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...@alibaba-inc.com: Force kswapd reclaim no more than needed] Link: http://lkml.kernel.org/r/1466518566-30034-12-git-send-email-mgor...@techsingularity.net Signed-off-by: Mel Gorman <mgor...@techsingularity.net> Signed-off-by: Hillf Danton <hillf...@alibaba-inc.com> Acked-by: Vlastimil Babka <vba...@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. + */ + 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)) + 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
