[PATCH 0/6] Prevent kswapd dumping excessive amounts of memory in response to high-order allocations V4

[PATCH 0/6] Prevent kswapd dumping excessive amounts of memory in response to high-order allocations V4

[Mel Gorman]

This is a primarily a rebase onto mmotm.

Changelog since V3
  o Rebase to mmotm-2010-12-02-16-34
  o Added reviewed-by's
  o pgdat_balanced() considers the number of present pages allowed by the
    classzone_idx instead of the full nodes pages
  o Clarify a number of comments

Changelog since V2
  o Add clarifying comments
  o Properly check that the zone is balanced for order-0
  o Treat zone->all_unreclaimable properly

Changelog since V1
  o Take classzone into account
  o Ensure that kswapd always balances at order-09
  o Reset classzone and order after reading
  o Require a percentage of a node be balanced for high-order allocations,
    not just any zone as ZONE_DMA could be balanced when the node in general
    is a mess

Simon Kirby reported the following problem

   We're seeing cases on a number of servers where cache never fully
   grows to use all available memory.  Sometimes we see servers with 4
   GB of memory that never seem to have less than 1.5 GB free, even with
   a constantly-active VM.  In some cases, these servers also swap out
   while this happens, even though they are constantly reading the working
   set into memory.  We have been seeing this happening for a long time;
   I don't think it's anything recent, and it still happens on 2.6.36.

After some debugging work by Simon, Dave Hansen and others, the prevaling
theory became that kswapd is reclaiming order-3 pages requested by SLUB
too aggressive about it.

There are two apparent problems here. On the target machine, there is a small
Normal zone in comparison to DMA32. As kswapd tries to balance all zones, it
would continually try reclaiming for Normal even though DMA32 was balanced
enough for callers. The second problem is that sleeping_prematurely() does
not use the same logic as balance_pgdat() when deciding whether to sleep
or not.  This keeps kswapd artifically awake.

A number of tests were run and the figures from previous postings will look
very different for a few reasons. One, the old figures were forcing my network
card to use GFP_ATOMIC in attempt to replicate Simon's problem. Second, I
previous specified slub_min_order=3 again in an attempt to reproduce Simon's
problem. In this posting, I'm depending on Simon to say whether his problem is
fixed or not and these figures are to show the impact to the ordinary cases.
Finally, the "vmscan" figures are taken from /proc/vmstat instead of the
tracepoints.  There is less information but recording is less disruptive.

The first test of relevance was postmark with a process running in the
background reading a large amount of anonymous memory in blocks. The
objective was to vaguely simulate what was happening on Simon's machine
and it's memory intensive enough to have kswapd awake.

POSTMARK
                                            traceonly          kanyzone
Transactions per second:              156.00 ( 0.00%)   153.00 (-1.96%)
Data megabytes read per second:        21.51 ( 0.00%)    21.52 ( 0.05%)
Data megabytes written per second:     29.28 ( 0.00%)    29.11 (-0.58%)
Files created alone per second:       250.00 ( 0.00%)   416.00 (39.90%)
Files create/transact per second:      79.00 ( 0.00%)    76.00 (-3.95%)
Files deleted alone per second:       520.00 ( 0.00%)   420.00 (-23.81%)
Files delete/transact per second:      79.00 ( 0.00%)    76.00 (-3.95%)

MMTests Statistics: duration
User/Sys Time Running Test (seconds)         16.58      17.4
Total Elapsed Time (seconds)                218.48    222.47

VMstat Reclaim Statistics: vmscan
Direct reclaims                                  0          4
Direct reclaim pages scanned                     0        203
Direct reclaim pages reclaimed                   0        184
Kswapd pages scanned                        326631     322018
Kswapd pages reclaimed                      312632     309784
Kswapd low wmark quickly                         1          4
Kswapd high wmark quickly                      122        475
Kswapd skip congestion_wait                      1          0
Pages activated                             700040     705317
Pages deactivated                           212113     203922
Pages written                                 9875       6363

Total pages scanned                         326631    322221
Total pages reclaimed                       312632    309968
%age total pages scanned/reclaimed          95.71%    96.20%
%age total pages scanned/written             3.02%     1.97%

proc vmstat: Faults
Major Faults                                   300       254
Minor Faults                                645183    660284
Page ins                                    493588    486704
Page outs                                  4960088   4986704
Swap ins                                      1230       661
Swap outs                                     9869      6355

Performance is mildly affected because kswapd is no longer doing as much
work and the background memory consumer process is getting in the way. Note
that kswapd scanned and reclaimed fewer pages as it's less aggressive and
overall fewer pages were scanned and reclaimed. Swap in/out is particularly
reduced again reflecting kswapd throwing out fewer pages.

