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

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Message-ID: <20201126072402.GA1047005@google.com>
Date:   Thu, 26 Nov 2020 00:24:02 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     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 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.

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;
 	struct lruvec *lruvec = NULL;
 	unsigned long flags = 0;
 
 	for (i = 0; i < pagevec_count(pvec); i++) {
 		struct page *page = pvec->pages[i];
 
+		if (!page)
+			continue;
+
 		lruvec = relock_page_lruvec_irqsave(page, lruvec, &flags);
-		__pagevec_lru_add_fn(page, lruvec);
+
+		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;
+		}
 	}
 	if (lruvec)
 		unlock_page_lruvec_irqrestore(lruvec, flags);