[PATCH 1/2] mm: provide zone vmstat percpu drift bounds

[Konstantin Khlebnikov]

vmstat use per-cpu counters for accounting, so atomic part on struct zone has some drift.
Free-pages watermark logic has some protection against this innacuracy.
too-many-isolated checks has the same problem. This patch provides drift bounds for them.

Plus this patch reset zone->percpu_drift_mark if drift protection is no longer required,
this can happens after memory hotplug.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
---
 include/linux/mmzone.h |    3 +++
 mm/vmstat.c            |    6 +++++-
 2 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 188cb2f..401438d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -307,6 +307,9 @@ struct zone {
 	 */
 	unsigned long percpu_drift_mark;
 
+	/* Maximum vm_stat per-cpu counters drift */
+	unsigned long percpu_drift;
+
 	/*
 	 * We don't know if the memory that we're going to allocate will be freeable
 	 * or/and it will be released eventually, so to avoid totally wasting several
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 8fd603b..94540e1 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -172,16 +172,20 @@ void refresh_zone_stat_thresholds(void)
 			per_cpu_ptr(zone->pageset, cpu)->stat_threshold
 							= threshold;
 
+		max_drift = num_online_cpus() * threshold;
+		zone->percpu_drift = max_drift;
+
 		/*
 		 * 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;
+		else
+			zone->percpu_drift_mark = 0;
 	}
 }
 

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