The slight performance impact is unfortunate here but it looks like a direct
result of kswapd being less aggressive. As the bug report is about too many
pages being freed by kswapd, it may have to be accepted for now.

The second test is a streaming IO benchmark that was previously used by
Johannes to show regressions in page reclaim.

MICRO
					 traceonly  kanyzone
User/Sys Time Running Test (seconds)         29.29     28.87
Total Elapsed Time (seconds)                492.18    488.79

VMstat Reclaim Statistics: vmscan
Direct reclaims                               2128       1460
Direct reclaim pages scanned               2284822    1496067
Direct reclaim pages reclaimed              148919     110937
Kswapd pages scanned                      15450014   16202876
Kswapd pages reclaimed                     8503697    8537897
Kswapd low wmark quickly                      3100       3397
Kswapd high wmark quickly                     1860       7243
Kswapd skip congestion_wait                    708        801
Pages activated                               9635       9573
Pages deactivated                             1432       1271
Pages written                                  223       1130

Total pages scanned                       17734836  17698943
Total pages reclaimed                      8652616   8648834
%age total pages scanned/reclaimed          48.79%    48.87%
%age total pages scanned/written             0.00%     0.01%

proc vmstat: Faults
Major Faults                                   165       221
Minor Faults                               9655785   9656506
Page ins                                      3880      7228
Page outs                                 37692940  37480076
Swap ins                                         0        69
Swap outs                                       19        15

Again fewer pages are scanned and reclaimed as expected and this time the test
completed faster. Note that kswapd is hitting its watermarks faster (low and
high wmark quickly) which I expect is due to kswapd reclaiming fewer pages.

I also ran fs-mark, iozone and sysbench but there is nothing interesting
to report in the figures. Performance is not significantly changed and the
reclaim statistics look reasonable.

 include/linux/mmzone.h |    3 +-
 mm/page_alloc.c        |    8 ++-
 mm/vmscan.c            |  153 +++++++++++++++++++++++++++++++++++++++++-------
 3 files changed, 138 insertions(+), 26 deletions(-)

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

[Mel Gorman]

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@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 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

[PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[Mel Gorman]

When reclaiming for high-orders, kswapd is responsible for balancing a
node but it should not reclaim excessively. It avoids excessive reclaim by
considering if any zone in a node is balanced then the node is balanced. In
the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
one small zone was balanced.

This alters the sleep logic of kswapd slightly. It counts the number of pages
that make up the balanced zones. If the total number of balanced pages is
more than a quarter of the zone, kswapd will go back to sleep. This should
keep a node balanced without reclaiming an excessive number of pages.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/vmscan.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 files changed, 49 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 625dfba..6723101 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2191,10 +2191,40 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
 }
 #endif
 
+/*
+ * pgdat_balanced is used when checking if a node is 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
+ *     precentage 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, unsigned long balanced_pages,
+						int classzone_idx)
+{
+	unsigned long present_pages = 0;
+	int i;
+
+	for (i = 0; i <= classzone_idx; i++)
+		present_pages += pgdat->node_zones[i].present_pages;
+
+	return balanced_pages > (present_pages >> 2);
+}
+
 /* is kswapd sleeping prematurely? */
 static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 {
 	int i;
+	unsigned long balanced = 0;
+	bool all_zones_ok = true;
 
 	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
 	if (remaining)
@@ -2212,10 +2242,20 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 
 		if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
 								0, 0))
-			return 1;
+			all_zones_ok = false;
+		else
+			balanced += zone->present_pages;
 	}
 
-	return 0;
+	/*
+	 * For high-order requests, the balanced zones must contain at least
+	 * 25% of the nodes pages for kswapd to sleep. For order-0, all zones
+	 * must be balanced
+	 */
+	if (order)
+		return pgdat_balanced(pgdat, balanced, 0);
+	else
+		return !all_zones_ok;
 }
 
