Re: [PATCH] mm: Do not allocate duplicate stack variables in shrink_page_list()

[Michal Hocko]

On Thu 31-01-19 18:37:02, Kirill Tkhai wrote:
> On path shrink_inactive_list() ---> shrink_page_list()
> we allocate stack variables for the statistics twice.
> This is completely useless, and this just consumes stack
> much more, then we really need.
> 
> The patch kills duplicate stack variables from shrink_page_list(),
> and this reduce stack usage and object file size significantly:

significantly is a bit of an overstatement for 32B saved...

> Stack usage:
> Before: vmscan.c:1122:22:shrink_page_list	648	static
> After:  vmscan.c:1122:22:shrink_page_list	616	static
> 
> Size of vmscan.o:
>          text	   data	    bss	    dec	    hex	filename
> Before: 56866	   4720	    128	  61714	   f112	mm/vmscan.o
> After:  56770	   4720	    128	  61618	   f0b2	mm/vmscan.o
> 
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/vmscan.c |   44 ++++++++++++++------------------------------
>  1 file changed, 14 insertions(+), 30 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index dd9554f5d788..54a389fd91e2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1128,16 +1128,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
> -	int pgactivate = 0;
> -	unsigned nr_unqueued_dirty = 0;
> -	unsigned nr_dirty = 0;
> -	unsigned nr_congested = 0;
>  	unsigned nr_reclaimed = 0;
> -	unsigned nr_writeback = 0;
> -	unsigned nr_immediate = 0;
> -	unsigned nr_ref_keep = 0;
> -	unsigned nr_unmap_fail = 0;
>  
> +	memset(stat, 0, sizeof(*stat));
>  	cond_resched();
>  
>  	while (!list_empty(page_list)) {
> @@ -1181,10 +1174,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		 */
>  		page_check_dirty_writeback(page, &dirty, &writeback);
>  		if (dirty || writeback)
> -			nr_dirty++;
> +			stat->nr_dirty++;
>  
>  		if (dirty && !writeback)
> -			nr_unqueued_dirty++;
> +			stat->nr_unqueued_dirty++;
>  
>  		/*
>  		 * Treat this page as congested if the underlying BDI is or if
> @@ -1196,7 +1189,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		if (((dirty || writeback) && mapping &&
>  		     inode_write_congested(mapping->host)) ||
>  		    (writeback && PageReclaim(page)))
> -			nr_congested++;
> +			stat->nr_congested++;
>  
>  		/*
>  		 * If a page at the tail of the LRU is under writeback, there
> @@ -1245,7 +1238,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			if (current_is_kswapd() &&
>  			    PageReclaim(page) &&
>  			    test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
> -				nr_immediate++;
> +				stat->nr_immediate++;
>  				goto activate_locked;
>  
>  			/* Case 2 above */
> @@ -1263,7 +1256,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  				 * and it's also appropriate in global reclaim.
>  				 */
>  				SetPageReclaim(page);
> -				nr_writeback++;
> +				stat->nr_writeback++;
>  				goto activate_locked;
>  
>  			/* Case 3 above */
> @@ -1283,7 +1276,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		case PAGEREF_ACTIVATE:
>  			goto activate_locked;
>  		case PAGEREF_KEEP:
> -			nr_ref_keep++;
> +			stat->nr_ref_keep++;
>  			goto keep_locked;
>  		case PAGEREF_RECLAIM:
>  		case PAGEREF_RECLAIM_CLEAN:
> @@ -1348,7 +1341,7 @@ 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)) {
> -				nr_unmap_fail++;
> +				stat->nr_unmap_fail++;
>  				goto activate_locked;
>  			}
>  		}
> @@ -1496,7 +1489,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		VM_BUG_ON_PAGE(PageActive(page), page);
>  		if (!PageMlocked(page)) {
>  			SetPageActive(page);
> -			pgactivate++;
> +			stat->nr_activate++;
>  			count_memcg_page_event(page, PGACTIVATE);
>  		}
>  keep_locked:
> @@ -1511,18 +1504,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  	free_unref_page_list(&free_pages);
>  
>  	list_splice(&ret_pages, page_list);
> -	count_vm_events(PGACTIVATE, pgactivate);
> -
> -	if (stat) {
> -		stat->nr_dirty = nr_dirty;
> -		stat->nr_congested = nr_congested;
> -		stat->nr_unqueued_dirty = nr_unqueued_dirty;
> -		stat->nr_writeback = nr_writeback;
> -		stat->nr_immediate = nr_immediate;
> -		stat->nr_activate = pgactivate;
> -		stat->nr_ref_keep = nr_ref_keep;
> -		stat->nr_unmap_fail = nr_unmap_fail;
> -	}
> +	count_vm_events(PGACTIVATE, stat->nr_activate);
> +
>  	return nr_reclaimed;
>  }
>  
> @@ -1534,6 +1517,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>  		.priority = DEF_PRIORITY,
>  		.may_unmap = 1,
>  	};
> +	struct reclaim_stat dummy_stat;
>  	unsigned long ret;
>  	struct page *page, *next;
>  	LIST_HEAD(clean_pages);
> @@ -1547,7 +1531,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>  	}
>  
>  	ret = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
> -			TTU_IGNORE_ACCESS, NULL, true);
> +			TTU_IGNORE_ACCESS, &dummy_stat, true);
>  	list_splice(&clean_pages, page_list);
>  	mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE, -ret);
>  	return ret;
> @@ -1922,7 +1906,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>  	unsigned long nr_scanned;
>  	unsigned long nr_reclaimed = 0;
>  	unsigned long nr_taken;
> -	struct reclaim_stat stat = {};
> +	struct reclaim_stat stat;
>  	int file = is_file_lru(lru);
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
> 

-- 
Michal Hocko
SUSE Labs