linux-kernel - Re: [PATCH 3/7] mm, vmscan: show the number of skipped pages in mm_vmscan_lru

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Date:   Tue, 3 Jan 2017 18:21:48 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 3/7] mm, vmscan: show the number of skipped pages in
 mm_vmscan_lru_isolate

On 12/28/2016 04:30 PM, Michal Hocko wrote:
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1428,6 +1428,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	unsigned long nr_taken = 0;
>  	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>  	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
> +	unsigned long skipped = 0, total_skipped = 0;
>  	unsigned long scan, nr_pages;
>  	LIST_HEAD(pages_skipped);
>
> @@ -1479,14 +1480,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  	 */
>  	if (!list_empty(&pages_skipped)) {
>  		int zid;
> -		unsigned long total_skipped = 0;
>
>  		for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>  			if (!nr_skipped[zid])
>  				continue;
>
>  			__count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
> -			total_skipped += nr_skipped[zid];
> +			skipped += nr_skipped[zid];
>  		}
>
>  		/*
> @@ -1494,13 +1494,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  		 * close to unreclaimable. If the LRU list is empty, account
>  		 * skipped pages as a full scan.
>  		 */
> -		scan += list_empty(src) ? total_skipped : total_skipped >> 2;
> +		total_skipped = list_empty(src) ? skipped : skipped >> 2;

Should the tracepoint output reflect this halving heuristic or rather report the 
raw data? Or is each variant inferrable from the other?

>
>  		list_splice(&pages_skipped, src);
>  	}
> -	*nr_scanned = scan;
> +	*nr_scanned = scan + total_skipped;
>  	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,
> -				    nr_taken, mode, is_file_lru(lru));
> +				    skipped, nr_taken, mode, is_file_lru(lru));
>  	update_lru_sizes(lruvec, lru, nr_zone_taken, nr_taken);
>  	return nr_taken;
>  }
>