aboutsummaryrefslogtreecommitdiffstats
path: root/mm
diff options
context:
space:
mode:
authorChristoph Lameter <cl@linux.com>2010-09-09 16:38:17 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-09-09 18:57:25 -0700
commitaa45484031ddee09b06350ab8528bfe5b2c76d1c (patch)
tree6758072232db9a54453022ec3e6cede35d52001c /mm
parent72853e2991a2702ae93aaf889ac7db743a415dd3 (diff)
downloadmrst-s0i3-test-aa45484031ddee09b06350ab8528bfe5b2c76d1c.tar.gz
mrst-s0i3-test-aa45484031ddee09b06350ab8528bfe5b2c76d1c.tar.xz
mrst-s0i3-test-aa45484031ddee09b06350ab8528bfe5b2c76d1c.zip
mm: page allocator: calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake
Ordinarily watermark checks are based on the vmstat NR_FREE_PAGES as it is cheaper than scanning a number of lists. To avoid synchronization overhead, counter deltas are maintained on a per-cpu basis and drained both periodically and when the delta is above a threshold. On large CPU systems, the difference between the estimated and real value of NR_FREE_PAGES can be very high. If NR_FREE_PAGES is much higher than number of real free page in buddy, the VM can allocate pages below min watermark, at worst reducing the real number of pages to zero. Even if the OOM killer kills some victim for freeing memory, it may not free memory if the exit path requires a new page resulting in livelock. This patch introduces a zone_page_state_snapshot() function (courtesy of Christoph) that takes a slightly more accurate view of an arbitrary vmstat counter. It is used to read NR_FREE_PAGES while kswapd is awake to avoid the watermark being accidentally broken. The estimate is not perfect and may result in cache line bounces but is expected to be lighter than the IPI calls necessary to continually drain the per-cpu counters while kswapd is awake. Signed-off-by: Christoph Lameter <cl@linux.com> Signed-off-by: Mel Gorman <mel@csn.ul.ie> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r--mm/mmzone.c21
-rw-r--r--mm/page_alloc.c4
-rw-r--r--mm/vmstat.c15
3 files changed, 37 insertions, 3 deletions
diff --git a/mm/mmzone.c b/mm/mmzone.c
index f5b7d176021..e35bfb82c85 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -87,3 +87,24 @@ int memmap_valid_within(unsigned long pfn,
return 1;
}
#endif /* CONFIG_ARCH_HAS_HOLES_MEMORYMODEL */
+
+#ifdef CONFIG_SMP
+/* Called when a more accurate view of NR_FREE_PAGES is needed */
+unsigned long zone_nr_free_pages(struct zone *zone)
+{
+ unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES);
+
+ /*
+ * While kswapd is awake, it is considered the zone is under some
+ * memory pressure. Under pressure, there is a risk that
+ * per-cpu-counter-drift will allow the min watermark to be breached
+ * potentially causing a live-lock. While kswapd is awake and
+ * free pages are low, get a better estimate for free pages
+ */
+ if (nr_free_pages < zone->percpu_drift_mark &&
+ !waitqueue_active(&zone->zone_pgdat->kswapd_wait))
+ return zone_page_state_snapshot(zone, NR_FREE_PAGES);
+
+ return nr_free_pages;
+}
+#endif /* CONFIG_SMP */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 452e2ba06c7..b2d21e06d45 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1462,7 +1462,7 @@ int zone_watermark_ok(struct zone *z, int order, unsigned long mark,
{
/* free_pages my go negative - that's OK */
long min = mark;
- long free_pages = zone_page_state(z, NR_FREE_PAGES) - (1 << order) + 1;
+ long free_pages = zone_nr_free_pages(z) - (1 << order) + 1;
int o;
if (alloc_flags & ALLOC_HIGH)
@@ -2424,7 +2424,7 @@ void show_free_areas(void)
" all_unreclaimable? %s"
"\n",
zone->name,
- K(zone_page_state(zone, NR_FREE_PAGES)),
+ K(zone_nr_free_pages(zone)),
K(min_wmark_pages(zone)),
K(low_wmark_pages(zone)),
K(high_wmark_pages(zone)),
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a8d6b59e609..355a9e669aa 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -138,11 +138,24 @@ static void refresh_zone_stat_thresholds(void)
int threshold;
for_each_populated_zone(zone) {
+ unsigned long max_drift, tolerate_drift;
+
threshold = calculate_threshold(zone);
for_each_online_cpu(cpu)
per_cpu_ptr(zone->pageset, cpu)->stat_threshold
= threshold;
+
+ /*
+ * Only set percpu_drift_mark if there is a danger that
+ * NR_FREE_PAGES reports the low watermark is ok when in fact
+ * the min watermark could be breached by an allocation
+ */
+ tolerate_drift = low_wmark_pages(zone) - min_wmark_pages(zone);
+ max_drift = num_online_cpus() * threshold;
+ if (max_drift > tolerate_drift)
+ zone->percpu_drift_mark = high_wmark_pages(zone) +
+ max_drift;
}
}
@@ -813,7 +826,7 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
"\n scanned %lu"
"\n spanned %lu"
"\n present %lu",
- zone_page_state(zone, NR_FREE_PAGES),
+ zone_nr_free_pages(zone),
min_wmark_pages(zone),
low_wmark_pages(zone),
high_wmark_pages(zone),