[PATCH 08/27] mm, vmscan: Make kswapd reclaim in terms of nodes

From: Mel Gorman
Date: Tue Feb 23 2016 - 10:08:55 EST

Patch "mm: vmscan: Begin reclaiming pages on a per-node basis" started
thinking of reclaim in terms of nodes but kswapd is still zone-centric. This
patch gets rid of many of the node-based versus zone-based decisions.

o A node is considered balanced when any eligible lower zone is balanced.
This eliminates one class of age-inversion problem because we avoid
reclaiming a newer page just because it's in the wrong zone
o pgdat_balanced disappears because we now only care about one zone being
balanced.
o Some anomalies related to writeback and congestion tracking being based on
zones disappear.
o kswapd no longer has to take care to reclaim zones in the reverse order
that the page allocator uses.
o Most importantly of all, reclaim from node 0 with multiple zones will
have similar aging and reclaiming characteristics as every
other node.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 285 +++++++++++++++++++++---------------------------------------
1 file changed, 97 insertions(+), 188 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1decaa31c4c6..57114c130df1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2963,7 +2963,8 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
}
#endif

-static void age_active_anon(struct zone *zone, struct scan_control *sc)
+static void age_active_anon(struct pglist_data *pgdat,
+ struct zone *zone, struct scan_control *sc)
{
struct mem_cgroup *memcg;

@@ -2982,85 +2983,15 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
} while (memcg);
}

-static bool zone_balanced(struct zone *zone, int order, bool highorder,
+static bool zone_balanced(struct zone *zone, int order,
unsigned long balance_gap, int classzone_idx)
{
unsigned long mark = high_wmark_pages(zone) + balance_gap;

- /*
- * When checking from pgdat_balanced(), kswapd should stop and sleep
- * when it reaches the high order-0 watermark and let kcompactd take
- * over. Other callers such as wakeup_kswapd() want to determine the
- * true high-order watermark.
- */
- if (IS_ENABLED(CONFIG_COMPACTION) && !highorder) {
- mark += (1UL << order);
- order = 0;
- }
-
return zone_watermark_ok_safe(zone, order, mark, classzone_idx);
}

/*
- * pgdat_balanced() is used when checking if a node is balanced.
- *
- * For order-0, all zones must be balanced!
- *
- * For high-order allocations only zones that meet watermarks and are in a
- * zone allowed by the callers classzone_idx are added to balanced_pages. The
- * total of balanced pages must be at least 25% of the zones allowed by
- * classzone_idx for the node to be considered balanced. Forcing all zones to
- * be balanced for high orders can cause excessive reclaim when there are
- * imbalanced zones.
- * The choice of 25% is due to
- * o a 16M DMA zone that is balanced will not balance a zone on any
- * reasonable sized machine
- * o On all other machines, the top zone must be at least a reasonable
- * percentage of the middle zones. For example, on 32-bit x86, highmem
- * would need to be at least 256M for it to be balance a whole node.
- * Similarly, on x86-64 the Normal zone would need to be at least 1G
- * to balance a node on its own. These seemed like reasonable ratios.
- */
-static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
-{
- unsigned long managed_pages = 0;
- unsigned long balanced_pages = 0;
- int i;
-
- /* Check the watermark levels */
- for (i = 0; i <= classzone_idx; i++) {
- struct zone *zone = pgdat->node_zones + i;
-
- if (!populated_zone(zone))
- continue;
-
- managed_pages += zone->managed_pages;
-
- /*
- * A special case here:
- *
- * balance_pgdat() skips over all_unreclaimable after
- * DEF_PRIORITY. Effectively, it considers them balanced so
- * they must be considered balanced here as well!
- */
- if (!pgdat_reclaimable(zone->zone_pgdat)) {
- balanced_pages += zone->managed_pages;
- continue;
- }
-
- if (zone_balanced(zone, order, false, 0, i))
- balanced_pages += zone->managed_pages;
- else if (!order)
- return false;
- }
-
- if (order)
- return balanced_pages >= (managed_pages >> 2);
- else
- return true;
-}
-
-/*
* Prepare kswapd for sleeping. This verifies that there are no processes
* waiting in throttle_direct_reclaim() and that watermarks have been met.
*
@@ -3069,6 +3000,8 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
int classzone_idx)
{
+ int i;
+
/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
if (remaining)
return false;
@@ -3089,97 +3022,84 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
if (waitqueue_active(&pgdat->pfmemalloc_wait))
wake_up_all(&pgdat->pfmemalloc_wait);

