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

[Vlastimil Babka]

On 11/26/20 12:22 PM, Vlastimil Babka wrote:
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?

However, Matthew wanted to increase pagevec size [1] and once 15^2 becomes 63^2, 
it starts to be somewhat more worrying.

[1] https://lore.kernel.org/linux-mm/20201105172651.2455-1-wi...@infradead.org/

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);