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-willy@xxxxxxxxxxxxx/

> 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);