Re: [PATCH 06/27] mm, vmscan: Make kswapd reclaim in terms of nodes

Hillf Danton
2016-06-12T09:39:06Z

>
> /*
> - * kswapd shrinks the zone by the number of pages required to reach
> - * the high watermark.
> + * kswapd shrinks a node of pages that are at or below the highest usable
> + * zone that is currently unbalanced.
> *
> * Returns true if kswapd scanned at least the requested number of pages to
> * reclaim or if the lack of progress was due to pages under writeback.
> * This is used to determine if the scanning priority needs to be raised.
> */
> -static bool kswapd_shrink_zone(struct zone *zone,
> +static bool kswapd_shrink_node(pg_data_t *pgdat,
> int classzone_idx,
> struct scan_control *sc)
> {
> - unsigned long balance_gap;
> - bool lowmem_pressure;
> - struct pglist_data *pgdat = zone->zone_pgdat;
> + struct zone *zone;
> + unsigned long nr_to_reclaim = 0;
> + int z;
>
> - /* Reclaim above the high watermark. */
> - sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
> + /* Reclaim a number of pages proportional to the number of zones */
> + for (z = 0; z <= classzone_idx; z++) {
> + zone = pgdat->node_zones + z;
> + if (!populated_zone(zone))
> + continue;
>
> - /*
> - * 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), DIV_ROUND_UP(
> - zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
> + nr_to_reclaim += max(high_wmark_pages(zone), SWAP_CLUSTER_MAX);
> + }

Missing sc->nr_to_reclaim = nr_to_reclaim; ?

>
> /*
> - * If there is no low memory pressure or the zone is balanced then no
> - * reclaim is necessary
> + * Historically care was taken to put equal pressure on all zones but
> + * now pressure is applied based on node LRU order.
> */
> - lowmem_pressure = (buffer_heads_over_limit && is_highmem(zone));
> - if (!lowmem_pressure && zone_balanced(zone, sc->order, false,
> - balance_gap, classzone_idx))
> - return true;
> -
> - shrink_node(zone->zone_pgdat, sc, classzone_idx);
> -
> - /* TODO: ANOMALY */
> - clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
> + shrink_node(pgdat, sc, classzone_idx);
>
> /*
> - * 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.
> + * Fragmentation may mean that the system cannot be rebalanced for
> + * high-order allocations. If twice the allocation size has been
> + * reclaimed then recheck watermarks only at order-0 to prevent
> + * excessive reclaim. Assume that a process requested a high-order
> + * can direct reclaim/compact.
> */
> - if (pgdat_reclaimable(zone->zone_pgdat) &&
> - zone_balanced(zone, sc->order, false, 0, classzone_idx)) {
> - clear_bit(PGDAT_CONGESTED, &pgdat->flags);
> - clear_bit(PGDAT_DIRTY, &pgdat->flags);
> - }
> + if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
> + sc->order = 0;
>
> return sc->nr_scanned >= sc->nr_to_reclaim;
> }