[PATCH 0/2][V3] slab and general reclaim improvements

[PATCH 0/2][V3] slab and general reclaim improvements

[josef]

This is a new set of patches to address some slab reclaim issues I observed when
trying to convert btrfs over to a purely slab meta data system.  The problem is
our slab reclaim amounts are based on how page cache scanning goes, which is not
related to how much slab is in use on the system.  This means we waste a lot of
cycles trying to reclaim really small numbers of objects and thus induce a huge
latency hit when we hit memory pressure.

The second patch is to fix another problem I noticed, namely that we will
constantly evict active pages instead of trying to evict shorter lived pages and
objects.  You can run the following test to see the behavior

https://github.com/josefbacik/debug-scripts/blob/master/cache-pressure/cache-pressure.sh

Reading two files that fit nicely into RAM, and then add slab pressure from
fs_mark and you'll see that we will evict both files 3 or 4 times during the
run.  With my patches we don't evict any of the file pages at all, only the slab
pressure.

CHANGES SINCE V2:

After discussions with Minchan and Dave I went back to the drawing board.
Dave's suggestion of shrinker specific callbacks to allow shrinkers to reclaim
at their own rate was intriguing so I did this first.  Unfortunately this fell
apart as I couldn't figure out a scheme to make the reclaim stuff work without
needlessly inducing latency during 'stable' periods.  Without a view of the
whole system it's hard to know when to trigger scanning of active objects
without wasting CPU cycles that aren't actually needed.  We will end up starting
our background thread and scanning objects when in reality our workload would
fit perfectly fine in memory.

Minchan's main complaint of my stuff was that it was too aggressive, and any
attempt to short circuit the aggression would be unfair to other shrinkers.  So
instead use his idea of using sc->priority to determine our scan count.  But
instead of using the traditional ratio, just use scan = nr_objects >>
sc->priority.  This gets us what we do with the LRU, which is scan = pages >>
sc->priority, and gives us an appropriate amount of aggressiveness for slab
reclaim.

I've dropped the other two patches around stopping slab reclaim, and have
changed the slab pressure to be based on sc->priority, which is consistent with
every other LRU on the system, and gives me the same results in my testcases.
Thanks,

Josef

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[PATCH 1/2] mm: use sc->priority for slab shrink targets

[josef]

From: Josef Bacik <jbacik@fb.com>

Previously we were using the ratio of the number of lru pages scanned to
the number of eligible lru pages to determine the number of slab objects
to scan.  The problem with this is that these two things have nothing to
do with each other, so in slab heavy work loads where there is little to
no page cache we can end up with the pages scanned being a very low
number.  This means that we reclaim next to no slab pages and waste a
lot of time reclaiming small amounts of space.

Instead use sc->priority in the same way we use it to determine scan
amounts for the lru's.  This generally equates to pages.  Consider the
following

slab_pages = (nr_objects * object_size) / PAGE_SIZE

What we would like to do is

scan = slab_pages >> sc->priority

but we don't know the number of slab pages each shrinker controls, only
the objects.  However say that theoretically we knew how many pages a
shrinker controlled, we'd still have to convert this to objects, which
would look like the following

scan = shrinker_pages >> sc->priority
scan_objects = (PAGE_SIZE / object_size) * scan

or written another way

scan_objects = (shrinker_pages >> sc->priority) *
		(PAGE_SIZE / object_size)

which can thus be written

scan_objects = ((shrinker_pages * PAGE_SIZE) / object_size) >>
		sc->priority

which is just

scan_objects = nr_objects >> sc->priority

We don't need to know exactly how many pages each shrinker represents,
it's objects are all the information we need.  Making this change allows
us to place an appropriate amount of pressure on the shrinker pools for
their relative size.

