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

From: Alexander Duyck <[email-masked]>
Date: Mon, 09 Sep 2019 08:22:11 -0700

On Mon, 2019-09-09 at 12:47 +0300, Kirill A. Shutemov wrote:
> 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;

I can do that, although I would tweak this slightly and do something more
like:
        if (is_shuffle_order(order))
                to_tail = shuffle_pick_tail();
        else
                to_tail = buddy+_merge_likely(pfn, buddy_pfn, page, order);

> 
> 	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.

Because I had originally put the shuffle logic in an inline function. I
can undo that now as I when back to doing the randomness in the .c
sometime v5 I believe.

> > @@ -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.

That is for compilation purposes. The function is not used in the
shuffle_pick_tail below that always returns false.

> >  #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 */
> > 
> > 

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