[PATCH] mm: don't wake kswapd prematurely when watermark boosting is disabled

[Johannes Weiner]

On 2-node NUMA hosts we see bursts of kswapd reclaim and subsequent
pressure spikes and stalls from cache refaults while there is plenty
of free memory in the system.

Usually, kswapd is woken up when all eligible nodes in an allocation
are full. But the code related to watermark boosting can wake kswapd
on one full node while the other one is mostly empty. This may be
justified to fight fragmentation, but is currently unconditionally
done whether watermark boosting is occurring or not.

In our case, many of our workloads' throughput scales with available
memory, and pure utilization is a more tangible concern than trends
around longer-term fragmentation. As a result we generally disable
watermark boosting.

Wake kswapd only woken when watermark boosting is requested.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e74ca22baaa1..4f9d9f7e910c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2470,12 +2470,12 @@ static bool can_steal_fallback(unsigned int order, int start_mt)
 	return false;
 }
 
-static inline void boost_watermark(struct zone *zone)
+static inline bool boost_watermark(struct zone *zone)
 {
 	unsigned long max_boost;
 
 	if (!watermark_boost_factor)
-		return;
+		return false;
 	/*
 	 * Don't bother in zones that are unlikely to produce results.
 	 * On small machines, including kdump capture kernels running
@@ -2483,7 +2483,7 @@ static inline void boost_watermark(struct zone *zone)
 	 * memory situation immediately.
 	 */
 	if ((pageblock_nr_pages * 4) > zone_managed_pages(zone))
-		return;
+		return false;
 
 	max_boost = mult_frac(zone->_watermark[WMARK_HIGH],
 			watermark_boost_factor, 10000);
@@ -2497,12 +2497,14 @@ static inline void boost_watermark(struct zone *zone)
 	 * boosted watermark resulting in a hang.
 	 */
 	if (!max_boost)
-		return;
+		return false;
 
 	max_boost = max(pageblock_nr_pages, max_boost);
 
 	zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,
 		max_boost);
+
+	return true;
 }
 
 /*
@@ -2540,8 +2542,7 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
 	 * likelihood of future fallbacks. Wake kswapd now as the node
 	 * may be balanced overall and kswapd will not wake naturally.
 	 */
-	boost_watermark(zone);
-	if (alloc_flags & ALLOC_KSWAPD)
+	if (boost_watermark(zone) && (alloc_flags & ALLOC_KSWAPD))
 		set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
 
 	/* We are not allowed to try stealing from the whole block */
-- 
2.28.0