Signed-off-by: Josef Bacik <jbacik@fb.com>
---
 include/trace/events/vmscan.h | 23 ++++++++++------------
 mm/vmscan.c                   | 46 +++++++++++--------------------------------
 2 files changed, 22 insertions(+), 47 deletions(-)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index 27e8a5c..8c5a00a 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -187,12 +187,12 @@ DEFINE_EVENT(mm_vmscan_direct_reclaim_end_template, mm_vmscan_memcg_softlimit_re
 
 TRACE_EVENT(mm_shrink_slab_start,
 	TP_PROTO(struct shrinker *shr, struct shrink_control *sc,
-		long nr_objects_to_shrink, unsigned long pgs_scanned,
-		unsigned long lru_pgs, unsigned long cache_items,
-		unsigned long long delta, unsigned long total_scan),
+		long nr_objects_to_shrink, unsigned long cache_items,
+		unsigned long long delta, unsigned long total_scan,
+		int priority),
 
-	TP_ARGS(shr, sc, nr_objects_to_shrink, pgs_scanned, lru_pgs,
-		cache_items, delta, total_scan),
+	TP_ARGS(shr, sc, nr_objects_to_shrink, cache_items, delta, total_scan,
+		priority),
 
 	TP_STRUCT__entry(
 		__field(struct shrinker *, shr)
@@ -200,11 +200,10 @@ TRACE_EVENT(mm_shrink_slab_start,
 		__field(int, nid)
 		__field(long, nr_objects_to_shrink)
 		__field(gfp_t, gfp_flags)
-		__field(unsigned long, pgs_scanned)
-		__field(unsigned long, lru_pgs)
 		__field(unsigned long, cache_items)
 		__field(unsigned long long, delta)
 		__field(unsigned long, total_scan)
+		__field(int, priority)
 	),
 
 	TP_fast_assign(
@@ -213,24 +212,22 @@ TRACE_EVENT(mm_shrink_slab_start,
 		__entry->nid = sc->nid;
 		__entry->nr_objects_to_shrink = nr_objects_to_shrink;
 		__entry->gfp_flags = sc->gfp_mask;
-		__entry->pgs_scanned = pgs_scanned;
-		__entry->lru_pgs = lru_pgs;
 		__entry->cache_items = cache_items;
 		__entry->delta = delta;
 		__entry->total_scan = total_scan;
+		__entry->priority = priority;
 	),
 
-	TP_printk("%pF %p: nid: %d objects to shrink %ld gfp_flags %s pgs_scanned %ld lru_pgs %ld cache items %ld delta %lld total_scan %ld",
+	TP_printk("%pF %p: nid: %d objects to shrink %ld gfp_flags %s cache items %ld delta %lld total_scan %ld priority %d",
 		__entry->shrink,
 		__entry->shr,
 		__entry->nid,
 		__entry->nr_objects_to_shrink,
 		show_gfp_flags(__entry->gfp_flags),
-		__entry->pgs_scanned,
-		__entry->lru_pgs,
 		__entry->cache_items,
 		__entry->delta,
-		__entry->total_scan)
+		__entry->total_scan,
+		__entry->priority)
 );
 
 TRACE_EVENT(mm_shrink_slab_end,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f84cdd3..a6f33ef 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -306,9 +306,7 @@ EXPORT_SYMBOL(unregister_shrinker);
 #define SHRINK_BATCH 128
 
 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
-				    struct shrinker *shrinker,
-				    unsigned long nr_scanned,
-				    unsigned long nr_eligible)
+				    struct shrinker *shrinker, int priority)
 {
 	unsigned long freed = 0;
 	unsigned long long delta;
@@ -333,9 +331,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
 
 	total_scan = nr;
-	delta = (4 * nr_scanned) / shrinker->seeks;
-	delta *= freeable;
-	do_div(delta, nr_eligible + 1);
+	delta = freeable >> priority;
+	delta = (4 * freeable) / shrinker->seeks;
 	total_scan += delta;
 	if (total_scan < 0) {
 		pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
@@ -369,8 +366,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		total_scan = freeable * 2;
 
 	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
-				   nr_scanned, nr_eligible,
-				   freeable, delta, total_scan);
+				   freeable, delta, total_scan, priority);
 