 /*
@@ -2243,7 +2283,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 							int classzone_idx)
 {
 	int all_zones_ok;
-	int any_zone_ok;
+	unsigned long balanced;
 	int priority;
 	int i;
 	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
@@ -2277,7 +2317,7 @@ loop_again:
 			disable_swap_token();
 
 		all_zones_ok = 1;
-		any_zone_ok = 0;
+		balanced = 0;
 
 		/*
 		 * Scan in the highmem->dma direction for the highest
@@ -2397,11 +2437,11 @@ loop_again:
 				 */
 				zone_clear_flag(zone, ZONE_CONGESTED);
 				if (i <= classzone_idx)
-					any_zone_ok = 1;
+					balanced += zone->present_pages;
 			}
 
 		}
-		if (all_zones_ok || (order && any_zone_ok))
+		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, classzone_idx)))
 			break;		/* kswapd: all done */
 		/*
 		 * OK, kswapd is getting into trouble.  Take a nap, then take
@@ -2427,10 +2467,10 @@ out:
 
 	/*
 	 * 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
+	 * high-order: Balanced zones must make up at least 25% of the node
+	 *             for the node to be balanced
 	 */
-	if (!(all_zones_ok || (order && any_zone_ok))) {
+	if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, classzone_idx)))) {
 		cond_resched();
 
 		try_to_freeze();
-- 
1.7.1

[PATCH 3/6] mm: kswapd: Use the order that kswapd was reclaiming at for sleeping_prematurely()

[Mel Gorman]

Before kswapd goes to sleep, it uses sleeping_prematurely() to check if
there was a race pushing a zone below its watermark. If the race happened,
it stays awake. However, balance_pgdat() can decide to reclaim at order-0
if it decides that high-order reclaim is not working as expected. This
information is not passed back to sleeping_prematurely().  The impact is
that kswapd remains awake reclaiming pages long after it should have gone
to sleep. This patch passes the adjusted order to sleeping_prematurely and
uses the same logic as balance_pgdat to decide if it's ok to go to sleep.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/vmscan.c |   16 +++++++++++-----
 1 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6723101..4d968b0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2220,7 +2220,7 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
 }
 
 /* is kswapd sleeping prematurely? */
-static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
+static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 {
 	int i;
 	unsigned long balanced = 0;
@@ -2230,7 +2230,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 	if (remaining)
 		return 1;
 
-	/* If after HZ/10, a zone is below the high mark, it's premature */
+	/* Check the watermark levels */
 	for (i = 0; i < pgdat->nr_zones; i++) {
 		struct zone *zone = pgdat->node_zones + i;
 
@@ -2262,7 +2262,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
  * For kswapd, balance_pgdat() will work across all this node's zones until
  * they are all at high_wmark_pages(zone).
  *
- * Returns the number of pages which were actually freed.
+ * Returns the final order 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
@@ -2525,7 +2525,13 @@ out:
 		}
 	}
 
-	return sc.nr_reclaimed;
+	/*
+	 * Return the order we were reclaiming at so sleeping_prematurely()
+	 * 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 order;
 }
 
 static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
@@ -2652,7 +2658,7 @@ static int kswapd(void *p)
 		 */
 		if (!ret) {
 			trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
-			balance_pgdat(pgdat, order, classzone_idx);
+			order = balance_pgdat(pgdat, order, classzone_idx);
 		}
 	}
 	return 0;
-- 
1.7.1

[PATCH 4/6] mm: kswapd: Reset kswapd_max_order and classzone_idx after reading

[Mel Gorman]

When kswapd wakes up, it reads its order and classzone from pgdat and
calls balance_pgdat. While its awake, it potentially reclaimes at a high
order and a low classzone index. This might have been a once-off that
was not required by subsequent callers. However, because the pgdat
values were not reset, they remain artifically high while
balance_pgdat() is running and potentially kswapd enters a second
unnecessary reclaim cycle. Reset the pgdat order and classzone index
after reading.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/vmscan.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4d968b0..e1be4e8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2646,6 +2646,8 @@ static int kswapd(void *p)
 			kswapd_try_to_sleep(pgdat, order);
 			order = pgdat->kswapd_max_order;
 			classzone_idx = pgdat->classzone_idx;
+			pgdat->kswapd_max_order = 0;
+			pgdat->classzone_idx = MAX_NR_ZONES - 1;
 		}
 
 		ret = try_to_freeze();
-- 
1.7.1

[PATCH 5/6] mm: kswapd: Treat zone->all_unreclaimable in sleeping_prematurely similar to balance_pgdat()

[Mel Gorman]

