[PATCH] memcg: combine slab obj stock charging and accounting

[Vlastimil Babka]

When handing slab objects, we use obj_cgroup_[un]charge() for
(un)charging and mod_objcg_state() to account NR_SLAB_[UN]RECLAIMABLE_B.
All these operations use the percpu stock for performance. However with
the calls being separate, the stock_lock is taken twice in each case.

By refactoring the code, we can turn mod_objcg_state() into
__account_obj_stock() which is called on a stock that's already locked
and validated. On the charging side we can call this function from
consume_obj_stock() when it succeeds, and refill_obj_stock() in the
fallback. We just expand parameters of these functions as necessary.
The uncharge side from __memcg_slab_free_hook() is just the call to
refill_obj_stock().

Other callers of obj_cgroup_[un]charge() (i.e. not slab) simply pass the
extra parameters as NULL/zeroes to skip the __account_obj_stock()
operation.

In __memcg_slab_post_alloc_hook() we now charge each object separately,
but that's not a problem as we did call mod_objcg_state() for each
object separately, and most allocations are non-bulk anyway. This
could be improved by batching all operations until slab_pgdat(slab)
changes.

Some preliminary benchmarking with a kfree(kmalloc()) loop of 10M
iterations with/without __GFP_ACCOUNT:

Before the patch:
kmalloc/kfree !memcg:    581390144 cycles
kmalloc/kfree memcg:     783689984 cycles

After the patch:
kmalloc/kfree memcg:     658723808 cycles

More than half of the overhead of __GFP_ACCOUNT relative to
non-accounted case seems eliminated.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/memcontrol.c | 77 +++++++++++++++++++++++++++++--------------------
 1 file changed, 46 insertions(+), 31 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dfe9c2eb7816..553eb1d7250a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2753,25 +2753,17 @@ static void replace_stock_objcg(struct memcg_stock_pcp *stock,
 	WRITE_ONCE(stock->cached_objcg, objcg);
 }
 
-static void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
-		     enum node_stat_item idx, int nr)
+static void __account_obj_stock(struct obj_cgroup *objcg,
+				struct memcg_stock_pcp *stock, int nr,
+				struct pglist_data *pgdat, enum node_stat_item idx)
 {
-	struct memcg_stock_pcp *stock;
-	unsigned long flags;
 	int *bytes;
 
-	localtry_lock_irqsave(&memcg_stock.stock_lock, flags);
-	stock = this_cpu_ptr(&memcg_stock);
-
 	/*
 	 * Save vmstat data in stock and skip vmstat array update unless
-	 * accumulating over a page of vmstat data or when pgdat or idx
-	 * changes.
+	 * accumulating over a page of vmstat data or when pgdat changes.
 	 */
-	if (READ_ONCE(stock->cached_objcg) != objcg) {
-		replace_stock_objcg(stock, objcg);
-		stock->cached_pgdat = pgdat;
-	} else if (stock->cached_pgdat != pgdat) {
+	if (stock->cached_pgdat != pgdat) {
 		/* Flush the existing cached vmstat data */
 		struct pglist_data *oldpg = stock->cached_pgdat;
 
@@ -2808,11 +2800,10 @@ static void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	}
 	if (nr)
 		__mod_objcg_mlstate(objcg, pgdat, idx, nr);
-
-	localtry_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 }
 
-static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
+static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
+			      struct pglist_data *pgdat, enum node_stat_item idx)
 {
 	struct memcg_stock_pcp *stock;
 	unsigned long flags;
@@ -2824,6 +2815,9 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 	if (objcg == READ_ONCE(stock->cached_objcg) && stock->nr_bytes >= nr_bytes) {
 		stock->nr_bytes -= nr_bytes;
 		ret = true;
+
+		if (pgdat)
+			__account_obj_stock(objcg, stock, nr_bytes, pgdat, idx);
 	}
 
 	localtry_unlock_irqrestore(&memcg_stock.stock_lock, flags);
@@ -2908,7 +2902,8 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 }
 
 static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
-			     bool allow_uncharge)
+		bool allow_uncharge, int nr_acct, struct pglist_data *pgdat,
+		enum node_stat_item idx)
 {
 	struct memcg_stock_pcp *stock;
 	unsigned long flags;
@@ -2923,6 +2918,9 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 	}
 	stock->nr_bytes += nr_bytes;
 