 	/*
 	 * Normally, we should not scan less than batch_size objects in one
@@ -429,8 +425,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
  * @gfp_mask: allocation context
  * @nid: node whose slab caches to target
  * @memcg: memory cgroup whose slab caches to target
- * @nr_scanned: pressure numerator
- * @nr_eligible: pressure denominator
+ * @priority: the reclaim priority
  *
  * Call the shrink functions to age shrinkable caches.
  *
@@ -442,20 +437,14 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
  * objects from the memory cgroup specified. Otherwise, only unaware
  * shrinkers are called.
  *
- * @nr_scanned and @nr_eligible form a ratio that indicate how much of
- * the available objects should be scanned.  Page reclaim for example
- * passes the number of pages scanned and the number of pages on the
- * LRU lists that it considered on @nid, plus a bias in @nr_scanned
- * when it encountered mapped pages.  The ratio is further biased by
- * the ->seeks setting of the shrink function, which indicates the
- * cost to recreate an object relative to that of an LRU page.
+ * @priority is sc->priority, we take the number of objects and >> by priority
+ * in order to get the scan target.
  *
  * Returns the number of reclaimed slab objects.
  */
 static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 				 struct mem_cgroup *memcg,
-				 unsigned long nr_scanned,
-				 unsigned long nr_eligible)
+				 int priority)
 {
 	struct shrinker *shrinker;
 	unsigned long freed = 0;
@@ -463,9 +452,6 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 	if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
 		return 0;
 
-	if (nr_scanned == 0)
-		nr_scanned = SWAP_CLUSTER_MAX;
-
 	if (!down_read_trylock(&shrinker_rwsem)) {
 		/*
 		 * If we would return 0, our callers would understand that we
@@ -496,7 +482,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 		if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
 			sc.nid = 0;
 
-		freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
+		freed += do_shrink_slab(&sc, shrinker, priority);
 	}
 
 	up_read(&shrinker_rwsem);
@@ -514,8 +500,7 @@ void drop_slab_node(int nid)
 
 		freed = 0;
 		do {
-			freed += shrink_slab(GFP_KERNEL, nid, memcg,
-					     1000, 1000);
+			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
 		} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
 	} while (freed > 10);
 }
@@ -2592,14 +2577,12 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 
 			reclaimed = sc->nr_reclaimed;
 			scanned = sc->nr_scanned;
-
 			shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
 			node_lru_pages += lru_pages;
 
 			if (memcg)
 				shrink_slab(sc->gfp_mask, pgdat->node_id,
-					    memcg, sc->nr_scanned - scanned,
-					    lru_pages);
+					    memcg, sc->priority);
 
 			/* Record the group's reclaim efficiency */
 			vmpressure(sc->gfp_mask, memcg, false,
@@ -2623,14 +2606,9 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			}
 		} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
 
-		/*
-		 * Shrink the slab caches in the same proportion that
-		 * the eligible LRU pages were scanned.
-		 */
 		if (global_reclaim(sc))
 			shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
-				    sc->nr_scanned - nr_scanned,
-				    node_lru_pages);
+				    sc->priority);
 
 		/*
 		 * Record the subtree's reclaim efficiency. The reclaimed
-- 
2.7.4

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[PATCH 2/2] mm: make kswapd try harder to keep active pages in cache

[josef]

From: Josef Bacik <jbacik@fb.com>

While testing slab reclaim I noticed that if we were running a workload
that used most of the system memory for it's working set and we start
putting a lot of reclaimable slab pressure on the system (think find /,
or some other silliness), we will happily evict the active pages over
the slab cache.  This is kind of backwards as we want to do all that we
can to keep the active working set in memory, and instead evict these
short lived objects.  The same thing occurs when say you do a yum
update of a few packages while your working set takes up most of RAM,
you end up with inactive lists being relatively small and so we reclaim
active pages even though we could reclaim these short lived inactive
pages.