After DEF_PRIORITY, balance_pgdat() considers all_unreclaimable zones to
be balanced but sleeping_prematurely does not. This can force kswapd to
stay awake longer than it should. This patch fixes it.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
 mm/vmscan.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index e1be4e8..5995121 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2237,8 +2237,16 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 		if (!populated_zone(zone))
 			continue;
 
-		if (zone->all_unreclaimable)
+		/*
+		 * balance_pgdat() skips over all_unreclaimable after
+		 * DEF_PRIORITY. Effectively, it considers them balanced so
+		 * they must be considered balanced here as well if kswapd
+		 * is to sleep
+		 */
+		if (zone->all_unreclaimable) {
+			balanced += zone->present_pages;
 			continue;
+		}
 
 		if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
 								0, 0))
-- 
1.7.1

[PATCH 6/6] mm: kswapd: Use the classzone idx that kswapd was using for sleeping_prematurely()

[Mel Gorman]

When kswapd is woken up for a high-order allocation, it takes account of
the highest usable zone by the caller (the classzone idx). During
allocation, this index is used to select the lowmem_reserve[] that
should be applied to the watermark calculation in zone_watermark_ok().

When balancing a node, kswapd considers the highest unbalanced zone to be the
classzone index. This will always be at least be the callers classzone_idx
and can be higher. However, sleeping_prematurely() always considers the
lowest zone (e.g. ZONE_DMA) to be the classzone index. This means that
sleeping_prematurely() can consider a zone to be balanced that is unusable
by the allocation request that originally woke kswapd. This patch changes
sleeping_prematurely() to use a classzone_idx matching the value it used
in balance_pgdat().

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/vmscan.c |   29 ++++++++++++++++-------------
 1 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5995121..cf03a11 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2220,7 +2220,8 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
 }
 
 /* is kswapd sleeping prematurely? */
-static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
+static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
+					int classzone_idx)
 {
 	int i;
 	unsigned long balanced = 0;
@@ -2228,7 +2229,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 
 	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
 	if (remaining)
-		return 1;
+		return true;
 
 	/* Check the watermark levels */
 	for (i = 0; i < pgdat->nr_zones; i++) {
@@ -2249,7 +2250,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 		}
 
 		if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
-								0, 0))
+							classzone_idx, 0))
 			all_zones_ok = false;
 		else
 			balanced += zone->present_pages;
@@ -2261,7 +2262,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
 	 * must be balanced
 	 */
 	if (order)
-		return pgdat_balanced(pgdat, balanced, 0);
+		return pgdat_balanced(pgdat, balanced, classzone_idx);
 	else
 		return !all_zones_ok;
 }
@@ -2288,7 +2289,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
  * of pages is balanced across the zones.
  */
 static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
-							int classzone_idx)
+							int *classzone_idx)
 {
 	int all_zones_ok;
 	unsigned long balanced;
@@ -2351,6 +2352,7 @@ loop_again:
 			if (!zone_watermark_ok_safe(zone, order,
 					high_wmark_pages(zone), 0, 0)) {
 				end_zone = i;
+				*classzone_idx = i;
 				break;
 			}
 		}
@@ -2444,12 +2446,12 @@ loop_again:
 				 * spectulatively avoid congestion waits
 				 */
 				zone_clear_flag(zone, ZONE_CONGESTED);
-				if (i <= classzone_idx)
+				if (i <= *classzone_idx)
 					balanced += zone->present_pages;
 			}
 
 		}
-		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, classzone_idx)))
+		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
 			break;		/* kswapd: all done */
 		/*
 		 * OK, kswapd is getting into trouble.  Take a nap, then take
@@ -2478,7 +2480,7 @@ out:
 	 * high-order: Balanced zones must make up at least 25% of the node
 	 *             for the node to be balanced
 	 */
-	if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, classzone_idx)))) {
+	if (!(all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))) {
 		cond_resched();
 
 		try_to_freeze();
@@ -2539,10 +2541,11 @@ out:
 	 * if another caller entered the allocator slow path while kswapd
 	 * was awake, order will remain at the higher level
 	 */
+	*classzone_idx = end_zone;
 	return order;
 }
 
