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

Kent Overstreet
2023-12-07T05:43:12Z

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.

Seems Waiman was missed on the CC

it looks like there's some duplication of this with list_lru
functionality - similar list-sharded-by-node idea.

list_lru does the sharding by page_to_nid() of the item, which saves a
pointer and allows just using a list_head in the item. OTOH, it's less
granular than what dlock-list is doing?

I think some attempt ought to be made to factor out the common ideas
hear; perhaps reworking list_lru to use this thing, and I hope someone
has looked at the page_nid idea vs. dlock_list using the current core.

But it's nice and small, and I'd like to use it elsewhere.

Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>