My approach here is twofold.  First, keep track of the difference in
inactive and slab pages since the last time kswapd ran.  In the first
run this will just be the overall counts of inactive and slab, but for
each subsequent run we'll have a good idea of where the memory pressure
is coming from.  Then we use this information to put pressure on either
the inactive lists or the slab caches, depending on where the pressure
is coming from.

I have two tests I was using to watch either side of this problem.  The
first test kept 2 files that took up 3/4 of the memory, and then started
creating a bunch of empty files.  Without this patch we would have to
re-read both files in their entirety at least 3 times during the run.
With this patch the active pages are never evicted.

The second test was a test that would read and stat all the files in a
directory, which again would take up about 3/4 of the memory with slab
cache.  Then I cat'ed a 100gib file into /dev/null and checked to see if
any of the files were evicted and verified that none of the files were
evicted.

Signed-off-by: Josef Bacik <jbacik@fb.com>
---
 mm/vmscan.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 158 insertions(+), 14 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a6f33ef..aa73368 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -110,11 +110,20 @@ struct scan_control {
 	/* One of the zones is ready for compaction */
 	unsigned int compaction_ready:1;
 
+	/* Only reclaim inactive page cache or slab. */
+	unsigned int inactive_only:1;
+
 	/* Incremented by the number of inactive pages that were scanned */
 	unsigned long nr_scanned;
 
 	/* Number of pages freed so far during a call to shrink_zones() */
 	unsigned long nr_reclaimed;
+
+	/* Number of inactive pages added since last kswapd run. */
+	unsigned long inactive_diff;
+
+	/* Number of slab pages added since last kswapd run. */
+	unsigned long slab_diff;
 };
 
 #ifdef ARCH_HAS_PREFETCH
@@ -306,7 +315,8 @@ EXPORT_SYMBOL(unregister_shrinker);
 #define SHRINK_BATCH 128
 
 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
-				    struct shrinker *shrinker, int priority)
+				    struct shrinker *shrinker, int priority,
+				    unsigned long *slab_scanned)
 {
 	unsigned long freed = 0;
 	unsigned long long delta;
@@ -405,6 +415,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		next_deferred -= scanned;
 	else
 		next_deferred = 0;
+	if (slab_scanned)
+		(*slab_scanned) += scanned;
+
 	/*
 	 * move the unused scan count back into the shrinker in a
 	 * manner that handles concurrent updates. If we exhausted the
@@ -444,7 +457,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
  */
 static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 				 struct mem_cgroup *memcg,
-				 int priority)
+				 int priority, unsigned long *slab_scanned)
 {
 	struct shrinker *shrinker;
 	unsigned long freed = 0;
@@ -482,7 +495,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 		if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
 			sc.nid = 0;
 
-		freed += do_shrink_slab(&sc, shrinker, priority);
+		freed += do_shrink_slab(&sc, shrinker, priority, slab_scanned);
 	}
 
 	up_read(&shrinker_rwsem);
@@ -500,7 +513,7 @@ void drop_slab_node(int nid)
 
 		freed = 0;
 		do {
-			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
+			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0, NULL);
 		} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
 	} while (freed > 10);
 }
@@ -2131,6 +2144,7 @@ enum scan_balance {
 	SCAN_FRACT,
 	SCAN_ANON,
 	SCAN_FILE,
+	SCAN_INACTIVE,
 };
 
 /*
@@ -2157,6 +2171,11 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 	unsigned long ap, fp;
 	enum lru_list lru;
 
