Re: [PATCH next] mm/swap.c: reduce lock contention in lru_cache_add

[Yu Zhao]

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);