Re: [PATCH v5 2/2] mm/page_alloc: remove software prefetching in __free_pages_core

[Vlastimil Babka]

On 10/5/18 10:10 AM, Arun KS wrote:
> They not only increase the code footprint, they actually make things
> slower rather than faster. Remove them as contemporary hardware doesn't
> need any hint.
> 
> Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>

Yeah, a tight loop with fixed stride is a trivial case for hw prefetcher.

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/page_alloc.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7ab5274..90db431 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1258,14 +1258,10 @@ void __free_pages_core(struct page *page, unsigned int order)
>  	struct page *p = page;
>  	unsigned int loop;
>  
> -	prefetchw(p);
> -	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
> -		prefetchw(p + 1);
> +	for (loop = 0; loop < nr_pages ; loop++, p++) {
>  		__ClearPageReserved(p);
>  		set_page_count(p, 0);
>  	}
> -	__ClearPageReserved(p);
> -	set_page_count(p, 0);
>  
>  	page_zone(page)->managed_pages += nr_pages;
>  	set_page_refcounted(page);
>