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

Kent Overstreet
2023-12-07T04:31:48Z

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).