Re: [PATCH 4/4] mm/page_alloc : relieve zone->lock's pressure for memory free

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

>   Move the {__mod_zone_page_state, pages_scanned, clear zone's flags}
> out the guard region of the spinlock to relieve the pressure for memory free.
> 
> Signed-off-by: Huang Shijie <shijie8@gmail.com>
> ---
>  mm/page_alloc.c |   27 +++++++++++++++++----------
>  1 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 290dfc3..1c7e32e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -530,12 +530,9 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  {
>  	int migratetype = 0;
>  	int batch_free = 0;
> +	int free_ok = 0;
>  
>  	spin_lock(&zone->lock);
> -	zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
> -	zone->pages_scanned = 0;
> -
> -	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
>  	while (count) {
>  		struct page *page;
>  		struct list_head *list;
> @@ -558,23 +555,33 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			page = list_entry(list->prev, struct page, lru);
>  			/* must delete as __free_one_page list manipulates */
>  			list_del(&page->lru);
> -			__free_one_page(page, zone, 0, migratetype);
> +			free_ok += __free_one_page(page, zone, 0, migratetype);
>  			trace_mm_page_pcpu_drain(page, 0, migratetype);
>  		} while (--count && --batch_free && !list_empty(list));
>  	}
>  	spin_unlock(&zone->lock);
> +
> +	if (likely(free_ok)) {
> +		zone_clear_flag(zone, ZONE_ALL_UNRECLAIMABLE);
> +		zone->pages_scanned = 0;
> +		__mod_zone_page_state(zone, NR_FREE_PAGES, free_ok);
> +	}
>  }

No. To introduce new branch have big performance degression risk. I don't
think this patch improve performance.

Why can't we remove this "if (free_ok)" statement?



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