Re: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone

[Minchan Kim]

On Fri, Jul 01, 2016 at 09:01:19PM +0100, Mel Gorman wrote:
> kswapd scans from highest to lowest for a zone that requires balancing.
> This was necessary when reclaim was per-zone to fairly age pages on lower
> zones.  Now that we are reclaiming on a per-node basis, any eligible zone
> can be used and pages will still be aged fairly.  This patch avoids
> reclaiming excessively unless buffer_heads are over the limit and it's
> necessary to reclaim from a higher zone than requested by the waker of
> kswapd to relieve low memory pressure.
> 
> [hillf.zj@xxxxxxxxxxxxxxx: Force kswapd reclaim no more than needed]
> Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgorman@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/vmscan.c | 56 ++++++++++++++++++++++++--------------------------------
>  1 file changed, 24 insertions(+), 32 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 911142d25de2..2f898ba2ee2e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3141,31 +3141,36 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  
>  		sc.nr_reclaimed = 0;
>  
> -		/* Scan from the highest requested zone to dma */
> -		for (i = classzone_idx; i >= 0; i--) {
> -			zone = pgdat->node_zones + i;
> -			if (!populated_zone(zone))
> -				continue;
> -
> -			/*
> -			 * If the number of buffer_heads in the machine
> -			 * exceeds the maximum allowed level and this node
> -			 * has a highmem zone, force kswapd to reclaim from
> -			 * it to relieve lowmem pressure.
> -			 */
> -			if (buffer_heads_over_limit && is_highmem_idx(i)) {
> -				classzone_idx = i;
> -				break;
> -			}
> +		/*
> +		 * If the number of buffer_heads in the machine exceeds the
> +		 * maximum allowed level then reclaim from all zones. This is
> +		 * not specific to highmem as highmem may not exist but it is
> +		 * it is expected that buffer_heads are stripped in writeback.
> +		 */
> +		if (buffer_heads_over_limit) {
> +			for (i = MAX_NR_ZONES - 1; i >= 0; i--) {
> +				zone = pgdat->node_zones + i;
> +				if (!populated_zone(zone))
> +					continue;
>  
> -			if (!zone_balanced(zone, order, 0)) {
>  				classzone_idx = i;
>  				break;
>  			}
>  		}
>  
> -		if (i < 0)
> -			goto out;
> +		/*
> +		 * Only reclaim if there are no eligible zones. Check from
> +		 * high to low zone to avoid prematurely clearing pgdat
> +		 * congested state.

I cannot understand "prematurely clearing pgdat congested state".
Could you add more words to clear it out?

> +		 */
> +		for (i = classzone_idx; i >= 0; i--) {
> +			zone = pgdat->node_zones + i;
> +			if (!populated_zone(zone))
> +				continue;
> +
> +			if (zone_balanced(zone, sc.order, classzone_idx))

If buffer_head is over limit, old logic force to reclaim highmem but
this zone_balanced logic will prevent it.

> +				goto out;
> +		}
>  
>  		/*
>  		 * Do some background aging of the anon list, to give
> @@ -3211,19 +3216,6 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  			break;
>  
>  		/*
> -		 * Stop reclaiming if any eligible zone is balanced and clear
> -		 * node writeback or congested.
> -		 */
> -		for (i = 0; i <= classzone_idx; i++) {
> -			zone = pgdat->node_zones + i;
> -			if (!populated_zone(zone))
> -				continue;
> -
> -			if (zone_balanced(zone, sc.order, classzone_idx))
> -				goto out;
> -		}
> -
> -		/*
>  		 * Raise priority if scanning rate is too low or there was no
>  		 * progress in reclaiming pages
>  		 */
> -- 
> 2.6.4
> 
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