On Fri, Oct 29, 2010 at 12:40:02PM -0700, Andrew Morton wrote:
> > ...
> >
> > Follow-on patch?
>
> Sometime, please.
>
How does this look?
==== CUT HERE ====
mm: vmscan: Comment on why kswapd reduces the per-cpu vmstat threshold
While kswapd is awake, the per-cpu vmstat threshold is reduced to
reduce per-cpu drift to acceptable levels. Add a comment explaining
why.
Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
---
mm/vmscan.c | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7966110..ba39948 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2378,6 +2378,19 @@ static int kswapd(void *p)
*/
if (!sleeping_prematurely(pgdat, order, remaining)) {
trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
+
+ /*
+ * vmstat counters are not perfectly
+ * accurate and the estimated value
+ * for counters such as NR_FREE_PAGES
+ * can deviate from the true value by
+ * nr_online_cpus * threshold. To
+ * avoid the zone watermarks being
+ * breached while under pressure, we
+ * reduce the per-cpu vmstat threshold
+ * while kswapd is awake and restore
+ * them before going back to sleep.
+ */
set_pgdat_percpu_threshold(pgdat,
calculate_normal_threshold);
schedule();
--
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