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SubjectRe: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone
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@alibaba-inc.com: Force kswapd reclaim no more than needed]
> Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgorman@techsingularity.net
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Hillf Danton <hillf.zj@alibaba-inc.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> 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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