+	if (pgdat)
+		__account_obj_stock(objcg, stock, nr_acct, pgdat, idx);
+
 	if (allow_uncharge && (stock->nr_bytes > PAGE_SIZE)) {
 		nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		stock->nr_bytes &= (PAGE_SIZE - 1);
@@ -2934,12 +2932,13 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
 }
 
-int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
+static int obj_cgroup_charge_account(struct obj_cgroup *objcg, gfp_t gfp, size_t size,
+				     struct pglist_data *pgdat, enum node_stat_item idx)
 {
 	unsigned int nr_pages, nr_bytes;
 	int ret;
 
-	if (consume_obj_stock(objcg, size))
+	if (likely(consume_obj_stock(objcg, size, pgdat, idx)))
 		return 0;
 
 	/*
@@ -2972,15 +2971,21 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 		nr_pages += 1;
 
 	ret = obj_cgroup_charge_pages(objcg, gfp, nr_pages);
-	if (!ret && nr_bytes)
-		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, false);
+	if (!ret && (nr_bytes || pgdat))
+		refill_obj_stock(objcg, nr_bytes ? PAGE_SIZE - nr_bytes : 0,
+					 false, size, pgdat, idx);
 
 	return ret;
 }
 
+int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
+{
+	return obj_cgroup_charge_account(objcg, gfp, size, NULL, 0);
+}
+
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
 {
-	refill_obj_stock(objcg, size, true);
+	refill_obj_stock(objcg, size, true, 0, NULL, 0);
 }
 
 static inline size_t obj_full_size(struct kmem_cache *s)
@@ -3032,23 +3037,32 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 			return false;
 	}
 
-	if (obj_cgroup_charge(objcg, flags, size * obj_full_size(s)))
-		return false;
-
 	for (i = 0; i < size; i++) {
 		slab = virt_to_slab(p[i]);
 
 		if (!slab_obj_exts(slab) &&
 		    alloc_slab_obj_exts(slab, s, flags, false)) {
-			obj_cgroup_uncharge(objcg, obj_full_size(s));
 			continue;
 		}
 
+		/*
+		 * if we fail and size is 1, memcg_alloc_abort_single() will
+		 * just free the object, which is ok as we have not assigned
+		 * objcg to its obj_ext yet
+		 *
+		 * for larger sizes, kmem_cache_free_bulk() will uncharge
+		 * any objects that were already charged and obj_ext assigned
+		 *
+		 * TODO: we could batch this until slab_pgdat(slab) changes
+		 * between iterations, with a more complicated undo
+		 */
+		if (obj_cgroup_charge_account(objcg, flags, obj_full_size(s),
+					slab_pgdat(slab), cache_vmstat_idx(s)))
+			return false;
+
 		off = obj_to_index(s, slab, p[i]);
 		obj_cgroup_get(objcg);
 		slab_obj_exts(slab)[off].objcg = objcg;
-		mod_objcg_state(objcg, slab_pgdat(slab),
-				cache_vmstat_idx(s), obj_full_size(s));
 	}
 
 	return true;
@@ -3057,6 +3071,8 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 			    void **p, int objects, struct slabobj_ext *obj_exts)
 {
+	size_t obj_size = obj_full_size(s);
+
 	for (int i = 0; i < objects; i++) {
 		struct obj_cgroup *objcg;
 		unsigned int off;
@@ -3067,9 +3083,8 @@ void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 			continue;
 
 		obj_exts[off].objcg = NULL;
-		obj_cgroup_uncharge(objcg, obj_full_size(s));
-		mod_objcg_state(objcg, slab_pgdat(slab), cache_vmstat_idx(s),
-				-obj_full_size(s));
+		refill_obj_stock(objcg, obj_size, true, -obj_size,
+				 slab_pgdat(slab), cache_vmstat_idx(s));
 		obj_cgroup_put(objcg);
 	}
 }
-- 
2.48.1