Re: [PATCH v1 1/6] mm: Adjust shuffle code to allow for future coalescing

David Hildenbrand
2019-06-25T07:56:05Z

On 20.06.19 00:33, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> This patch is meant to 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.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
> include/linux/mmzone.h | 12 --------
> mm/page_alloc.c | 70 +++++++++++++++++++++++++++---------------------
> mm/shuffle.c | 24 ----------------
> mm/shuffle.h | 35 ++++++++++++++++++++++++
> 4 files changed, 74 insertions(+), 67 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 427b79c39b3c..4c07af2cfc2f 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)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f4651a09948c..ec344ce46587 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -830,6 +830,36 @@ 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 (is_shuffle_order(order) || order >= (MAX_ORDER - 2))

My intuition tells me you can drop the () around "MAX_ORDER - 2"

> + return false;

Guess the "is_shuffle_order(order)" check should rather be performed by
the caller, before calling this function.

> +
> + 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);
> +}
> +
> +/*
> * Freeing function for a buddy system allocator.
> *
> * The concept of a buddy system is to maintain direct-mapped table
> @@ -858,11 +888,12 @@ 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);
>
> @@ -931,35 +962,12 @@ 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 (buddy_merge_likely(pfn, buddy_pfn, page, order) ||
> + is_shuffle_tail_page(order))
> + add_to_free_area_tail(page, area, migratetype);

I would prefer here something like

if (is_shuffle_order(order)) {
if (add_shuffle_order_to_tail(order))
add_to_free_area_tail(page, area, migratetype);
else
add_to_free_area(page, area, migratetype);
} else if (buddy_merge_likely(pfn, buddy_pfn, page, order)) {
add_to_free_area_tail(page, area, migratetype);
} else {
add_to_free_area(page, area, migratetype);
}

dropping "is_shuffle_order()" from buddy_merge_likely()

Especially, the name "is_shuffle_tail_page(order)" suggests that you are
passing a page.

> 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 3ce12481b1dc..55d592e62526 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"
> @@ -182,26 +181,3 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
> for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
> shuffle_zone(z);
> }
> -
> -void add_to_free_area_random(struct page *page, struct free_area *area,
> - int migratetype)
> -{
> - static u64 rand;
> - static u8 rand_bits;
> -
> - /*
> - * The lack of locking is deliberate. If 2 threads race to
> - * update the rand state it just adds to the entropy.
> - */
> - if (rand_bits == 0) {
> - rand_bits = 64;
> - rand = get_random_u64();
> - }
> -
> - if (rand & 1)
> - add_to_free_area(page, area, migratetype);
> - else
> - add_to_free_area_tail(page, area, migratetype);
> - rand_bits--;
> - rand >>= 1;
> -}
> diff --git a/mm/shuffle.h b/mm/shuffle.h
> index 777a257a0d2f..3f4edb60a453 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
> @@ -43,6 +44,35 @@ static inline bool is_shuffle_order(int order)
> return false;
> return order >= SHUFFLE_ORDER;
> }
> +
> +static inline bool is_shuffle_tail_page(int order)
> +{
> + static u64 rand;
> + static u8 rand_bits;
> + u64 rand_old;
> +
> + if (!is_shuffle_order(order))
> + return false;
> +
> + /*
> + * The lack of locking is deliberate. If 2 threads race to
> + * update the rand state it just adds to the entropy.
> + */
> + if (rand_bits-- == 0) {
> + rand_bits = 64;
> + rand = get_random_u64();
> + }
> +
> + /*
> + * Test highest order bit while shifting our random value. This
> + * should result in us testing for the carry flag following the
> + * shift.
> + */
> + rand_old = rand;
> + rand <<= 1;
> +
> + return rand < rand_old;
> +}
> #else
> static inline void shuffle_free_memory(pg_data_t *pgdat)
> {
> @@ -60,5 +90,10 @@ static inline bool is_shuffle_order(int order)
> {
> return false;
> }
> +
> +static inline bool is_shuffle_tail_page(int order)
> +{
> + return false;
> +}
> #endif
> #endif /* _MM_SHUFFLE_H */
>

--

Thanks,

David / dhildenb