-static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
+static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
 {
 	long remaining = 0;
 	DEFINE_WAIT(wait);
@@ -2553,7 +2556,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
 	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 
 	/* Try to sleep for a short interval */
-	if (!sleeping_prematurely(pgdat, order, remaining)) {
+	if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {
 		remaining = schedule_timeout(HZ/10);
 		finish_wait(&pgdat->kswapd_wait, &wait);
 		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
@@ -2563,7 +2566,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order)
 	 * After a short sleep, check if it was a premature sleep. If not, then
 	 * go fully to sleep until explicitly woken up.
 	 */
-	if (!sleeping_prematurely(pgdat, order, remaining)) {
+	if (!sleeping_prematurely(pgdat, order, remaining, classzone_idx)) {
 		trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
 
 		/*
@@ -2651,7 +2654,7 @@ static int kswapd(void *p)
 			order = new_order;
 			classzone_idx = new_classzone_idx;
 		} else {
-			kswapd_try_to_sleep(pgdat, order);
+			kswapd_try_to_sleep(pgdat, order, classzone_idx);
 			order = pgdat->kswapd_max_order;
 			classzone_idx = pgdat->classzone_idx;
 			pgdat->kswapd_max_order = 0;
@@ -2668,7 +2671,7 @@ static int kswapd(void *p)
 		 */
 		if (!ret) {
 			trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
-			order = balance_pgdat(pgdat, order, classzone_idx);
+			order = balance_pgdat(pgdat, order, &classzone_idx);
 		}
 	}
 	return 0;
-- 
1.7.1

Re: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[KAMEZAWA Hiroyuki]

On Fri, 10 Dec 2010 15:46:21 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> When reclaiming for high-orders, kswapd is responsible for balancing a
> node but it should not reclaim excessively. It avoids excessive reclaim by
> considering if any zone in a node is balanced then the node is balanced. In
> the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> one small zone was balanced.
> 
> This alters the sleep logic of kswapd slightly. It counts the number of pages
> that make up the balanced zones. If the total number of balanced pages is
> more than a quarter of the zone, kswapd will go back to sleep. This should
> keep a node balanced without reclaiming an excessive number of pages.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> 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@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Started reviewing before I saw this series.

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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Re: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> When reclaiming for high-orders, kswapd is responsible for balancing a
> node but it should not reclaim excessively. It avoids excessive reclaim by
> considering if any zone in a node is balanced then the node is balanced. In
> the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> one small zone was balanced.
> 
> This alters the sleep logic of kswapd slightly. It counts the number of pages
> that make up the balanced zones. If the total number of balanced pages is
> more than a quarter of the zone, kswapd will go back to sleep. This should
> keep a node balanced without reclaiming an excessive number of pages.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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Re: [PATCH 3/6] mm: kswapd: Use the order that kswapd was reclaiming at for sleeping_prematurely()

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> Before kswapd goes to sleep, it uses sleeping_prematurely() to check if
> there was a race pushing a zone below its watermark. If the race happened,
> it stays awake. However, balance_pgdat() can decide to reclaim at order-0
> if it decides that high-order reclaim is not working as expected. This
> information is not passed back to sleeping_prematurely().  The impact is
> that kswapd remains awake reclaiming pages long after it should have gone
> to sleep. This patch passes the adjusted order to sleeping_prematurely and
> uses the same logic as balance_pgdat to decide if it's ok to go to sleep.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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Re: [PATCH 4/6] mm: kswapd: Reset kswapd_max_order and classzone_idx after reading

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> When kswapd wakes up, it reads its order and classzone from pgdat and
> calls balance_pgdat. While its awake, it potentially reclaimes at a high
> order and a low classzone index. This might have been a once-off that
> was not required by subsequent callers. However, because the pgdat
> values were not reset, they remain artifically high while
> balance_pgdat() is running and potentially kswapd enters a second
> unnecessary reclaim cycle. Reset the pgdat order and classzone index
> after reading.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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Re: [PATCH 5/6] mm: kswapd: Treat zone->all_unreclaimable in sleeping_prematurely similar to balance_pgdat()

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> After DEF_PRIORITY, balance_pgdat() considers all_unreclaimable zones to
> be balanced but sleeping_prematurely does not. This can force kswapd to
> stay awake longer than it should. This patch fixes it.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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Re: [PATCH 6/6] mm: kswapd: Use the classzone idx that kswapd was using for sleeping_prematurely()

[Eric B Munson]

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On Fri, 10 Dec 2010, Mel Gorman wrote:

