Re: [PATCH 2/9] mm: vmscan: Obey proportional scanning requirements for kswapd

[Michal Hocko <mhocko@suse.cz>]

On Mon 13-05-13 09:12:33, Mel Gorman wrote:
> Simplistically, the anon and file LRU lists are scanned proportionally
> depending on the value of vm.swappiness although there are other factors
> taken into account by get_scan_count().  The patch "mm: vmscan: Limit
> the number of pages kswapd reclaims" limits the number of pages kswapd
> reclaims but it breaks this proportional scanning and may evenly shrink
> anon/file LRUs regardless of vm.swappiness.
> 
> This patch preserves the proportional scanning and reclaim. It does mean
> that kswapd will reclaim more than requested but the number of pages will
> be related to the high watermark.
> 
> [mhocko@suse.cz: Correct proportional reclaim for memcg and simplify]
> [kamezawa.hiroyu@jp.fujitsu.com: Recalculate scan based on target]
> [hannes@cmpxchg.org: Account for already scanned pages properly]
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> Acked-by: Rik van Riel <riel@redhat.com>

active vs. inactive might get skewed a bit AFAICS because both of them
are zeroed but file vs. anon should be scanned proportionally based on
swappiness now which sounds like it is good enough.

Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/vmscan.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 59 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cdbc069..26ad67f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1822,17 +1822,25 @@ out:
>  static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  {
>  	unsigned long nr[NR_LRU_LISTS];
> +	unsigned long targets[NR_LRU_LISTS];
>  	unsigned long nr_to_scan;
>  	enum lru_list lru;
>  	unsigned long nr_reclaimed = 0;
>  	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
>  	struct blk_plug plug;
> +	bool scan_adjusted = false;
>  
>  	get_scan_count(lruvec, sc, nr);
>  
> +	/* Record the original scan target for proportional adjustments later */
> +	memcpy(targets, nr, sizeof(nr));
> +
>  	blk_start_plug(&plug);
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
>  					nr[LRU_INACTIVE_FILE]) {
> +		unsigned long nr_anon, nr_file, percentage;
> +		unsigned long nr_scanned;
> +
>  		for_each_evictable_lru(lru) {
>  			if (nr[lru]) {
>  				nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
> @@ -1842,17 +1850,60 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  							    lruvec, sc);
>  			}
>  		}
> +
> +		if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
> +			continue;
> +
>  		/*
> -		 * On large memory systems, scan >> priority can become
> -		 * really large. This is fine for the starting priority;
> -		 * we want to put equal scanning pressure on each zone.
> -		 * However, if the VM has a harder time of freeing pages,
> -		 * with multiple processes reclaiming pages, the total
> -		 * freeing target can get unreasonably large.
> +		 * For global direct reclaim, reclaim only the number of pages
> +		 * requested. Less care is taken to scan proportionally as it
> +		 * is more important to minimise direct reclaim stall latency
> +		 * than it is to properly age the LRU lists.
>  		 */
> -		if (nr_reclaimed >= nr_to_reclaim &&
> -		    sc->priority < DEF_PRIORITY)
> +		if (global_reclaim(sc) && !current_is_kswapd())
>  			break;
> +
> +		/*
> +		 * For kswapd and memcg, reclaim at least the number of pages
> +		 * requested. Ensure that the anon and file LRUs shrink
> +		 * proportionally what was requested by get_scan_count(). We
> +		 * stop reclaiming one LRU and reduce the amount scanning
> +		 * proportional to the original scan target.
> +		 */
> +		nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
> +		nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
> +
> +		if (nr_file > nr_anon) {
> +			unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
> +						targets[LRU_ACTIVE_ANON] + 1;
> +			lru = LRU_BASE;
> +			percentage = nr_anon * 100 / scan_target;
> +		} else {
> +			unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
> +						targets[LRU_ACTIVE_FILE] + 1;
> +			lru = LRU_FILE;
> +			percentage = nr_file * 100 / scan_target;
> +		}
> +
> +		/* Stop scanning the smaller of the LRU */
> +		nr[lru] = 0;
> +		nr[lru + LRU_ACTIVE] = 0;
> +
> +		/*
> +		 * Recalculate the other LRU scan count based on its original
> +		 * scan target and the percentage scanning already complete
> +		 */
> +		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
> +		nr_scanned = targets[lru] - nr[lru];
> +		nr[lru] = targets[lru] * (100 - percentage) / 100;
> +		nr[lru] -= min(nr[lru], nr_scanned);
> +
> +		lru += LRU_ACTIVE;
> +		nr_scanned = targets[lru] - nr[lru];
> +		nr[lru] = targets[lru] * (100 - percentage) / 100;
> +		nr[lru] -= min(nr[lru], nr_scanned);
> +
> +		scan_adjusted = true;
>  	}
>  	blk_finish_plug(&plug);
>  	sc->nr_reclaimed += nr_reclaimed;
> -- 
> 1.8.1.4
> 

-- 
Michal Hocko
SUSE Labs

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