Re: [PATCH v9 2/8] mm: Adjust shuffle code to allow for future coalescing

Kirill A. Shutemov
2019-09-09T09:47:06Z

On Sat, Sep 07, 2019 at 10:25:20AM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> Move the head/tail adding logic out of the shuffle code and into the
> __free_one_page function since ultimately that is where it is really
> needed anyway. By doing this we should be able to reduce the overhead
> and can consolidate all of the list addition bits in one spot.
>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
> include/linux/mmzone.h | 12 --------
> mm/page_alloc.c | 70 +++++++++++++++++++++++++++---------------------
> mm/shuffle.c | 9 +-----
> mm/shuffle.h | 12 ++++++++
> 4 files changed, 53 insertions(+), 50 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index bda20282746b..125f300981c6 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -116,18 +116,6 @@ static inline void add_to_free_area_tail(struct page *page, struct free_area *ar
> area->nr_free++;
> }
>
> -#ifdef CONFIG_SHUFFLE_PAGE_ALLOCATOR
> -/* Used to preserve page allocation order entropy */
> -void add_to_free_area_random(struct page *page, struct free_area *area,
> - int migratetype);
> -#else
> -static inline void add_to_free_area_random(struct page *page,
> - struct free_area *area, int migratetype)
> -{
> - add_to_free_area(page, area, migratetype);
> -}
> -#endif
> -
> /* Used for pages which are on another list */
> static inline void move_to_free_area(struct page *page, struct free_area *area,
> int migratetype)

Looks like add_to_free_area() and add_to_free_area_tail() can be moved to
mm/page_alloc.c as all users are there now. And the same for struct
free_area definition (but not declaration).

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c5d62f1c2851..4e4356ba66c7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -878,6 +878,36 @@ static inline struct capture_control *task_capc(struct zone *zone)
> #endif /* CONFIG_COMPACTION */
>
> /*
> + * If this is not the largest possible page, check if the buddy
> + * of the next-highest order is free. If it is, it's possible
> + * that pages are being freed that will coalesce soon. In case,
> + * that is happening, add the free page to the tail of the list
> + * so it's less likely to be used soon and more likely to be merged
> + * as a higher order page
> + */
> +static inline bool
> +buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
> + struct page *page, unsigned int order)
> +{
> + struct page *higher_page, *higher_buddy;
> + unsigned long combined_pfn;
> +
> + if (order >= MAX_ORDER - 2)
> + return false;
> +
> + if (!pfn_valid_within(buddy_pfn))
> + return false;
> +
> + combined_pfn = buddy_pfn & pfn;
> + higher_page = page + (combined_pfn - pfn);
> + buddy_pfn = __find_buddy_pfn(combined_pfn, order + 1);
> + higher_buddy = higher_page + (buddy_pfn - combined_pfn);
> +
> + return pfn_valid_within(buddy_pfn) &&
> + page_is_buddy(higher_page, higher_buddy, order + 1);
> +}

Okay, that's much easier to read.

> +
> +/*
> * Freeing function for a buddy system allocator.
> *
> * The concept of a buddy system is to maintain direct-mapped table
> @@ -906,11 +936,12 @@ static inline void __free_one_page(struct page *page,
> struct zone *zone, unsigned int order,
> int migratetype)
> {
> - unsigned long combined_pfn;
> + struct capture_control *capc = task_capc(zone);
> unsigned long uninitialized_var(buddy_pfn);
> - struct page *buddy;
> + unsigned long combined_pfn;
> + struct free_area *area;
> unsigned int max_order;
> - struct capture_control *capc = task_capc(zone);
> + struct page *buddy;
>
> max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
>
> @@ -979,35 +1010,12 @@ static inline void __free_one_page(struct page *page,
> done_merging:
> set_page_order(page, order);
>
> - /*
> - * If this is not the largest possible page, check if the buddy
> - * of the next-highest order is free. If it is, it's possible
> - * that pages are being freed that will coalesce soon. In case,
> - * that is happening, add the free page to the tail of the list
> - * so it's less likely to be used soon and more likely to be merged
> - * as a higher order page
> - */
> - if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)
> - && !is_shuffle_order(order)) {
> - struct page *higher_page, *higher_buddy;
> - combined_pfn = buddy_pfn & pfn;
> - higher_page = page + (combined_pfn - pfn);
> - buddy_pfn = __find_buddy_pfn(combined_pfn, order + 1);
> - higher_buddy = higher_page + (buddy_pfn - combined_pfn);
> - if (pfn_valid_within(buddy_pfn) &&
> - page_is_buddy(higher_page, higher_buddy, order + 1)) {
> - add_to_free_area_tail(page, &zone->free_area[order],
> - migratetype);
> - return;
> - }
> - }
> -
> - if (is_shuffle_order(order))
> - add_to_free_area_random(page, &zone->free_area[order],
> - migratetype);
> + area = &zone->free_area[order];
> + if (is_shuffle_order(order) ? shuffle_pick_tail() :
> + buddy_merge_likely(pfn, buddy_pfn, page, order))

