Re: [PATCH 4/9] mm: vmscan: Decide whether to compact the pgdatbased on reclaim progress

[Michal Hocko]

On Mon 13-05-13 09:12:35, Mel Gorman wrote:
> In the past, kswapd makes a decision on whether to compact memory after the
> pgdat was considered balanced. This more or less worked but it is late to
> make such a decision and does not fit well now that kswapd makes a decision
> whether to exit the zone scanning loop depending on reclaim progress.
> 
> This patch will compact a pgdat if at least the requested number of pages
> were reclaimed from unbalanced zones for a given priority. If any zone is
> currently balanced, kswapd will not call compaction as it is expected the
> necessary pages are already available.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/vmscan.c | 59 ++++++++++++++++++++++++++++++-----------------------------
>  1 file changed, 30 insertions(+), 29 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1c10ee5..cd09803 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2661,7 +2661,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>   */
>  static bool kswapd_shrink_zone(struct zone *zone,
>  			       struct scan_control *sc,
> -			       unsigned long lru_pages)
> +			       unsigned long lru_pages,
> +			       unsigned long *nr_attempted)
>  {
>  	unsigned long nr_slab;
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
> @@ -2677,6 +2678,9 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  	nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
>  	sc->nr_reclaimed += reclaim_state->reclaimed_slab;
>  
> +	/* Account for the number of pages attempted to reclaim */
> +	*nr_attempted += sc->nr_to_reclaim;
> +
>  	if (nr_slab == 0 && !zone_reclaimable(zone))
>  		zone->all_unreclaimable = 1;
>  
> @@ -2724,7 +2728,9 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  
>  	do {
>  		unsigned long lru_pages = 0;
> +		unsigned long nr_attempted = 0;
>  		bool raise_priority = true;
> +		bool pgdat_needs_compaction = (order > 0);
>  
>  		sc.nr_reclaimed = 0;
>  
> @@ -2774,7 +2780,21 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  		for (i = 0; i <= end_zone; i++) {
>  			struct zone *zone = pgdat->node_zones + i;
>  
> +			if (!populated_zone(zone))
> +				continue;
> +
>  			lru_pages += zone_reclaimable_pages(zone);
> +
> +			/*
> +			 * If any zone is currently balanced then kswapd will
> +			 * not call compaction as it is expected that the
> +			 * necessary pages are already available.
> +			 */
> +			if (pgdat_needs_compaction &&
> +					zone_watermark_ok(zone, order,
> +						low_wmark_pages(zone),
> +						*classzone_idx, 0))
> +				pgdat_needs_compaction = false;
>  		}
>  
>  		/*
> @@ -2843,7 +2863,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  				 * already being scanned that high
>  				 * watermark would be met at 100% efficiency.
>  				 */
> -				if (kswapd_shrink_zone(zone, &sc, lru_pages))
> +				if (kswapd_shrink_zone(zone, &sc, lru_pages,
> +						       &nr_attempted))
>  					raise_priority = false;
>  			}
>  
> @@ -2896,6 +2917,13 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  			break;
>  
>  		/*
> +		 * Compact if necessary and kswapd is reclaiming at least the
> +		 * high watermark number of pages as requsted
> +		 */
> +		if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
> +			compact_pgdat(pgdat, order);
> +
> +		/*
>  		 * Raise priority if scanning rate is too low or there was no
>  		 * progress in reclaiming pages
>  		 */
> @@ -2904,33 +2932,6 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  	} while (sc.priority >= 0 &&
>  		 !pgdat_balanced(pgdat, order, *classzone_idx));
>  
> -	/*
> -	 * If kswapd was reclaiming at a higher order, it has the option of
> -	 * sleeping without all zones being balanced. Before it does, it must
> -	 * ensure that the watermarks for order-0 on *all* zones are met and
> -	 * that the congestion flags are cleared. The congestion flag must
> -	 * be cleared as kswapd is the only mechanism that clears the flag
> -	 * and it is potentially going to sleep here.
> -	 */
> -	if (order) {
> -		int zones_need_compaction = 1;
> -
> -		for (i = 0; i <= end_zone; i++) {
> -			struct zone *zone = pgdat->node_zones + i;
> -
> -			if (!populated_zone(zone))
> -				continue;
> -
> -			/* Check if the memory needs to be defragmented. */
> -			if (zone_watermark_ok(zone, order,
> -				    low_wmark_pages(zone), *classzone_idx, 0))
> -				zones_need_compaction = 0;
> -		}
> -
> -		if (zones_need_compaction)
> -			compact_pgdat(pgdat, order);
> -	}
> -
>  out:
>  	/*
>  	 * Return the order we were reclaiming at so prepare_kswapd_sleep()
> -- 
> 1.8.1.4
> 

-- 
Michal Hocko
SUSE Labs
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