linux-kernel - Re: [PATCH 04/11] lib/dlock-list: Make sibling CPUs share the same linked list

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Message-ID: <20231207064918.GZ1674809@ZenIV>
Date:   Thu, 7 Dec 2023 06:49:18 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-cachefs@...hat.com, dhowells@...hat.com,
        gfs2@...ts.linux.dev, dm-devel@...ts.linux.dev,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/11] lib/dlock-list: Make sibling CPUs share the same
 linked list

On Wed, Dec 06, 2023 at 05:05:33PM +1100, Dave Chinner wrote:
> From: Waiman Long <longman@...hat.com>
> 
> The dlock list needs one list for each of the CPUs available. However,
> for sibling CPUs, they are sharing the L2 and probably L1 caches
> too. As a result, there is not much to gain in term of avoiding
> cacheline contention while increasing the cacheline footprint of the
> L1/L2 caches as separate lists may need to be in the cache.
> 
> This patch makes all the sibling CPUs share the same list, thus
> reducing the number of lists that need to be maintained in each
> dlock list without having any noticeable impact on performance. It
> also improves dlock list iteration performance as fewer lists need
> to be iterated.

Probably a dumb question, but... "available" != "possible"; the code
actually goes for the latter, which avoids nasty questions about
CPU hotplug interations.  Is the sibling relation on CPUs unchanging
on CPU hotplug?