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Subject[PATCH 03/16] mm: supporting variables and functions for balanced zone aging
The zone aging rates are currently imbalanced, the gap can be as large as 3
times, which can severely damage read-ahead requests and shorten their
effective life time.

This patch adds three variables in struct zone
- aging_total
- aging_milestone
- page_age
to keep track of page aging rate, and keep it in sync on page reclaim time.

The aging_total is just a per-zone counter-part to the per-cpu
pgscan_{kswapd,direct}_{zone name}. But it is not direct comparable between
zones, so the aging_milestone/page_age are maintained based on aging_total.
The page_age is a normalized value that can be direct compared between zones
with the helper macro age_ge/age_gt. The goal of balancing logics are to keep
this normalized value in sync between zones.

One can check the balanced aging progress by running:
tar c / | cat > /dev/null &
watch -n1 'grep "age " /proc/zoneinfo'
Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
include/linux/mmzone.h | 14 ++++++++++++++
mm/page_alloc.c | 11 +++++++++++
mm/vmscan.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 73 insertions(+)
--- linux.orig/include/linux/mmzone.h
+++ linux/include/linux/mmzone.h
@@ -149,6 +149,20 @@ struct zone {
unsigned long pages_scanned; /* since last reclaim */
int all_unreclaimable; /* All pages pinned */

+ /* Fields for balanced page aging:
+ * aging_total - The accumulated number of activities that may
+ * cause page aging, that is, make some pages closer
+ * to the tail of inactive_list.
+ * aging_milestone - A snapshot of total_scan every time a full
+ * inactive_list of pages become aged.
+ * page_age - A normalized value showing the percent of pages
+ * have been aged. It is compared between zones to
+ * balance the rate of page aging.
+ */
+ unsigned long aging_total;
+ unsigned long aging_milestone;
+ unsigned long page_age;
+
/*
* Does the allocator try to reclaim pages from the zone as soon
* as it fails a watermark_ok() in __alloc_pages?
--- linux.orig/mm/vmscan.c
+++ linux/mm/vmscan.c
@@ -123,6 +123,53 @@ static long total_memory;
static LIST_HEAD(shrinker_list);
static DECLARE_RWSEM(shrinker_rwsem);

+#ifdef CONFIG_HIGHMEM64G
+#define PAGE_AGE_SHIFT 8
+#elif BITS_PER_LONG == 32
+#define PAGE_AGE_SHIFT 12
+#elif BITS_PER_LONG == 64
+#define PAGE_AGE_SHIFT 20
+#else
+#error unknown BITS_PER_LONG
+#endif
+#define PAGE_AGE_SIZE (1 << PAGE_AGE_SHIFT)
+#define PAGE_AGE_MASK (PAGE_AGE_SIZE - 1)
+
+/*
+ * The simplified code is:
+ * age_ge: (@a->page_age >= @b->page_age)
+ * age_gt: (@a->page_age > @b->page_age)
+ * The complexity deals with the wrap-around problem.
+ * Two page ages not close enough(gap >= 1/8) should also be ignored:
+ * they are out of sync and the comparison may be nonsense.
+ *
+ * Return value depends on the position of @a relative to @b:
+ * -1/8 b +1/8
+ * |--------|--------|-----------------------------------------------|
+ * 0 1 0
+ */
+#define age_ge(a, b) \
+ (((a->page_age - b->page_age) & PAGE_AGE_MASK) < PAGE_AGE_SIZE / 8)
+#define age_gt(a, b) \
+ (((b->page_age - a->page_age) & PAGE_AGE_MASK) > PAGE_AGE_SIZE * 7 / 8)
+
+/*
+ * Keep track of the percent of cold pages that have been scanned / aged.
+ * It's not really ##%, but a high resolution normalized value.
+ */
+static inline void update_zone_age(struct zone *z, int nr_scan)
+{
+ unsigned long len = z->nr_inactive | 1;
+
+ z->aging_total += nr_scan;
+
+ if (z->aging_total - z->aging_milestone > len)
+ z->aging_milestone += len;
+
+ z->page_age = ((z->aging_total - z->aging_milestone)
+ << PAGE_AGE_SHIFT) / len;
+}
+
/*
* Add a shrinker callback to be called from the vm
*/
@@ -887,6 +934,7 @@ static void shrink_cache(struct zone *zo
&page_list, &nr_scan);
zone->nr_inactive -= nr_taken;
zone->pages_scanned += nr_scan;
+ update_zone_age(zone, nr_scan);
spin_unlock_irq(&zone->lru_lock);

if (nr_taken == 0)
--- linux.orig/mm/page_alloc.c
+++ linux/mm/page_alloc.c
@@ -1522,6 +1522,8 @@ void show_free_areas(void)
" active:%lukB"
" inactive:%lukB"
" present:%lukB"
+ " aging:%lukB"
+ " age:%lu"
" pages_scanned:%lu"
" all_unreclaimable? %s"
"\n",
@@ -1533,6 +1535,8 @@ void show_free_areas(void)
K(zone->nr_active),
K(zone->nr_inactive),
K(zone->present_pages),
+ K(zone->aging_total),
+ zone->page_age,
zone->pages_scanned,
(zone->all_unreclaimable ? "yes" : "no")
);
@@ -2144,6 +2148,9 @@ static void __init free_area_init_core(s
zone->nr_scan_inactive = 0;
zone->nr_active = 0;
zone->nr_inactive = 0;
+ zone->aging_total = 0;
+ zone->aging_milestone = 0;
+ zone->page_age = 0;
atomic_set(&zone->reclaim_in_progress, 0);
if (!size)
continue;
@@ -2292,6 +2299,8 @@ static int zoneinfo_show(struct seq_file
"\n high %lu"
"\n active %lu"
"\n inactive %lu"
+ "\n aging %lu"
+ "\n age %lu"
"\n scanned %lu (a: %lu i: %lu)"
"\n spanned %lu"
"\n present %lu",
@@ -2301,6 +2310,8 @@ static int zoneinfo_show(struct seq_file
zone->pages_high,
zone->nr_active,
zone->nr_inactive,
+ zone->aging_total,
+ zone->page_age,
zone->pages_scanned,
zone->nr_scan_active, zone->nr_scan_inactive,
zone->spanned_pages,
--
-
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