Re: [PATCH v3 2/2] lru: allow large batched add large folio to lru list

["Yin, Fengwei" <fengwei.yin@intel.com>]

Hi Matthew,

On 4/30/2023 6:35 AM, Matthew Wilcox wrote:
> On Sat, Apr 29, 2023 at 04:27:59PM +0800, Yin Fengwei wrote:
>> @@ -22,6 +23,7 @@ struct address_space;
>>  struct pagevec {
>>  	unsigned char nr;
>>  	bool percpu_pvec_drained;
>> +	unsigned short nr_pages;
> 
> I still don't like storing nr_pages in the pagevec/folio_batch.
> 

What about the change like following:

diff --git a/mm/swap.c b/mm/swap.c
index 57cb01b042f6..5e7e9c0734ab 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -228,8 +228,10 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
 static void folio_batch_add_and_move(struct folio_batch *fbatch,
                struct folio *folio, move_fn_t move_fn)
 {
-       if (folio_batch_add(fbatch, folio) && !folio_test_large(folio) &&
-           !lru_cache_disabled())
+       int nr_pages = folio_nr_pages(folio);
+
+       if (folio_batch_add(fbatch, folio) && !lru_cache_disabled() &&
+           (!folio_test_large(folio) || (nr_pages <= (PAGEVEC_SIZE + 1))))
                return;
        folio_batch_move_lru(fbatch, move_fn);
 }


I did testing about the lru lock contention with different folio size
with will-it-scale + deferred queue lock contention mitigated:
  - If large folio size is 16K (order 2), the lru lock takes 64.31% cpu runtime
  - If large folio size is 64K (order 4), the lru lock takes 24.24% cpu runtime
This is as our expectation: The larger size of folio, the less lru lock
contention.

It's acceptable to not batched operate on large folio which is large
enough. PAGEVEC_SIZE + 1 is chosen here based on following reasons:
  - acceptable max memory size per batch: 15 x 16 x 4096 = 983040 bytes
  - the folios with size larger than it will not apply batched operation.
    But the lru lock contention is not high already.


I collected data with lru contention when run will-it-scale.page_fault1:

folio with order 2:
  Without the change:
  -   64.31%     0.23%  page_fault1_pro  [kernel.kallsyms]           [k] folio_lruvec_lock_irqsave
     + 64.07% folio_lruvec_lock_irqsave

  With the change:
  -   21.55%     0.21%  page_fault1_pro  [kernel.kallsyms]           [k] folio_lruvec_lock_irqsave
     + 21.34% folio_lruvec_lock_irqsave

folio with order 4:
  Without the change:
  -   24.24%     0.15%  page_fault1_pro  [kernel.kallsyms]           [k] folio_lruvec_lock_irqsave
     + 24.09% folio_lruvec_lock_irqsave

  With the change:
  -   2.20%     0.09%  page_fault1_pro  [kernel.kallsyms]            [k] folio_lruvec_lock_irqsave
     + 2.11% folio_lruvec_lock_irqsave

folio with order 5:
  -   8.21%     0.16%  page_fault1_pro  [kernel.kallsyms]  [k] folio_lruvec_lock_irqsave
     + 8.05% folio_lruvec_lock_irqsave


Regards
Yin, Fengwei