[PATCH 21/34] kswapd: assign new_order and new_classzone_idx after wakeup in sleeping

[Mel Gorman]

From: "Alex,Shi" <alex.shi@xxxxxxxxx>

commit d2ebd0f6b89567eb93ead4e2ca0cbe03021f344b upstream.

Stable note: Fixes https://bugzilla.redhat.com/show_bug.cgi?id=712019. This
	patch reduces kswapd CPU usage.

There 2 places to read pgdat in kswapd.  One is return from a successful
balance, another is waked up from kswapd sleeping.  The new_order and
new_classzone_idx represent the balance input order and classzone_idx.

But current new_order and new_classzone_idx are not assigned after
kswapd_try_to_sleep(), that will cause a bug in the following scenario.

1: after a successful balance, kswapd goes to sleep, and new_order = 0;
   new_classzone_idx = __MAX_NR_ZONES - 1;

2: kswapd waked up with order = 3 and classzone_idx = ZONE_NORMAL

3: in the balance_pgdat() running, a new balance wakeup happened with
   order = 5, and classzone_idx = ZONE_NORMAL

4: the first wakeup(order = 3) finished successufly, return order = 3
   but, the new_order is still 0, so, this balancing will be treated as a
   failed balance.  And then the second tighter balancing will be missed.

So, to avoid the above problem, the new_order and new_classzone_idx need
to be assigned for later successful comparison.

Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
Tested-by: PÃdraig Brady <P@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/vmscan.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bf85e4d..b8c1fc0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2905,6 +2905,8 @@ static int kswapd(void *p)
 						balanced_classzone_idx);
 			order = pgdat->kswapd_max_order;
 			classzone_idx = pgdat->classzone_idx;
+			new_order = order;
+			new_classzone_idx = classzone_idx;
 			pgdat->kswapd_max_order = 0;
 			pgdat->classzone_idx = pgdat->nr_zones - 1;
 		}
-- 
1.7.9.2

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