Re: [PATCH v2] mm/page_alloc: optimize find_suitable_fallback() and fallbacks array

[Zi Yan]

On 9 Feb 2023, at 5:11, Yajun Deng wrote:

> There is no need to execute the next loop if it not return in the first
> loop. So add a break at the end of the loop.

Can you explain why? If it is the case, MIGRATE_UNMOVABLE cannot fall back
to MIGRATE_MOVABLE? And MIGRATE_MOVABLE cannot fall back to MIGRATE_UNMOVABLE?
And MIGRATE_RECLAIMABLE cannot fall back to MIGRATE_MOVABLE?

>
> At the same time, add !migratetype_is_mergeable() before the loop and
> reduce the first index size from MIGRATE_TYPES to MIGRATE_PCPTYPES in
> fallbacks array.

You sent a patch: https://lore.kernel.org/all/20230203100132.1627787-1-yajun.deng@xxxxxxxxx/T/#u, why not squash this one into that? Why do
we need two separate small patches working on the same code?

Thanks.
>
> Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  include/linux/mmzone.h |  2 +-
>  mm/page_alloc.c        | 11 +++++------
>  2 files changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index ab94985ee7d9..0a817b8c7fb2 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -85,7 +85,7 @@ static inline bool is_migrate_movable(int mt)
>   * Check whether a migratetype can be merged with another migratetype.
>   *
>   * It is only mergeable when it can fall back to other migratetypes for
> - * allocation. See fallbacks[MIGRATE_TYPES][3] in page_alloc.c.
> + * allocation. See fallbacks[][] array in page_alloc.c.
>   */
>  static inline bool migratetype_is_mergeable(int mt)
>  {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1113483fa6c5..536e8d838fb5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2603,7 +2603,7 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>   *
>   * The other migratetypes do not have fallbacks.
>   */
> -static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {
> +static int fallbacks[MIGRATE_PCPTYPES][MIGRATE_PCPTYPES - 1] = {
>  	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE   },
>  	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
>  	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
> @@ -2861,7 +2861,7 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
>  	int i;
>  	int fallback_mt;
>
> -	if (area->nr_free == 0)
> +	if (area->nr_free == 0 || !migratetype_is_mergeable(migratetype))
>  		return -1;
>
>  	*can_steal = false;
> @@ -2873,11 +2873,10 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
>  		if (can_steal_fallback(order, migratetype))
>  			*can_steal = true;
>
> -		if (!only_stealable)
> -			return fallback_mt;
> -
> -		if (*can_steal)
> +		if (!only_stealable || *can_steal)
>  			return fallback_mt;
> +		else
> +			break;
>  	}
>
>  	return -1;
> -- 
> 2.25.1

--
Best Regards,
Yan, Zi

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