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

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Message-ID: <20191213184759.GH2844@hirez.programming.kicks-ass.net>
Date:   Fri, 13 Dec 2019 19:47:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.