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

Vlastimil Babka
2020-11-25T15:38:42Z

On 11/20/20 9:27 AM, Alex Shi wrote:
> The current relock logical will change lru_lock when found a new
> lruvec, so if 2 memcgs are reading file or alloc page at same time,
> they could hold the lru_lock alternately, and wait for each other for
> fairness attribute of ticket spin lock.
>
> This patch will sort that all lru_locks and only hold them once in
> above scenario. That could reduce fairness waiting for lock reget.
> Than, vm-scalability/case-lru-file-readtwice could get ~5% performance
> gain on my 2P*20core*HT machine.

Hm, once you sort the pages like this, it's a shame not to splice them
instead of more list_del() + list_add() iterations. update_lru_size()
could be also called once?

> Suggested-by: Konstantin Khlebnikov <koct9i@gmail.com>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> mm/swap.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 49 insertions(+), 8 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 490553f3f9ef..c787b38bf9c0 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -1009,24 +1009,65 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec)
> trace_mm_lru_insertion(page, lru);
> }
>
> +struct lruvecs {
> + struct list_head lists[PAGEVEC_SIZE];
> + struct lruvec *vecs[PAGEVEC_SIZE];
> +};
> +
> +/* Sort pvec pages on their lruvec */
> +int sort_page_lruvec(struct lruvecs *lruvecs, struct pagevec *pvec)
> +{
> + int i, j, nr_lruvec;
> + struct page *page;
> + struct lruvec *lruvec = NULL;
> +
> + lruvecs->vecs[0] = NULL;
> + for (i = nr_lruvec = 0; i < pagevec_count(pvec); i++) {
> + page = pvec->pages[i];
> + lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
> +
> + /* Try to find a same lruvec */
> + for (j = 0; j <= nr_lruvec; j++)
> + if (lruvec == lruvecs->vecs[j])
> + break;
> +
> + /* A new lruvec */
> + if (j > nr_lruvec) {
> + INIT_LIST_HEAD(&lruvecs->lists[nr_lruvec]);
> + lruvecs->vecs[nr_lruvec] = lruvec;
> + j = nr_lruvec++;
> + lruvecs->vecs[nr_lruvec] = 0;
> + }
> +
> + list_add_tail(&page->lru, &lruvecs->lists[j]);
> + }
> +
> + return nr_lruvec;
> +}
> +
> /*
> * Add the passed pages to the LRU, then drop the caller's refcount
> * on them. Reinitialises the caller's pagevec.
> */
> 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);
>
> - lruvec = relock_page_lruvec_irqsave(page, lruvec, &flags);
> - __pagevec_lru_add_fn(page, lruvec);
> + for (i = 0; i < nr_lruvec; i++) {
> + spin_lock_irqsave(&lruvecs.vecs[i]->lru_lock, flags);
> + while (!list_empty(&lruvecs.lists[i])) {
> + page = lru_to_page(&lruvecs.lists[i]);
> + list_del(&page->lru);
> + __pagevec_lru_add_fn(page, lruvecs.vecs[i]);
> + }
> + spin_unlock_irqrestore(&lruvecs.vecs[i]->lru_lock, flags);
> }
> - if (lruvec)
> - unlock_page_lruvec_irqrestore(lruvec, flags);
> +
> release_pages(pvec->pages, pvec->nr);
> pagevec_reinit(pvec);
> }
>