- return pgdat_balanced(pgdat, order, classzone_idx);
+ for (i = 0; i <= classzone_idx; i++) {
+ struct zone *zone = pgdat->node_zones + i;
+
+ if (!populated_zone(zone))
+ continue;
+
+ if (zone_balanced(zone, order, 0, classzone_idx))
+ return true;
+ }
+
+ return false;
}

/*
- * 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)
{
- int testorder = sc->order;
- 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);
+ }

/*
- * 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, testorder, false,
- balance_gap, classzone_idx))
- return true;
-
- shrink_node(zone->zone_pgdat, sc, zone_idx(zone), classzone_idx);
-
- /* TODO: ANOMALY */
- clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
+ shrink_node(pgdat, sc, classzone_idx, 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, testorder, 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;
}

/*
- * For kswapd, balance_pgdat() will work across all this node's zones until
- * they are all at high_wmark_pages(zone).
+ * For kswapd, balance_pgdat() will reclaim pages across a node from zones
+ * that are eligible for use by the caller until at least one zone is
+ * balanced.
*
* Returns the highest zone idx kswapd was reclaiming at
*
- * There is special handling here for zones which are full of pinned pages.
- * This can happen if the pages are all mlocked, or if they are all used by
- * device drivers (say, ZONE_DMA). Or if they are all in use by hugetlb.
- * What we do is to detect the case where all pages in the zone have been
- * scanned twice and there has been zero successful reclaim. Mark the zone as
- * dead and from now on, only perform a short scan. Basically we're polling
- * the zone for when the problem goes away.
- *
* kswapd scans the zones in the highmem->normal->dma direction. It skips
* zones which have free_pages > high_wmark_pages(zone), but once a zone is
- * found to have free_pages <= high_wmark_pages(zone), we scan that zone and the
- * lower zones regardless of the number of free pages in the lower zones. This
- * interoperates with the page allocator fallback scheme to ensure that aging
- * of pages is balanced across the zones.
+ * found to have free_pages <= high_wmark_pages(zone), any page is that zone
+ * or lower is eligible for reclaim until at least one usable zone is
+ * balanced.
*/
static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
{
int i;
- int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
unsigned long nr_soft_reclaimed;
unsigned long nr_soft_scanned;
+ struct zone *zone;
struct scan_control sc = {
.gfp_mask = GFP_KERNEL,
- .reclaim_idx = MAX_NR_ZONES - 1,
.order = order,
.priority = DEF_PRIORITY,
.may_writepage = !laptop_mode,
@@ -3195,21 +3115,10 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)

/* Scan from the highest requested zone to dma */
for (i = classzone_idx; i >= 0; i--) {
- struct zone *zone = pgdat->node_zones + i;
-
+ zone = pgdat->node_zones + i;
if (!populated_zone(zone))
continue;

- if (sc.priority != DEF_PRIORITY &&
- !pgdat_reclaimable(zone->zone_pgdat))
- continue;
-
- /*
- * Do some background aging of the anon list, to give
- * pages a chance to be referenced before reclaiming.
- */
- age_active_anon(zone, &sc);
-
/*
* If the number of buffer_heads in the machine
* exceeds the maximum allowed level and this node
@@ -3217,19 +3126,17 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
* it to relieve lowmem pressure.
*/
if (buffer_heads_over_limit && is_highmem_idx(i)) {
- end_zone = i;
+ classzone_idx = i;
break;
}

- if (!zone_balanced(zone, order, true, 0, 0)) {
- end_zone = i;
+ if (!zone_balanced(zone, order, 0, 0)) {
+ classzone_idx = i;
break;
} else {
/*
- * If balanced, clear the dirty and congested
- * flags
- *
- * TODO: ANOMALY
+ * If any eligible zone is balanced then the
+ * node is not considered congested or dirty.
*/
clear_bit(PGDAT_CONGESTED, &zone->zone_pgdat->flags);
clear_bit(PGDAT_DIRTY, &zone->zone_pgdat->flags);
@@ -3240,50 +3147,35 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
goto out;