+	if (sc->inactive_only) {
+		scan_balance = SCAN_INACTIVE;
+		goto out;
+	}
+
 	/* If we have no swap space, do not bother scanning anon pages. */
 	if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
 		scan_balance = SCAN_FILE;
@@ -2330,6 +2349,14 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 				scan = 0;
 			}
 			break;
+		case SCAN_INACTIVE:
+			if (file && !is_active_lru(lru)) {
+				scan = max(scan, sc->nr_to_reclaim);
+			} else {
+				size = 0;
+				scan = 0;
+			}
+			break;
 		default:
 			/* Look ma, no brain */
 			BUG();
@@ -2547,7 +2574,61 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 {
 	struct reclaim_state *reclaim_state = current->reclaim_state;
 	unsigned long nr_reclaimed, nr_scanned;
+	unsigned long greclaim = 1, gslab = 1, total_high_wmark = 0, nr_inactive;
+	int priority_adj = 1;
 	bool reclaimable = false;
+	bool skip_slab = false;
+
+	if (global_reclaim(sc)) {
+		int z;
+		for (z = 0; z < MAX_NR_ZONES; z++) {
+			struct zone *zone = &pgdat->node_zones[z];
+			if (!managed_zone(zone))
+				continue;
+			total_high_wmark += high_wmark_pages(zone);
+		}
+		nr_inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
+		gslab = node_page_state(pgdat, NR_SLAB_RECLAIMABLE);
+		greclaim = pgdat_reclaimable_pages(pgdat);
+	} else {
+		struct lruvec *lruvec =
+			mem_cgroup_lruvec(pgdat, sc->target_mem_cgroup);
+		total_high_wmark = sc->nr_to_reclaim;
+		nr_inactive = lruvec_page_state(lruvec, NR_INACTIVE_FILE);
+		gslab = lruvec_page_state(lruvec, NR_SLAB_RECLAIMABLE);
+	}
+
+	/*
+	 * If we don't have a lot of inactive or slab pages then there's no
+	 * point in trying to free them exclusively, do the normal scan stuff.
+	 */
+	if (nr_inactive + gslab < total_high_wmark)
+		sc->inactive_only = 0;
+
+	/*
+	 * We still want to slightly prefer slab over inactive, so if the
+	 * inactive on this node is large enough and what is pushing us into
+	 * reclaim territory then limit our flushing to the inactive list for
+	 * the first go around.
+	 *
+	 * The idea is that with a memcg configured system we will still reclaim
+	 * memcg aware shrinkers, which includes the super block shrinkers.  So
+	 * if our steady state is keeping fs objects in cache for our workload
+	 * we'll still put a certain amount of pressure on them anyway.  To
+	 * avoid evicting things we actually care about we want to skip slab
+	 * reclaim altogether.
+	 *
+	 * However we still want to account for slab and inactive growing at the
+	 * same rate, so if that is the case just carry on shrinking inactive
+	 * and slab together.
+	 */
+	if (nr_inactive > total_high_wmark &&
+	    sc->inactive_diff > sc->slab_diff) {
+		unsigned long tmp = sc->inactive_diff >> 1;
+
+		if (tmp >= sc->slab_diff)
+			skip_slab = true;
+	}
 
 	do {
 		struct mem_cgroup *root = sc->target_mem_cgroup;
@@ -2556,6 +2637,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			.priority = sc->priority,
 		};
 		unsigned long node_lru_pages = 0;
+		unsigned long slab_reclaimed = 0;
+		unsigned long slab_scanned = 0;
 		struct mem_cgroup *memcg;
 
 		nr_reclaimed = sc->nr_reclaimed;
@@ -2580,9 +2663,16 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
 			node_lru_pages += lru_pages;
 
-			if (memcg)
-				shrink_slab(sc->gfp_mask, pgdat->node_id,
-					    memcg, sc->priority);
+			if (memcg && !skip_slab) {
+				int priority = sc->priority;
+				if (sc->inactive_only)
+					priority -= priority_adj;
+				priority = max(0, priority);
+				slab_reclaimed +=
+					shrink_slab(sc->gfp_mask,
+						    pgdat->node_id, memcg,
+						    priority, &slab_scanned);
+			}
 
 			/* Record the group's reclaim efficiency */
 			vmpressure(sc->gfp_mask, memcg, false,
@@ -2606,9 +2696,16 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			}
 		} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
 