Too loaded condition to my taste. Maybe

bool to_tail;
...
if (is_shuffle_order(order))
to_tail = shuffle_pick_tail();
else if (buddy_merge_likely(pfn, buddy_pfn, page, order))
to_tail = true;
else
to_tail = false;

if (to_tail)
add_to_free_area_tail(page, area, migratetype);
else
add_to_free_area(page, area, migratetype);

> + add_to_free_area_tail(page, area, migratetype);
> else
> - add_to_free_area(page, &zone->free_area[order], migratetype);
> -
> + add_to_free_area(page, area, migratetype);
> }
>
> /*
> diff --git a/mm/shuffle.c b/mm/shuffle.c
> index 9ba542ecf335..345cb4347455 100644
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -4,7 +4,6 @@
> #include <linux/mm.h>
> #include <linux/init.h>
> #include <linux/mmzone.h>
> -#include <linux/random.h>
> #include <linux/moduleparam.h>
> #include "internal.h"
> #include "shuffle.h"

Why do you move #include <linux/random.h> from .c to .h?
It's not obvious to me.

> @@ -190,8 +189,7 @@ struct batched_bit_entropy {
>
> static DEFINE_PER_CPU(struct batched_bit_entropy, batched_entropy_bool);
>
> -void add_to_free_area_random(struct page *page, struct free_area *area,
> - int migratetype)
> +bool __shuffle_pick_tail(void)
> {
> struct batched_bit_entropy *batch;
> unsigned long entropy;
> @@ -213,8 +211,5 @@ void add_to_free_area_random(struct page *page, struct free_area *area,
> batch->position = position;
> entropy = batch->entropy_bool;
>
> - if (1ul & (entropy >> position))
> - add_to_free_area(page, area, migratetype);
> - else
> - add_to_free_area_tail(page, area, migratetype);
> + return 1ul & (entropy >> position);
> }
> diff --git a/mm/shuffle.h b/mm/shuffle.h
> index 777a257a0d2f..0723eb97f22f 100644
> --- a/mm/shuffle.h
> +++ b/mm/shuffle.h
> @@ -3,6 +3,7 @@
> #ifndef _MM_SHUFFLE_H
> #define _MM_SHUFFLE_H
> #include <linux/jump_label.h>
> +#include <linux/random.h>
>
> /*
> * SHUFFLE_ENABLE is called from the command line enabling path, or by
> @@ -22,6 +23,7 @@ enum mm_shuffle_ctl {
> DECLARE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
> extern void page_alloc_shuffle(enum mm_shuffle_ctl ctl);
> extern void __shuffle_free_memory(pg_data_t *pgdat);
> +extern bool __shuffle_pick_tail(void);
> static inline void shuffle_free_memory(pg_data_t *pgdat)
> {
> if (!static_branch_unlikely(&page_alloc_shuffle_key))
> @@ -43,6 +45,11 @@ static inline bool is_shuffle_order(int order)
> return false;
> return order >= SHUFFLE_ORDER;
> }
> +
> +static inline bool shuffle_pick_tail(void)
> +{
> + return __shuffle_pick_tail();
> +}

I don't see a reason in __shuffle_pick_tail() existing if you call it
unconditionally.

> #else
> static inline void shuffle_free_memory(pg_data_t *pgdat)
> {
> @@ -60,5 +67,10 @@ static inline bool is_shuffle_order(int order)
> {
> return false;
> }
> +
> +static inline bool shuffle_pick_tail(void)
> +{
> + return false;
> +}
> #endif
> #endif /* _MM_SHUFFLE_H */
>
>

--
Kirill A. Shutemov