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From: Kent Overstreet <kent.overstreet@linux.dev>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-cachefs@redhat.com, dhowells@redhat.com, gfs2@lists.linux.dev,
	dm-devel@lists.linux.dev, linux-security-module@vger.kernel.org,
	selinux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 04/11] lib/dlock-list: Make sibling CPUs share the same
 linked list
Message-ID: <20231207043129.cswpxpwow43b35lj@moria.home.lan>
References: <20231206060629.2827226-1-david@fromorbit.com>
 <20231206060629.2827226-5-david@fromorbit.com>
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On Wed, Dec 06, 2023 at 05:05:33PM +1100, Dave Chinner wrote:
> From: Waiman Long <longman@redhat.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.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

We badly need this done in a more generic way. Besides shared caches,
I've done a bunch of percpu algorithms where "amount of x stranded on
percpu lists" is a major consideration and this would be preferable over
percpu lists (including in fs/aio.c).