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Subject[PATCH 2/5] mm: kswapd: Use the order that kswapd was reclaiming at for sleeping_prematurely()
Date
Before kswapd goes to sleep, it uses sleeping_prematurely() to check if
there was a race pushing a zone below its watermark. If the race
happened, it stays awake. However, balance_pgdat() can decide to reclaim
at a lower order if it decides that high-order reclaim is not working as
expected. This information is not passed back to sleeping_prematurely().
The impact is that kswapd remains awake reclaiming pages long after it
should have gone to sleep. This patch passes the adjusted order to
sleeping_prematurely and uses the same logic as balance_pgdat to decide
if it's ok to go to sleep.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
mm/vmscan.c | 29 +++++++++++++++++++++++------
1 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d070d19..193feeb 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2118,15 +2118,17 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
#endif

/* is kswapd sleeping prematurely? */
-static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
+static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
{
int i;
+ bool all_zones_ok = true;
+ bool any_zone_ok = false;

/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
if (remaining)
return 1;

- /* If after HZ/10, a zone is below the high mark, it's premature */
+ /* Check the watermark levels */
for (i = 0; i < pgdat->nr_zones; i++) {
struct zone *zone = pgdat->node_zones + i;

@@ -2138,10 +2140,19 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)

if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
0, 0))
- return 1;
+ all_zones_ok = false;
+ else
+ any_zone_ok = true;
}

- return 0;
+ /*
+ * For high-order requests, any zone meeting the watermark allows
+ * kswapd to sleep. For order-0, all zones must be balanced
+ */
+ if (order)
+ return !any_zone_ok;
+ else
+ return !all_zones_ok;
}

/*
@@ -2391,7 +2402,13 @@ out:
}
}

- return sc.nr_reclaimed;
+ /*
+ * Return the order we were reclaiming at so sleeping_prematurely()
+ * makes a decision on the order we were last reclaiming at. However,
+ * if another caller entered the allocator slow path while kswapd
+ * was awake, order will remain at the higher level
+ */
+ return order;
}

/*
@@ -2501,7 +2518,7 @@ static int kswapd(void *p)
*/
if (!ret) {
trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
- balance_pgdat(pgdat, order);
+ order = balance_pgdat(pgdat, order, classzone_idx);
}
}
return 0;
--
1.7.1


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