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

Vlastimil Babka
2020-11-26T11:22:56Z

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);
>