/*
+ * Do some background aging of the anon list, to give
+ * pages a chance to be referenced before reclaiming. All
+ * pages are rotated regardless of classzone as this is
+ * about consistent aging.
+ */
+ sc.reclaim_idx = MAX_NR_ZONES - 1;
+ age_active_anon(pgdat, &pgdat->node_zones[sc.reclaim_idx], &sc);
+
+ /*
* If we're getting trouble reclaiming, start doing writepage
* even in laptop mode.
*/
- if (sc.priority < DEF_PRIORITY - 2)
+ if (sc.priority < DEF_PRIORITY - 2 || !pgdat_reclaimable(pgdat))
sc.may_writepage = 1;

+ /* Call soft limit reclaim before calling shrink_node. */
+ sc.nr_scanned = 0;
+ nr_soft_scanned = 0;
+ nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone, order,
+ sc.gfp_mask, &nr_soft_scanned);
+ sc.nr_reclaimed += nr_soft_reclaimed;
+
/*
- * Continue scanning in the highmem->dma direction stopping at
- * the last zone which needs scanning. This may reclaim lowmem
- * pages that are not necessary for zone balancing but it
- * preserves LRU ordering. It is assumed that the bulk of
- * allocation requests can use arbitrary zones with the
- * possible exception of big highmem:lowmem configurations.
+ * There should be no need to raise the scanning priority if
+ * enough pages are already being scanned that that high
+ * watermark would be met at 100% efficiency.
*/
- for (i = end_zone; i >= end_zone; i--) {
- struct zone *zone = pgdat->node_zones + i;
-
- if (!populated_zone(zone))
- continue;
-
- if (sc.priority != DEF_PRIORITY &&
- !pgdat_reclaimable(zone->zone_pgdat))
- continue;
-
- sc.nr_scanned = 0;
-
- nr_soft_scanned = 0;
- /*
- * Call soft limit reclaim before calling shrink_zone.
- */
- nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
- order, sc.gfp_mask,
- &nr_soft_scanned);
- sc.nr_reclaimed += nr_soft_reclaimed;
-
- /*
- * There should be no need to raise the scanning
- * priority if enough pages are already being scanned
- * that that high watermark would be met at 100%
- * efficiency.
- */
- if (kswapd_shrink_zone(zone, end_zone, &sc))
- raise_priority = false;
- }
+ if (kswapd_shrink_node(pgdat, classzone_idx, &sc))
+ raise_priority = false;

/*
* If the low watermark is met there is no need for processes
@@ -3299,20 +3191,37 @@ 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, 0, classzone_idx)) {
+ clear_bit(PGDAT_CONGESTED, &pgdat->flags);
+ clear_bit(PGDAT_DIRTY, &pgdat->flags);
+ goto out;
+ }
+ }
+
+ /*
* Raise priority if scanning rate is too low or there was no
* progress in reclaiming pages
*/
if (raise_priority || !sc.nr_reclaimed)
sc.priority--;
- } while (sc.priority >= 1 &&
- !pgdat_balanced(pgdat, order, classzone_idx));
+ } while (sc.priority >= 1);

out:
/*
- * Return the highest zone idx we were reclaiming at so
- * prepare_kswapd_sleep() makes the same decisions as here.
+ * Return the order we were reclaiming at so prepare_kswapd_sleep()
+ * makes a decision on the order we were last reclaiming at. However,
+ * if another caller entered the allocator slow path while kswapd
+ * was awake, order will remain at the higher level
*/
- return end_zone;
+ return order;
}

static void kswapd_try_to_sleep(pg_data_t *pgdat, int order,
@@ -3500,7 +3409,7 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
}
if (!waitqueue_active(&pgdat->kswapd_wait))
return;
- if (zone_balanced(zone, order, true, 0, 0))
+ if (zone_balanced(zone, order, 0, 0))
return;

trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
--
2.6.4