM -0500, Waiman Long wrote:
>> On 12/13/19 1:12 PM, Peter Zijlstra wrote:
>>>> In this way, the wasted space will be k bytes where k is the number of
>>>> 1-entry chains. I don't think merging adjacent blocks will be that
>>>> useful at this point. We can always add this capability later on if it
>>>> is found to be useful.
>>> I'm thinking 1 entry isn't much of a chain. My brain is completely fried
>>> atm, but are we really storing single entry 'chains' ? It seems to me we
>>> could skip that.
>>>
>> Indeed, the current code can produce a 1-entry chain. I also thought
>> that a chain had to be at least 2 entries. I got tripped up assuming
>> that. It could be a bug somewhere that allow a 1-entry chain to happen,
>> but I am not focusing on that right now.
> If we need the minimum 2 entry granularity, it might make sense to spend
> a little time on that. If we can get away with single entry markers,
> then maybe write a comment so we'll not forget about it.
>
I will take a look at why an 1-entry chain happes and see if it is a bug
that need to be fixed.

Cheers,
Longman