Re: [PATCH 4/5] locking/lockdep: Reuse free chain_hlocks entries

From: Peter Zijlstra
Date: Fri Dec 13 2019 - 13:48:08 EST

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.