> When kswapd is woken up for a high-order allocation, it takes account of
> the highest usable zone by the caller (the classzone idx). During
> allocation, this index is used to select the lowmem_reserve[] that
> should be applied to the watermark calculation in zone_watermark_ok().
> 
> When balancing a node, kswapd considers the highest unbalanced zone to be the
> classzone index. This will always be at least be the callers classzone_idx
> and can be higher. However, sleeping_prematurely() always considers the
> lowest zone (e.g. ZONE_DMA) to be the classzone index. This means that
> sleeping_prematurely() can consider a zone to be balanced that is unusable
> by the allocation request that originally woke kswapd. This patch changes
> sleeping_prematurely() to use a classzone_idx matching the value it used
> in balance_pgdat().
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

Reviewed-by: Eric B Munson <emunson@mgebm.net>

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

[Andrew Morton]

On Fri, 10 Dec 2010 15:46:20 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> 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.

Why was it doing this?  

> The user-visible behaviour is that kswapd is awake and reclaiming
> even though plenty of pages are free from a suitable zone.

Suitable for what?  I assume you refer to a future allocation which can
be satisfied from more than one of the zones?

But what if that allocation wanted to allocate a high-order page from
a zone which we just abandoned?

> 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

again, suitable for what?

> it reclaims from other zones. kswapd still tries to ensure that order-0
> watermarks for all zones are met before sleeping.

Handling order-0 pages differently from higher-order pages sounds weird
and wrong.

I don't think I understand this patch.

Re: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[Andrew Morton]

On Fri, 10 Dec 2010 15:46:21 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> When reclaiming for high-orders, kswapd is responsible for balancing a
> node but it should not reclaim excessively. It avoids excessive reclaim by
> considering if any zone in a node is balanced then the node is balanced.

Here you're referring to your [patch 1/6] yes?  Not to current upstream.

> In
> the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> one small zone was balanced.

Since [1/6]?

> This alters the sleep logic of kswapd slightly. It counts the number of pages
> that make up the balanced zones. If the total number of balanced pages is

Define "balanced page"?  Seems to be the sum of the total sizes of all
zones which have reached their desired free-pages threshold?

But this includes all page orders, whereas here we're targetting a
particular order.  Although things should work out OK due to the
scaling/sizing proportionality.

> more than a quarter of the zone, kswapd will go back to sleep. This should
> keep a node balanced without reclaiming an excessive number of pages.

ick.

> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/vmscan.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++++---------
>  1 files changed, 49 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 625dfba..6723101 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2191,10 +2191,40 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  }
>  #endif
>  
> +/*
> + * pgdat_balanced is used when checking if a node is balanced for high-order
> + * allocations.

Is this the correct use of the term "balanced"?  I think "balanced" is
something that happens *between* zones: They've all achieved the same
(perhaps weighted) ratio of free pages.  

>  Only zones that meet watermarks and are in a zone allowed
> + * by the callers classzone_idx are added to balanced_pages. The total of

caller's

> + * 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.

Excessive reclaim of what?

If one particular zone is having trouble achieving its desired level of
free pages of a partocular order, are you saying that kswapd sits there
madly scanning other zones, which have already reached their desired
level?  If so, that would be bad.

I think you're saying that we just keep on scanning away at this one
zone.  But what was wrong with doing that?

> + * 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

How does a zone balance another zone?

> + *   o On all other machines, the top zone must be at least a reasonable
> + *     precentage 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, unsigned long balanced_pages,
> +						int classzone_idx)
> +{
> +	unsigned long present_pages = 0;
> +	int i;
> +
> +	for (i = 0; i <= classzone_idx; i++)
> +		present_pages += pgdat->node_zones[i].present_pages;
> +
> +	return balanced_pages > (present_pages >> 2);
> +}
> +
>
> ...
>

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

[Mel Gorman]

On Tue, Dec 14, 2010 at 02:33:06PM -0800, Andrew Morton wrote:
> On Fri, 10 Dec 2010 15:46:20 +0000
> Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > 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.
> 
> Why was it doing this?  
> 

Partially because of lumpy reclaim but mostly because it simply stays
awake. If the zone is unbalanced, kswapd will reclaim in there,
shrinking slabs, rotating lists etc. even if ultimately it cannot
balance that zone.

> > The user-visible behaviour is that kswapd is awake and reclaiming
> > even though plenty of pages are free from a suitable zone.
> 
> Suitable for what?  I assume you refer to a future allocation which can
> be satisfied from more than one of the zones?
> 

Yes.

