Re: [PATCH 3/3] mm: page_alloc: batch vmstat updates in expand()

[Vlastimil Babka]

On 3/27/24 8:01 PM, Johannes Weiner wrote:
> On Wed, Mar 27, 2024 at 09:54:01AM +0100, Vlastimil Babka wrote:
>> > @@ -1314,10 +1349,10 @@ static inline void expand(struct zone *zone, struct page *page,
>> >  		 * Corresponding page table entries will not be touched,
>> >  		 * pages will stay not present in virtual address space
>> >  		 */
>> > -		if (set_page_guard(zone, &page[size], high, migratetype))
>> > +		if (set_page_guard(zone, &page[size], high))
>> >  			continue;
>> >  
>> > -		add_to_free_list(&page[size], zone, high, migratetype);
>> > +		add_to_free_list(&page[size], zone, high, migratetype, false);
>> 
>> This is account_freepages() in the hot loop, what if we instead used
>> __add_to_free_list(), sum up nr_pages and called account_freepages() once
>> outside of the loop?
> 
> How about this on top of the series? Could be folded into
> mm-page_alloc-consolidate-free-page-accounting, but for credit and
> bisectability (just in case) I think stand-alone makes sense.
> 
> ---
> 
> From 361f5df28183d85c7718fe0b579438d3d58777be Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@xxxxxxxxxxx>
> Date: Wed, 27 Mar 2024 12:29:25 -0400
> Subject: [PATCH 3/3] mm: page_alloc: batch vmstat updates in expand()
> 
> expand() currently updates vmstat for every subpage. This is
> unnecessary, since they're all of the same zone and migratetype.
> 
> Count added pages locally, then do a single vmstat update.
> 
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/page_alloc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8987e8869f6d..13fe5c612fbe 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1341,6 +1341,7 @@ static inline void expand(struct zone *zone, struct page *page,
>  	int low, int high, int migratetype)
>  {
>  	unsigned long size = 1 << high;
> +	unsigned long nr_added = 0;
>  
>  	while (high > low) {
>  		high--;
> @@ -1356,9 +1357,11 @@ static inline void expand(struct zone *zone, struct page *page,
>  		if (set_page_guard(zone, &page[size], high))
>  			continue;
>  
> -		add_to_free_list(&page[size], zone, high, migratetype, false);
> +		__add_to_free_list(&page[size], zone, high, migratetype, false);
>  		set_buddy_order(&page[size], high);
> +		nr_added += size;
>  	}
> +	account_freepages(zone, nr_added, migratetype);
>  }
>  
>  static void check_new_page_bad(struct page *page)