[patch 3/3] mm: page_alloc: fair zone allocator policy

[Johannes Weiner <hannes@cmpxchg.org>]

Each zone that holds userspace pages of one workload must be aged at a
speed proportional to the zone size.  Otherwise, the time an
individual page gets to stay in memory depends on the zone it happened
to be allocated in.  Asymmetry in the zone aging creates rather
unpredictable aging behavior and results in the wrong pages being
reclaimed, activated etc.

But exactly this happens right now because of the way the page
allocator and kswapd interact.  The page allocator uses per-node lists
of all zones in the system, ordered by preference, when allocating a
new page.  When the first iteration does not yield any results, kswapd
is woken up and the allocator retries.  Due to the way kswapd reclaims
zones below the high watermark while a zone can be allocated from when
it is above the low watermark, the allocator may keep kswapd running
while kswapd reclaim ensures that the page allocator can keep
allocating from the first zone in the zonelist for extended periods of
time.  Meanwhile the other zones rarely see new allocations and thus
get aged much slower in comparison.

The result is that the occasional page placed in lower zones gets
relatively more time in memory, even get promoted to the active list
after its peers have long been evicted.  Meanwhile, the bulk of the
working set may be thrashing on the preferred zone even though there
may be significant amounts of memory available in the lower zones.

Even the most basic test -- repeatedly reading a file slightly bigger
than memory -- shows how broken the zone aging is.  In this scenario,
no single page should be able stay in memory long enough to get
referenced twice and activated, but activation happens in spades:

  $ grep active_file /proc/zoneinfo
      nr_inactive_file 0
      nr_active_file 0
      nr_inactive_file 0
      nr_active_file 8
      nr_inactive_file 1582
      nr_active_file 11994
  $ cat data data data data >/dev/null
  $ grep active_file /proc/zoneinfo
      nr_inactive_file 0
      nr_active_file 70
      nr_inactive_file 258753
      nr_active_file 443214
      nr_inactive_file 149793
      nr_active_file 12021

Fix this with a very simple round robin allocator.  Each zone is
allowed a batch of allocations that is proportional to the zone's
size, after which it is treated as full.  The batch counters are reset
when all zones have been tried and the allocator enters the slowpath
and kicks off kswapd reclaim:

  $ grep active_file /proc/zoneinfo
      nr_inactive_file 0
      nr_active_file 0
      nr_inactive_file 174
      nr_active_file 4865
      nr_inactive_file 53
      nr_active_file 860
  $ cat data data data data >/dev/null
  $ grep active_file /proc/zoneinfo
      nr_inactive_file 0
      nr_active_file 0
      nr_inactive_file 666622
      nr_active_file 4988
      nr_inactive_file 190969
      nr_active_file 937

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        | 39 +++++++++++++++++++++++++++++----------
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index af4a3b7..0c41d59 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -352,6 +352,7 @@ struct zone {
 	 * free areas of different sizes
 	 */
 	spinlock_t		lock;
+	atomic_t		alloc_batch;
 	int                     all_unreclaimable; /* All pages pinned */
 #if defined CONFIG_COMPACTION || defined CONFIG_CMA
 	/* Set to true when the PG_migrate_skip bits should be cleared */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index af1d956b..d938b67 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1879,6 +1879,14 @@ zonelist_scan:
 		if (alloc_flags & ALLOC_NO_WATERMARKS)
 			goto try_this_zone;
 		/*
+		 * Distribute pages in proportion to the individual
+		 * zone size to ensure fair page aging.  The zone a
+		 * page was allocated in should have no effect on the
+		 * time the page has in memory before being reclaimed.
+		 */
+		if (atomic_read(&zone->alloc_batch) <= 0)
+			continue;
+		/*
 		 * When allocating a page cache page for writing, we
 		 * want to get it from a zone that is within its dirty
 		 * limit, such that no single zone holds more than its
@@ -1984,7 +1992,8 @@ this_zone_full:
 		goto zonelist_scan;
 	}
 
-	if (page)
+	if (page) {
+		atomic_sub(1U << order, &zone->alloc_batch);
 		/*
 		 * page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
 		 * necessary to allocate the page. The expectation is
@@ -1993,6 +2002,7 @@ this_zone_full:
 		 * for !PFMEMALLOC purposes.
 		 */
 		page->pfmemalloc = !!(alloc_flags & ALLOC_NO_WATERMARKS);
+	}
 
 	return page;
 }
@@ -2342,16 +2352,20 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
 	return page;
 }
 
-static inline
-void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
-						enum zone_type high_zoneidx,
-						enum zone_type classzone_idx)
+static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
+			     struct zonelist *zonelist,
+			     enum zone_type high_zoneidx,
+			     enum zone_type classzone_idx)
 {
 	struct zoneref *z;
 	struct zone *zone;
 
-	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
-		wakeup_kswapd(zone, order, classzone_idx);
+	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
+		atomic_set(&zone->alloc_batch,
+			   high_wmark_pages(zone) - low_wmark_pages(zone));
+		if (!(gfp_mask & __GFP_NO_KSWAPD))
+			wakeup_kswapd(zone, order, classzone_idx);
+	}
 }
 
 static inline int
@@ -2447,9 +2461,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		goto nopage;
 
 restart:
-	if (!(gfp_mask & __GFP_NO_KSWAPD))
-		wake_all_kswapd(order, zonelist, high_zoneidx,
-						zone_idx(preferred_zone));
+	prepare_slowpath(gfp_mask, order, zonelist,
+			 high_zoneidx, zone_idx(preferred_zone));
 
 	/*
 	 * OK, we're below the kswapd watermark and have kicked background
@@ -4758,6 +4771,9 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
 		zone_seqlock_init(zone);
 		zone->zone_pgdat = pgdat;
 
+		/* For bootup, initialized properly in watermark setup */
+		atomic_set(&zone->alloc_batch, zone->managed_pages);
+
 		zone_pcp_init(zone);
 		lruvec_init(&zone->lruvec);
 		if (!size)
@@ -5533,6 +5549,9 @@ static void __setup_per_zone_wmarks(void)
 		zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + (tmp >> 2);
 		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
 
+		atomic_set(&zone->alloc_batch,
+			   high_wmark_pages(zone) - low_wmark_pages(zone));
+
 		setup_zone_migrate_reserve(zone);
 		spin_unlock_irqrestore(&zone->lock, flags);
 	}
-- 
1.8.3.2

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