Re: [PATCH 10/10] mm: vmscan: Move logic from balance_pgdat() to kswapd_shrink_zone()

[Michal Hocko <mhocko@suse.cz>]

On Sun 17-03-13 13:04:16, Mel Gorman wrote:
> balance_pgdat() is very long and some of the logic can and should
> be internal to kswapd_shrink_zone(). Move it so the flow of
> balance_pgdat() is marginally easier to follow.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/vmscan.c | 104 +++++++++++++++++++++++++++++-------------------------------
>  1 file changed, 51 insertions(+), 53 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8c66e5a..d7cf384 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2660,18 +2660,53 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>   * reclaim or if the lack of process was due to pages under writeback.
>   */
>  static bool kswapd_shrink_zone(struct zone *zone,
> +			       int classzone_idx,
>  			       struct scan_control *sc,
>  			       unsigned long lru_pages,
>  			       bool shrinking_slab)
>  {
> +	int testorder = sc->order;
>  	unsigned long nr_slab = 0;
> +	unsigned long balance_gap;
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
>  	struct shrink_control shrink = {
>  		.gfp_mask = sc->gfp_mask,
>  	};
> +	bool lowmem_pressure;
>  
>  	/* Reclaim above the high watermark. */
>  	sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
> +
> +	/*
> +	 * Kswapd reclaims only single pages with compaction enabled. Trying
> +	 * too hard to reclaim until contiguous free pages have become
> +	 * available can hurt performance by evicting too much useful data
> +	 * from memory. Do not reclaim more than needed for compaction.
> +	 */
> +	if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
> +			compaction_suitable(zone, sc->order) !=
> +				COMPACT_SKIPPED)
> +		testorder = 0;
> +
> +	/*
> +	 * We put equal pressure on every zone, unless one zone has way too
> +	 * many pages free already. The "too many pages" is defined as the
> +	 * high wmark plus a "gap" where the gap is either the low
> +	 * watermark or 1% of the zone, whichever is smaller.
> +	 */
> +	balance_gap = min(low_wmark_pages(zone),
> +		(zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> +		KSWAPD_ZONE_BALANCE_GAP_RATIO);
> +
> +	/*
> +	 * If there is no low memory pressure or the zone is balanced then no
> +	 * reclaim is necessary
> +	 */
> +	lowmem_pressure = (buffer_heads_over_limit && is_highmem(zone));
> +	if (!(lowmem_pressure || !zone_balanced(zone, testorder,
> +						balance_gap, classzone_idx)))

if (!lowmem_pressure && zone_balanced) would be less cryptic I guess

> +		return true;
> +
>  	shrink_zone(zone, sc);
>  
>  	/*
> @@ -2689,6 +2724,16 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  
>  	zone_clear_flag(zone, ZONE_WRITEBACK);
>  
> +	/*
> +	 * If a zone reaches its high watermark, consider it to be no longer
> +	 * congested. It's possible there are dirty pages backed by congested
> +	 * BDIs but as pressure is relieved, speculatively avoid congestion
> +	 * waits.
> +	 */
> +	if (!zone->all_unreclaimable &&
> +	    zone_balanced(zone, testorder, 0, classzone_idx))
> +		zone_clear_flag(zone, ZONE_CONGESTED);
> +
>  	return sc->nr_scanned >= sc->nr_to_reclaim;
>  }
>  
> @@ -2821,8 +2866,6 @@ 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;
> -			int testorder;
> -			unsigned long balance_gap;
>  
>  			if (!populated_zone(zone))
>  				continue;
> @@ -2843,61 +2886,16 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  			sc.nr_reclaimed += nr_soft_reclaimed;
>  
>  			/*
> -			 * We put equal pressure on every zone, unless
> -			 * one zone has way too many pages free
> -			 * already. The "too many pages" is defined
> -			 * as the high wmark plus a "gap" where the
> -			 * gap is either the low watermark or 1%
> -			 * of the zone, whichever is smaller.
> -			 */
> -			balance_gap = min(low_wmark_pages(zone),
> -				(zone->managed_pages +
> -					KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> -				KSWAPD_ZONE_BALANCE_GAP_RATIO);
> -			/*
> -			 * Kswapd reclaims only single pages with compaction
> -			 * enabled. Trying too hard to reclaim until contiguous
> -			 * free pages have become available can hurt performance
> -			 * by evicting too much useful data from memory.
> -			 * Do not reclaim more than needed for compaction.
> +			 * There should be no need to raise the scanning
> +			 * priority if enough pages are already being scanned
> +			 * that that high watermark would be met at 100%
> +			 * efficiency.
>  			 */
> -			testorder = order;
> -			if (IS_ENABLED(CONFIG_COMPACTION) && order &&
> -					compaction_suitable(zone, order) !=
> -						COMPACT_SKIPPED)
> -				testorder = 0;
> -
> -			if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
> -			    !zone_balanced(zone, testorder,
> -					   balance_gap, end_zone)) {
> -				/*
> -				 * There should be no need to raise the
> -				 * scanning priority if enough pages are
> -				 * already being scanned that that high
> -				 * watermark would be met at 100% efficiency.
> -				 */
> -				if (kswapd_shrink_zone(zone, &sc,
> +			if (kswapd_shrink_zone(zone, end_zone, &sc,
>  						lru_pages, shrinking_slab))
>  					raise_priority = false;
>  
> -				nr_to_reclaim += sc.nr_to_reclaim;
> -			}
> -
> -			if (zone->all_unreclaimable) {
> -				if (end_zone && end_zone == i)
> -					end_zone--;
> -				continue;
> -			}
> -
> -			if (zone_balanced(zone, testorder, 0, end_zone))
> -				/*
> -				 * If a zone reaches its high watermark,
> -				 * consider it to be no longer congested. It's
> -				 * possible there are dirty pages backed by
> -				 * congested BDIs but as pressure is relieved,
> -				 * speculatively avoid congestion waits
> -				 */
> -				zone_clear_flag(zone, ZONE_CONGESTED);
> +			nr_to_reclaim += sc.nr_to_reclaim;
>  		}

nr_to_reclaim is updated if the zone is balanced an no reclaim is done
which break compaction condition AFAICS.

-- 
Michal Hocko
SUSE Labs

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