-		if (global_reclaim(sc))
-			shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
-				    sc->priority);
+		if (!skip_slab && global_reclaim(sc)) {
+			int priority = sc->priority;
+			if (sc->inactive_only)
+				priority -= priority_adj;
+			priority = max(0, priority);
+			slab_reclaimed +=
+				shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
+					    priority, &slab_scanned);
+		}
+
 
 		/*
 		 * Record the subtree's reclaim efficiency. The reclaimed
@@ -2627,9 +2724,28 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			reclaim_state->reclaimed_slab = 0;
 		}
 
-		if (sc->nr_reclaimed - nr_reclaimed)
+		if (sc->nr_reclaimed - nr_reclaimed) {
 			reclaimable = true;
+		} else if (sc->inactive_only && !skip_slab) {
+			unsigned long percent = 0;
 
+			/*
+			 * We didn't reclaim anything this go around, so the
+			 * inactive list is likely spent.  If we're reclaiming
+			 * less than half of the objects in slab that we're
+			 * scanning then just stop doing the inactive only scan.
+			 * Otherwise ramp up the pressure on the slab caches
+			 * hoping that eventually we'll start freeing enough
+			 * objects to reclaim space.
+			 */
+			if (slab_scanned)
+				percent = slab_reclaimed * 100 / slab_scanned;
+			if (percent < 50)
+				sc->inactive_only = 0;
+			else
+				priority_adj++;
+		}
+		skip_slab = false;
 	} while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
 					 sc->nr_scanned - nr_scanned, sc));
 
@@ -3272,7 +3388,8 @@ static bool kswapd_shrink_node(pg_data_t *pgdat,
  * or lower is eligible for reclaim until at least one usable zone is
  * balanced.
  */
-static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
+static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx,
+			 unsigned long inactive_diff, unsigned long slab_diff)
 {
 	int i;
 	unsigned long nr_soft_reclaimed;
@@ -3285,6 +3402,9 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 		.may_writepage = !laptop_mode,
 		.may_unmap = 1,
 		.may_swap = 1,
+		.inactive_only = 1,
+		.inactive_diff = inactive_diff,
+		.slab_diff = slab_diff,
 	};
 	count_vm_event(PAGEOUTRUN);
 
@@ -3504,7 +3624,7 @@ static int kswapd(void *p)
 	unsigned int classzone_idx = MAX_NR_ZONES - 1;
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
-
+	unsigned long nr_slab = 0, nr_inactive = 0;
 	struct reclaim_state reclaim_state = {
 		.reclaimed_slab = 0,
 	};
@@ -3534,6 +3654,7 @@ static int kswapd(void *p)
 	pgdat->kswapd_order = 0;
 	pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
 	for ( ; ; ) {
+		unsigned long slab_diff, inactive_diff;
 		bool ret;
 
 		alloc_order = reclaim_order = pgdat->kswapd_order;
@@ -3561,6 +3682,23 @@ static int kswapd(void *p)
 			continue;
 
 		/*
+		 * We want to know where we're adding pages so we can make
+		 * smarter decisions about where we're going to put pressure
+		 * when shrinking.
+		 */
+		slab_diff = sum_zone_node_page_state(pgdat->node_id,
+						     NR_SLAB_RECLAIMABLE);
+		inactive_diff = node_page_state(pgdat, NR_INACTIVE_FILE);
+		if (nr_slab > slab_diff)
+			slab_diff = 0;
+		else
+			slab_diff -= nr_slab;
+		if (inactive_diff < nr_inactive)
+			inactive_diff = 0;
+		else
+			inactive_diff -= nr_inactive;
+
+		/*
 		 * Reclaim begins at the requested order but if a high-order
 		 * reclaim fails then kswapd falls back to reclaiming for
 		 * order-0. If that happens, kswapd will consider sleeping
@@ -3570,7 +3708,11 @@ static int kswapd(void *p)
 		 */
 		trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
 						alloc_order);
-		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
+		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx,
+					      inactive_diff, slab_diff);
+		nr_inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
+		nr_slab = sum_zone_node_page_state(pgdat->node_id,
+						   NR_SLAB_RECLAIMABLE);
 		if (reclaim_order < alloc_order)
 			goto kswapd_try_sleep;
 	}
@@ -3822,6 +3964,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
 		.may_swap = 1,
 		.reclaim_idx = gfp_zone(gfp_mask),
+		.slab_diff = 1,
+		.inactive_diff = 1,
 	};
 
