[PATCH 1/6] mm: kswapd: Stop high-order balancing when any suitable zone is balanced

From: Mel Gorman
Date: Fri Dec 10 2010 - 10:47:36 EST

When the allocator enters its slow path, kswapd is woken up to balance the
node. It continues working until all zones within the node are balanced. For
order-0 allocations, this makes perfect sense but for higher orders it can
have unintended side-effects. If the zone sizes are imbalanced, kswapd may
reclaim heavily within a smaller zone discarding an excessive number of
pages. The user-visible behaviour is that kswapd is awake and reclaiming
even though plenty of pages are free from a suitable zone.

This patch alters the "balance" logic for high-order reclaim allowing kswapd
to stop if any suitable zone becomes balanced to reduce the number of pages
it reclaims from other zones. kswapd still tries to ensure that order-0
watermarks for all zones are met before sleeping.

Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
include/linux/mmzone.h | 3 +-
mm/page_alloc.c | 8 +++--
mm/vmscan.c | 68 +++++++++++++++++++++++++++++++++++++++++------
3 files changed, 66 insertions(+), 13 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 4890662..dad3612 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -639,6 +639,7 @@ typedef struct pglist_data {
wait_queue_head_t kswapd_wait;
struct task_struct *kswapd;
int kswapd_max_order;
+ enum zone_type classzone_idx;
} pg_data_t;

#define node_present_pages(nid) (NODE_DATA(nid)->node_present_pages)
@@ -654,7 +655,7 @@ typedef struct pglist_data {

extern struct mutex zonelists_mutex;
void build_all_zonelists(void *data);
-void wakeup_kswapd(struct zone *zone, int order);
+void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx);
bool zone_watermark_ok(struct zone *z, int order, unsigned long mark,
int classzone_idx, int alloc_flags);
bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1845a97..1497fe8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1936,13 +1936,14 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,

static inline
void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
- enum zone_type high_zoneidx)
+ enum zone_type high_zoneidx,
+ enum zone_type classzone_idx)
{
struct zoneref *z;
struct zone *zone;

for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
- wakeup_kswapd(zone, order);
+ wakeup_kswapd(zone, order, classzone_idx);
}

static inline int
@@ -2020,7 +2021,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
goto nopage;

restart:
- wake_all_kswapd(order, zonelist, high_zoneidx);
+ wake_all_kswapd(order, zonelist, high_zoneidx,
+ zone_idx(preferred_zone));

/*
* OK, we're below the kswapd watermark and have kicked background
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 42a4859..625dfba 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2239,11 +2239,14 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
* interoperates with the page allocator fallback scheme to ensure that aging
* of pages is balanced across the zones.
*/
-static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
+static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
+ int classzone_idx)
{
int all_zones_ok;
+ int any_zone_ok;
int priority;
int i;
+ int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
unsigned long total_scanned;
struct reclaim_state *reclaim_state = current->reclaim_state;
struct scan_control sc = {
@@ -2266,7 +2269,6 @@ loop_again:
count_vm_event(PAGEOUTRUN);

for (priority = DEF_PRIORITY; priority >= 0; priority--) {
- int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
unsigned long lru_pages = 0;
int has_under_min_watermark_zone = 0;

@@ -2275,6 +2277,7 @@ loop_again:
disable_swap_token();

all_zones_ok = 1;
+ any_zone_ok = 0;

/*
* Scan in the highmem->dma direction for the highest
@@ -2393,10 +2396,12 @@ loop_again:
* spectulatively avoid congestion waits
*/
zone_clear_flag(zone, ZONE_CONGESTED);
+ if (i <= classzone_idx)
+ any_zone_ok = 1;
}

}
- if (all_zones_ok)
+ if (all_zones_ok || (order && any_zone_ok))
break; /* kswapd: all done */
/*
* OK, kswapd is getting into trouble. Take a nap, then take
@@ -2419,7 +2424,13 @@ loop_again:
break;
}
out:
- if (!all_zones_ok) {
+
+ /*
+ * order-0: All zones must meet high watermark for a balanced node
+ * high-order: Any zone below pgdats classzone_idx must meet the high
+ * watermark for a balanced node
+ */
+ if (!(all_zones_ok || (order && any_zone_ok))) {
cond_resched();

try_to_freeze();
@@ -2444,6 +2455,36 @@ out:
goto loop_again;
}

+ /*
+ * If kswapd was reclaiming at a higher order, it has the option of
+ * sleeping without all zones being balanced. Before it does, it must
+ * ensure that the watermarks for order-0 on *all* zones are met and
+ * that the congestion flags are cleared. The congestion flag must
+ * be cleared as kswapd is the only mechanism that clears the flag
+ * and it is potentially going to sleep here.
+ */
+ if (order) {
+ for (i = 0; i <= end_zone; i++) {
+ struct zone *zone = pgdat->node_zones + i;
+
+ if (!populated_zone(zone))
+ continue;
+
+ if (zone->all_unreclaimable && priority != DEF_PRIORITY)
+ continue;
+
+ /* Confirm the zone is balanced for order-0 */
+ if (!zone_watermark_ok(zone, 0,
+ high_wmark_pages(zone), 0, 0)) {
+ order = sc.order = 0;
+ goto loop_again;
+ }
+
+ /* If balanced, clear the congested flag */
+ zone_clear_flag(zone, ZONE_CONGESTED);
+ }
+ }
+
return sc.nr_reclaimed;
}

@@ -2507,6 +2548,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
static int kswapd(void *p)
{
unsigned long order;
+ int classzone_idx;
pg_data_t *pgdat = (pg_data_t*)p;
struct task_struct *tsk = current;

@@ -2537,21 +2579,27 @@ static int kswapd(void *p)
set_freezable();

order = 0;
+ classzone_idx = MAX_NR_ZONES - 1;
for ( ; ; ) {
unsigned long new_order;
+ int new_classzone_idx;
int ret;

new_order = pgdat->kswapd_max_order;
+ new_classzone_idx = pgdat->classzone_idx;
pgdat->kswapd_max_order = 0;
- if (order < new_order) {
+ pgdat->classzone_idx = MAX_NR_ZONES - 1;
+ if (order < new_order || classzone_idx > new_classzone_idx) {
/*
* Don't sleep if someone wants a larger 'order'
- * allocation
+ * allocation or has tigher zone constraints
*/
order = new_order;
+ classzone_idx = new_classzone_idx;
} else {
kswapd_try_to_sleep(pgdat, order);
order = pgdat->kswapd_max_order;
+ classzone_idx = pgdat->classzone_idx;
}

ret = try_to_freeze();
@@ -2564,7 +2612,7 @@ static int kswapd(void *p)
*/
if (!ret) {
trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
- balance_pgdat(pgdat, order);
+ balance_pgdat(pgdat, order, classzone_idx);
}
}
return 0;
@@ -2573,7 +2621,7 @@ static int kswapd(void *p)
/*
* A zone is low on free memory, so wake its kswapd task to service it.
*/
-void wakeup_kswapd(struct zone *zone, int order)
+void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
{
pg_data_t *pgdat;

@@ -2583,8 +2631,10 @@ void wakeup_kswapd(struct zone *zone, int order)
if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
return;
pgdat = zone->zone_pgdat;
- if (pgdat->kswapd_max_order < order)
+ if (pgdat->kswapd_max_order < order) {
pgdat->kswapd_max_order = order;
+ pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
+ }
if (!waitqueue_active(&pgdat->kswapd_wait))
return;
if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0))
--
1.7.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/