Re: [PATCH 02/28] mm, page_alloc: Use new PageAnonHead helper in the free page fast path

[Vlastimil Babka]

On 04/15/2016 10:58 AM, Mel Gorman wrote:
> The PageAnon check always checks for compound_head but this is a relatively
> expensive check if the caller already knows the page is a head page. This
> patch creates a helper and uses it in the page free path which only operates
> on head pages.
>
> With this patch and "Only check PageCompound for high-order pages", the
> performance difference on a page allocator microbenchmark is;
[...]
> There is a sizable boost to the free allocator performance. While there
> is an apparent boost on the allocation side, it's likely a co-incidence
> or due to the patches slightly reducing cache footprint.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

This again highlights the cost of thp rework due to those 
compound_head() calls, and a more general solution would benefit other 
places, but this can always be converted later if such solution happens.

> ---
>   include/linux/page-flags.h | 7 ++++++-
>   mm/page_alloc.c            | 2 +-
>   2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f4ed4f1b0c77..ccd04ee1ba2d 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -371,10 +371,15 @@ PAGEFLAG(Idle, idle, PF_ANY)
>   #define PAGE_MAPPING_KSM	2
>   #define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM)
>
> +static __always_inline int PageAnonHead(struct page *page)
> +{
> +	return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
> +}
> +
>   static __always_inline int PageAnon(struct page *page)
>   {
>   	page = compound_head(page);
> -	return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
> +	return PageAnonHead(page);
>   }
>
>   #ifdef CONFIG_KSM
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5d205bcfe10d..6812de41f698 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1048,7 +1048,7 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>   			bad += free_pages_check(page + i);
>   		}
>   	}
> -	if (PageAnon(page))
> +	if (PageAnonHead(page))
>   		page->mapping = NULL;
>   	bad += free_pages_check(page);
>   	if (bad)