Re: [PATCH v9 07/20] mm/lru: introduce TestClearPageLRU

[Andrew Morton]

On Mon,  2 Mar 2020 19:00:17 +0800 Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote:

> Combined PageLRU check and ClearPageLRU into one function by new
> introduced func TestClearPageLRU. This function will be used as page
> isolation precondition.
> 
> No functional change yet.
> 
> ...
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2588,9 +2588,8 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>  		pgdat = page_pgdat(page);
>  		spin_lock_irq(&pgdat->lru_lock);
>  
> -		if (PageLRU(page)) {
> +		if (TestClearPageLRU(page)) {
>  			lruvec = mem_cgroup_page_lruvec(page, pgdat);
> -			ClearPageLRU(page);
>  			del_page_from_lru_list(page, lruvec, page_lru(page));
>  		} else

The code will now get exclusive access of the page->flags cacheline and
will dirty that cacheline, even for !PageLRU() pages.  What is the
performance impact of this?