> But what if that allocation wanted to allocate a high-order page from
> a zone which we just abandoned?
> 

classzone_idx is taken into account by the series overall and it doesn't
count zones above the classzone_idx.

> > 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
> 
> again, suitable for what?
> 

Suitable for a future allocation of the same type that woke kswapd.

> > it reclaims from other zones. kswapd still tries to ensure that order-0
> > watermarks for all zones are met before sleeping.
> 
> Handling order-0 pages differently from higher-order pages sounds weird
> and wrong.
> 
> I don't think I understand this patch.
> 

The objective is that kswapd will go to sleep again. It has been found
when there is a constant source of high-order allocations that kswapd
stays awake constantly trying to reclaim even though a suitable zone had
free pages.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

Re: [PATCH 2/6] mm: kswapd: Keep kswapd awake for high-order allocations until a percentage of the node is balanced

[Mel Gorman]

On Tue, Dec 14, 2010 at 02:43:41PM -0800, Andrew Morton wrote:
> On Fri, 10 Dec 2010 15:46:21 +0000
> Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > When reclaiming for high-orders, kswapd is responsible for balancing a
> > node but it should not reclaim excessively. It avoids excessive reclaim by
> > considering if any zone in a node is balanced then the node is balanced.
> 
> Here you're referring to your [patch 1/6] yes?  Not to current upstream.
> 

Yes.

> > In
> > the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> > ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> > one small zone was balanced.
> 
> Since [1/6]?
> 

Yes.

> > This alters the sleep logic of kswapd slightly. It counts the number of pages
> > that make up the balanced zones. If the total number of balanced pages is
> 
> Define "balanced page"?  Seems to be the sum of the total sizes of all
> zones which have reached their desired free-pages threshold?
> 

Correct.

> But this includes all page orders, whereas here we're targetting a
> particular order.  Although things should work out OK due to the
> scaling/sizing proportionality.
> 

It's the size of the whole zone that is being accounted for and as it's
a watermark check, the order is being taken into account.

> > more than a quarter of the zone, kswapd will go back to sleep. This should
> > keep a node balanced without reclaiming an excessive number of pages.
> 
> ick.
> 
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> > ---
> >  mm/vmscan.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++++---------
> >  1 files changed, 49 insertions(+), 9 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 625dfba..6723101 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2191,10 +2191,40 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> >  }
> >  #endif
> >  
> > +/*
> > + * pgdat_balanced is used when checking if a node is balanced for high-order
> > + * allocations.
> 
> Is this the correct use of the term "balanced"?  I think "balanced" is
> something that happens *between* zones: They've all achieved the same
> (perhaps weighted) ratio of free pages.  
> 

What would be a better term?  pgdat_sufficiently_but_not_fully_balanced()? If
it returns true, it can mean the node is either fully "balanced" as you
define it or that enough zones have enough free suitably-ordered pages for
allocations to succeed.

> >  Only zones that meet watermarks and are in a zone allowed
> > + * by the callers classzone_idx are added to balanced_pages. The total of
> 
> caller's
> 

Right.

> > + * 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.
> 
> Excessive reclaim of what?
> 

slab, list rotations and pages within the imbalanced zones that may never
become balanced. Minimally, kswapd just stays awake consuming CPU.

> If one particular zone is having trouble achieving its desired level of
> free pages of a partocular order, are you saying that kswapd sits there
> madly scanning other zones, which have already reached their desired
> level?  If so, that would be bad.
> 

As far as I can gather, yes, this is what is happening. I don't have a local
reproduction case so I'm basing this on a bug report. He has two problems -
kswapd stays awake constantly and way too many pages are free.

> I think you're saying that we just keep on scanning away at this one
> zone.  But what was wrong with doing that?
> 

It wastes CPU.

> > + * 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
> 
> How does a zone balance another zone?
> 

That should have been "will not balance a node".

> > + *   o On all other machines, the top zone must be at least a reasonable
> > + *     precentage 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, unsigned long balanced_pages,
> > +						int classzone_idx)
> > +{
> > +	unsigned long present_pages = 0;
> > +	int i;
> > +
> > +	for (i = 0; i <= classzone_idx; i++)
> > +		present_pages += pgdat->node_zones[i].present_pages;
> > +
> > +	return balanced_pages > (present_pages >> 2);
> > +}
> > +
> >
> > ...
> >
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab