Re: [RFC PATCH v4 06/40] mm: Demarcate and maintain pageblocks inregion-order in the zones' freelists

[Dave Hansen]

On 09/25/2013 04:14 PM, Srivatsa S. Bhat wrote:
> @@ -605,16 +713,22 @@ static inline void __free_one_page(struct page *page,
>  		buddy_idx = __find_buddy_index(combined_idx, order + 1);
>  		higher_buddy = higher_page + (buddy_idx - combined_idx);
>  		if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
> -			list_add_tail(&page->lru,
> -				&zone->free_area[order].free_list[migratetype].list);
> +
> +			/*
> +			 * Implementing an add_to_freelist_tail() won't be
> +			 * very useful because both of them (almost) add to
> +			 * the tail within the region. So we could potentially
> +			 * switch off this entire "is next-higher buddy free?"
> +			 * logic when memory regions are used.
> +			 */
> +			add_to_freelist(page, &area->free_list[migratetype]);
>  			goto out;
>  		}
>  	}

Commit 6dda9d55b says that this had some discrete performance gains.
It's a bummer that this deoptimizes it, and I think that (expected)
performance degredation at least needs to be referenced _somewhere_.

I also find it very hard to take code seriously which stuff like this:

> +#ifdef CONFIG_DEBUG_PAGEALLOC
> +		WARN(region->nr_free == 0, "%s: nr_free messed up\n", __func__);
> +#endif

nine times.
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