 	cond_resched();
-- 
2.7.4

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Re: [PATCH 1/2] mm: use sc->priority for slab shrink targets

[Andrew Morton]

On Thu, 20 Jul 2017 14:45:30 -0400 josef@toxicpanda.com wrote:

> From: Josef Bacik <jbacik@fb.com>
> 
> Previously we were using the ratio of the number of lru pages scanned to
> the number of eligible lru pages to determine the number of slab objects
> to scan.  The problem with this is that these two things have nothing to
> do with each other,

"nothing"?

> so in slab heavy work loads where there is little to
> no page cache we can end up with the pages scanned being a very low
> number.

In this case the "number of eligible lru pages" will also be low, so
these things do have something to do with each other?

>  This means that we reclaim next to no slab pages and waste a
> lot of time reclaiming small amounts of space.
> 
> Instead use sc->priority in the same way we use it to determine scan
> amounts for the lru's.

That sounds like a good idea.

Alternatively did you consider hooking into the vmpressure code (or
hannes's new memdelay code) to determine how hard to scan slab?

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Re: [PATCH 0/2][V3] slab and general reclaim improvements

[Andrew Morton]

On Thu, 20 Jul 2017 14:45:29 -0400 josef@toxicpanda.com wrote:

> This is a new set of patches to address some slab reclaim issues I observed when
> trying to convert btrfs over to a purely slab meta data system.

Without more review-n-test I'm a bit reluctant to apply these even on a
see-how-it-goes basis.  Anyone?

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Re: [PATCH 1/2] mm: use sc->priority for slab shrink targets

[Josef Bacik]

On Thu, Jul 27, 2017 at 04:53:48PM -0700, Andrew Morton wrote:
> On Thu, 20 Jul 2017 14:45:30 -0400 josef@toxicpanda.com wrote:
> 
> > From: Josef Bacik <jbacik@fb.com>
> > 
> > Previously we were using the ratio of the number of lru pages scanned to
> > the number of eligible lru pages to determine the number of slab objects
> > to scan.  The problem with this is that these two things have nothing to
> > do with each other,
> 
> "nothing"?
> 
> > so in slab heavy work loads where there is little to
> > no page cache we can end up with the pages scanned being a very low
> > number.
> 
> In this case the "number of eligible lru pages" will also be low, so
> these things do have something to do with each other?
> 

The problem is scanned doesn't correlate to the scanned count we calculate, but
rather the pages we're able to actually scan.  With almost no page cache we end
up with really low scanned counts to "relatively" high lru count, which makes
the ratio really really low.  Anecdotally we would have 10 million inodes in
cache, but the ratios were such that our scan target was like 8k.

> >  This means that we reclaim next to no slab pages and waste a
> > lot of time reclaiming small amounts of space.
> > 
> > Instead use sc->priority in the same way we use it to determine scan
> > amounts for the lru's.
> 
> That sounds like a good idea.
> 
> Alternatively did you consider hooking into the vmpressure code (or
> hannes's new memdelay code) to determine how hard to scan slab?
> 

Vmpressure requires memcg to be turned on.  As for memdelay that might be a good
direction in the future, but right now it's just per task.  We could probably
use it for direct reclaim, but I really want this to make kswapd better so we
avoid direct reclaim.  If it's expanded to be system wide so we could have an
idea of the effect of memory reclaim on the whole system that would tie in
nicely here.  But for now I think staying consistent with everything else is
good enough.  Thanks,

Josef

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