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

From: "Kirill A. Shutemov" <[email-masked]>
Date: Mon, 9 Sep 2019 12:47:00 +0300

On Sat, Sep 07, 2019 at 10:25:20AM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: Alexander Duyck <[email-masked]>
> ---
>  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

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel