kswapd: avoid unnecessary rebalance after an unsuccessful balancing

In commit 215ddd66 ("mm: vmscan: only read new_classzone_idx from pgdat
when reclaiming successfully") , Mel Gorman said kswapd is better to sleep
after a unsuccessful balancing if there is tighter reclaim request pending
in the balancing.  But in the following scenario, kswapd do something that
is not matched our expectation.  The patch fixes this issue.

1, Read pgdat request A (classzone_idx, order = 3)
2, balance_pgdat()
3, During pgdat, a new pgdat request B (classzone_idx, order = 5) is placed
4, balance_pgdat() returns but failed since returned order = 0
5, pgdat of request A assigned to balance_pgdat(), and do balancing again.
   While the expectation behavior of kswapd should try to sleep.

Signed-off-by: Alex Shi <alex.shi@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Tested-by: Pádraig Brady <P@draigBrady.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ed78ccf63aaf1a672883f1e2fa37b08f58b0fd7b..a90b0573fc4c7430dbab44fbfe8f1af3f0cb26ed 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2830,7 +2830,9 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
 static int kswapd(void *p)
 {
        unsigned long order, new_order;
+       unsigned balanced_order;
        int classzone_idx, new_classzone_idx;
+       int balanced_classzone_idx;
        pg_data_t *pgdat = (pg_data_t*)p;
        struct task_struct *tsk = current;
 
@@ -2861,7 +2863,9 @@ static int kswapd(void *p)
        set_freezable();
 
        order = new_order = 0;
+       balanced_order = 0;
        classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
+       balanced_classzone_idx = classzone_idx;
        for ( ; ; ) {
                int ret;
 
@@ -2870,7 +2874,8 @@ static int kswapd(void *p)
                 * new request of a similar or harder type will succeed soon
                 * so consider going to sleep on the basis we reclaimed at
                 */
-               if (classzone_idx >= new_classzone_idx && order == new_order) {
+               if (balanced_classzone_idx >= new_classzone_idx &&
+                                       balanced_order == new_order) {
                        new_order = pgdat->kswapd_max_order;
                        new_classzone_idx = pgdat->classzone_idx;
                        pgdat->kswapd_max_order =  0;
@@ -2885,7 +2890,8 @@ static int kswapd(void *p)
                        order = new_order;
                        classzone_idx = new_classzone_idx;
                } else {
-                       kswapd_try_to_sleep(pgdat, order, classzone_idx);
+                       kswapd_try_to_sleep(pgdat, balanced_order,
+                                               balanced_classzone_idx);
                        order = pgdat->kswapd_max_order;
                        classzone_idx = pgdat->classzone_idx;
                        pgdat->kswapd_max_order = 0;
@@ -2902,7 +2908,9 @@ static int kswapd(void *p)
                 */
                if (!ret) {
                        trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
-                       order = balance_pgdat(pgdat, order, &classzone_idx);
+                       balanced_classzone_idx = classzone_idx;
+                       balanced_order = balance_pgdat(pgdat, order,
+                                               &balanced_classzone_idx);
                }
        }
        return 0;