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

Mel Gorman
2012-07-23T13:42:58Z

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

commit f0dfcde099453aa4c0dc42473828d15a6d492936 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@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Tested-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
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