Re: [v3 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Johannes Weiner]

On Tue, May 21, 2019 at 05:40:42PM +0800, Yang Shi wrote:
> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
> swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
> and some other vm counters still get inc'ed by one even though a whole
> THP (512 pages) gets swapped out.
> 
> This doesn't make too much sense to memory reclaim.  For example, direct
> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
> could fulfill it.  But, if nr_reclaimed is not increased correctly,
> direct reclaim may just waste time to reclaim more pages,
> SWAP_CLUSTER_MAX * 512 pages in worst case.
> 
> And, it may cause pgsteal_{kswapd|direct} is greater than
> pgscan_{kswapd|direct}, like the below:
> 
> pgsteal_kswapd 122933
> pgsteal_direct 26600225
> pgscan_kswapd 174153
> pgscan_direct 14678312
> 
> nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would
> break some page reclaim logic, e.g.
> 
> vmpressure: this looks at the scanned/reclaimed ratio so it won't
> change semantics as long as scanned & reclaimed are fixed in parallel.
> 
> compaction/reclaim: compaction wants a certain number of physical pages
> freed up before going back to compacting.
> 
> kswapd priority raising: kswapd raises priority if we scan fewer pages
> than the reclaim target (which itself is obviously expressed in order-0
> pages). As a result, kswapd can falsely raise its aggressiveness even
> when it's making great progress.
> 
> Other than nr_scanned and nr_reclaimed, some other counters, e.g.
> pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed
> too since they are user visible via cgroup, /proc/vmstat or trace
> points, otherwise they would be underreported.
> 
> When isolating pages from LRUs, nr_taken has been accounted in base
> page, but nr_scanned and nr_skipped are still accounted in THP.  It
> doesn't make too much sense too since this may cause trace point
> underreport the numbers as well.
> 
> So accounting those counters in base page instead of accounting THP as
> one page.
> 
> This change may result in lower steal/scan ratio in some cases since
> THP may get split during page reclaim, then a part of tail pages get
> reclaimed instead of the whole 512 pages, but nr_scanned is accounted
> by 512, particularly for direct reclaim.  But, this should be not a
> significant issue.
> 
> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly
>     Switched back to use compound_order per Matthew
>     Fixed more counters per Johannes
> v2: Added Shakeel's Reviewed-by
>     Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
> 
>  mm/vmscan.c | 40 ++++++++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b65bc50..1044834 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1250,7 +1250,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		case PAGEREF_ACTIVATE:
>  			goto activate_locked;
>  		case PAGEREF_KEEP:
> -			stat->nr_ref_keep++;
> +			stat->nr_ref_keep += (1 << compound_order(page));
>  			goto keep_locked;
>  		case PAGEREF_RECLAIM:
>  		case PAGEREF_RECLAIM_CLEAN:
> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  						goto activate_locked;
>  				}
>  
> +				/*
> +				 * Account all tail pages when THP is added
> +				 * into swap cache successfully.
> +				 * The head page has been accounted at the
> +				 * first place.
> +				 */
> +				if (PageTransHuge(page))
> +					sc->nr_scanned +=
> +						((1 << compound_order(page)) -
> +							1);
> +
>  				may_enter_fs = 1;

Even if we don't split and reclaim the page, we should always account
the number of base pages in nr_scanned. Otherwise it's not clear what
nr_scanned means.

>  				/* Adding to swap updated mapping */
> @@ -1315,7 +1326,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			if (unlikely(PageTransHuge(page)))
>  				flags |= TTU_SPLIT_HUGE_PMD;
>  			if (!try_to_unmap(page, flags)) {
> -				stat->nr_unmap_fail++;
> +				stat->nr_unmap_fail +=
> +					(1 << compound_order(page));
>  				goto activate_locked;
>  			}
>  		}
> @@ -1442,7 +1454,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  
>  		unlock_page(page);
>  free_it:
> -		nr_reclaimed++;
> +		/*
> +		 * THP may get swapped out in a whole, need account
> +		 * all base pages.
> +		 */
> +		nr_reclaimed += (1 << compound_order(page));

This expression is quite repetitive. Why not do

		int nr_pages;

		page = lru_to_page(page_list);
		nr_pages = 1 << compound_order(page);
		list_del(&page->lru);

		if (!trylock_page(page))
			...

at the head of the loop and add nr_pages to all these counters
instead?

> @@ -1642,14 +1659,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>  	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
>  	unsigned long skipped = 0;
> -	unsigned long scan, total_scan, nr_pages;
> +	unsigned long scan, nr_pages;
>  	LIST_HEAD(pages_skipped);
>  	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>  
>  	scan = 0;
> -	for (total_scan = 0;
> -	     scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
> -	     total_scan++) {
> +	while (scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src)) {
>  		struct page *page;

Once you fixed the units, scan < nr_to_scan && nr_taken >= nr_to_scan
is an impossible condition. You should be able to write:

	while (scan < nr_to_scan && !list_empty(src))

Also, you need to keep total_scan. The trace point wants to know how
many pages were actually looked at, including the ones from ineligible
zones that were skipped over.

>  
>  		page = lru_to_page(src);
> @@ -1659,7 +1674,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  
>  		if (page_zonenum(page) > sc->reclaim_idx) {
>  			list_move(&page->lru, &pages_skipped);
> -			nr_skipped[page_zonenum(page)]++;
> +			nr_skipped[page_zonenum(page)] +=
> +				(1 << compound_order(page));
>  			continue;
>  		}
>  
> @@ -1669,7 +1685,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  		 * ineligible pages.  This causes the VM to not reclaim any
>  		 * pages, triggering a premature OOM.
>  		 */
> -		scan++;
> +		scan += (1 << compound_order(page));
>  		switch (__isolate_lru_page(page, mode)) {
>  		case 0:
>  			nr_pages = hpage_nr_pages(page);

Same here, you can calculate nr_pages at the top of the loop and use
it throughout.

> @@ -1707,9 +1723,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  			skipped += nr_skipped[zid];
>  		}
>  	}
> -	*nr_scanned = total_scan;
> +	*nr_scanned = scan;
>  	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
> -				    total_scan, skipped, nr_taken, mode, lru);
> +				    scan, skipped, nr_taken, mode, lru);
>  	update_lru_sizes(lruvec, lru, nr_zone_taken);
>  	return nr_taken;
>  }
> -- 
> 1.8.3.1
>