Re: [PATCH 11/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone

Vlastimil Babka
2016-06-16T10:09:05Z

On 06/09/2016 08:04 PM, 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.

Looks like the code was even wrong before... if classzone_idx wasn't
already set to the highmem zone in the first place, it wouldn't look at it.

> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

After fixing the bug below,

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/vmscan.c | 32 +++++++++++++++++++-------------
> 1 file changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e4f3e068b7a0..6663fc75c3bc 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3102,24 +3102,30 @@ 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 */
> + /*
> + * 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) {
> + for (i = MAX_NR_ZONES - 1; i >= 0; i++) {

i--

> + zone = pgdat->node_zones + i;
> + if (!populated_zone(zone))
> + continue;
> +
> + if (is_highmem_idx(i))
> + classzone_idx = i;
> + break;
> + }
> + }
> +
> + /* Only reclaim if there are no eligible zones */
> 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 (!zone_balanced(zone, order, 0)) {
> + if (!zone_balanced(zone, sc.order, classzone_idx)) {
> classzone_idx = i;
> break;
> }
>