linux-kernel - Re: [PATCH next] mm/swap.c: reduce lock contention in lru_cache

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Date:   Thu, 26 Nov 2020 12:22:30 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yu Zhao <yuzhao@...gle.com>, Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Konstantin Khlebnikov <koct9i@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next] mm/swap.c: reduce lock contention in lru_cache_add

On 11/26/20 8:24 AM, Yu Zhao wrote:
> On Thu, Nov 26, 2020 at 02:39:03PM +0800, Alex Shi wrote:
>> 
>> 
>> 在 2020/11/26 下午12:52, Yu Zhao 写道:
>> >>   */
>> >>  void __pagevec_lru_add(struct pagevec *pvec)
>> >>  {
>> >> -	int i;
>> >> -	struct lruvec *lruvec = NULL;
>> >> +	int i, nr_lruvec;
>> >>  	unsigned long flags = 0;
>> >> +	struct page *page;
>> >> +	struct lruvecs lruvecs;
>> >>  
>> >> -	for (i = 0; i < pagevec_count(pvec); i++) {
>> >> -		struct page *page = pvec->pages[i];
>> >> +	nr_lruvec = sort_page_lruvec(&lruvecs, pvec);
>> > Simply looping pvec multiple times (15 at most) for different lruvecs
>> > would be better because 1) it requires no extra data structures and
>> > therefore has better cache locality (theoretically faster) 2) it only
>> > loops once when !CONFIG_MEMCG and !CONFIG_NUMA and therefore has no
>> > impact on Android and Chrome OS.
>> > 
>> 
>> With multiple memcgs, it do help a lot, I had gotten 30% grain on readtwice
>> case. but yes, w/o MEMCG and NUMA, it's good to keep old behavior. So 
>> would you like has a proposal for this?
> 
> Oh, no, I'm not against your idea. I was saying it doesn't seem
> necessary to sort -- a nested loop would just do the job given
> pagevec is small.

Yeah that could work. The worst case doesn't look nice (O(n^2)) but it should be 
rather rare to have pages from really multiple memcgs/nodes?

Maybe with the following change? Avoids going through the nulls if we got lucky 
(or have !MEMCG !NUMA).

> diff --git a/mm/swap.c b/mm/swap.c
> index cb3794e13b48..1d238edc2907 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -996,15 +996,26 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec)
>    */
>   void __pagevec_lru_add(struct pagevec *pvec)
>   {
> -	int i;
> +	int i, j;

int i, j, n;

>   	struct lruvec *lruvec = NULL;
>   	unsigned long flags = 0;
>   

n = pagevec_count(pvec);
>   	for (i = 0; i < pagevec_count(pvec); i++) {

    	for (i = 0; n; i++) {

>   		struct page *page = pvec->pages[i];
>   
> +		if (!page)
> +			continue;
> +
>   		lruvec = relock_page_lruvec_irqsave(page, lruvec, &flags);
> -		__pagevec_lru_add_fn(page, lruvec);

		n--;

> +
> +		for (j = i; j < pagevec_count(pvec); j++) {
> +			if (page_to_nid(pvec->pages[j]) != page_to_nid(page) ||
> +			    page_memcg(pvec->pages[j]) != page_memcg(page))
> +				continue;
> +
> +			__pagevec_lru_add_fn(pvec->pages[j], lruvec);
> +			pvec->pages[j] = NULL;

			n--;
> +		}
>   	}
>   	if (lruvec)
>   		unlock_page_lruvec_irqrestore(lruvec, flags);
>