[PATCH RFC v2 07/10] slab: determine barn status racily outside of lock

[Vlastimil Babka <vbabka@suse.cz>]

The possibility of many barn operations is determined by the current
number of full or empty sheaves. Taking the barn->lock just to find out
that e.g. there are no empty sheaves results in unnecessary overhead and
lock contention. Thus perform these checks outside of the lock with a
data_race() annotated variable read and fail quickly without taking the
lock.

Checks for sheaf availability that racily succeed have to be obviously
repeated under the lock for correctness, but we can skip repeating
checks if there are too many sheaves on the given list as the limits
don't need to be strict.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 57 ++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 34 insertions(+), 23 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index c1df7cf22267f28f743404531bef921e25fac086..72e6437f1d74bfacbb1cd7642af42929c48cc66a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2685,9 +2685,12 @@ static struct slab_sheaf *barn_get_empty_sheaf(struct node_barn *barn)
 	struct slab_sheaf *empty = NULL;
 	unsigned long flags;
 
+	if (!data_race(barn->nr_empty))
+		return NULL;
+
 	spin_lock_irqsave(&barn->lock, flags);
 
-	if (barn->nr_empty) {
+	if (likely(barn->nr_empty)) {
 		empty = list_first_entry(&barn->sheaves_empty,
 					 struct slab_sheaf, barn_list);
 		list_del(&empty->barn_list);
@@ -2703,38 +2706,36 @@ static int barn_put_empty_sheaf(struct node_barn *barn,
 				struct slab_sheaf *sheaf, bool ignore_limit)
 {
 	unsigned long flags;
-	int ret = 0;
+
+	/* we don't repeat the check under barn->lock as it's not critical */
+	if (!ignore_limit && data_race(barn->nr_empty) >= MAX_EMPTY_SHEAVES)
+		return -E2BIG;
 
 	spin_lock_irqsave(&barn->lock, flags);
 
-	if (!ignore_limit && barn->nr_empty >= MAX_EMPTY_SHEAVES) {
-		ret = -E2BIG;
-	} else {
-		list_add(&sheaf->barn_list, &barn->sheaves_empty);
-		barn->nr_empty++;
-	}
+	list_add(&sheaf->barn_list, &barn->sheaves_empty);
+	barn->nr_empty++;
 
 	spin_unlock_irqrestore(&barn->lock, flags);
-	return ret;
+	return 0;
 }
 
 static int barn_put_full_sheaf(struct node_barn *barn, struct slab_sheaf *sheaf,
 			       bool ignore_limit)
 {
 	unsigned long flags;
-	int ret = 0;
+
+	/* we don't repeat the check under barn->lock as it's not critical */
+	if (!ignore_limit && data_race(barn->nr_full) >= MAX_FULL_SHEAVES)
+		return -E2BIG;
 
 	spin_lock_irqsave(&barn->lock, flags);
 
-	if (!ignore_limit && barn->nr_full >= MAX_FULL_SHEAVES) {
-		ret = -E2BIG;
-	} else {
-		list_add(&sheaf->barn_list, &barn->sheaves_full);
-		barn->nr_full++;
-	}
+	list_add(&sheaf->barn_list, &barn->sheaves_full);
+	barn->nr_full++;
 
 	spin_unlock_irqrestore(&barn->lock, flags);
-	return ret;
+	return 0;
 }
 
 static struct slab_sheaf *barn_get_full_or_empty_sheaf(struct node_barn *barn)
@@ -2742,6 +2743,9 @@ static struct slab_sheaf *barn_get_full_or_empty_sheaf(struct node_barn *barn)
 	struct slab_sheaf *sheaf = NULL;
 	unsigned long flags;
 
+	if (!data_race(barn->nr_full) && !data_race(barn->nr_empty))
+		return NULL;
+
 	spin_lock_irqsave(&barn->lock, flags);
 
 	if (barn->nr_full) {
@@ -2772,9 +2776,12 @@ barn_replace_empty_sheaf(struct node_barn *barn, struct slab_sheaf *empty)
 	struct slab_sheaf *full = NULL;
 	unsigned long flags;
 
+	if (!data_race(barn->nr_full))
+		return NULL;
+
 	spin_lock_irqsave(&barn->lock, flags);
 
-	if (barn->nr_full) {
+	if (likely(barn->nr_full)) {
 		full = list_first_entry(&barn->sheaves_full, struct slab_sheaf,
 					barn_list);
 		list_del(&full->barn_list);
@@ -2797,19 +2804,23 @@ barn_replace_full_sheaf(struct node_barn *barn, struct slab_sheaf *full)
 	struct slab_sheaf *empty;
 	unsigned long flags;
 
+	/* we don't repeat this check under barn->lock as it's not critical */
+	if (data_race(barn->nr_full) >= MAX_FULL_SHEAVES)
+		return ERR_PTR(-E2BIG);
+	if (!data_race(barn->nr_empty))
+		return ERR_PTR(-ENOMEM);
+
 	spin_lock_irqsave(&barn->lock, flags);
 
-	if (barn->nr_full >= MAX_FULL_SHEAVES) {
-		empty = ERR_PTR(-E2BIG);
-	} else if (!barn->nr_empty) {
-		empty = ERR_PTR(-ENOMEM);
-	} else {
+	if (likely(barn->nr_empty)) {
 		empty = list_first_entry(&barn->sheaves_empty, struct slab_sheaf,
 					 barn_list);
 		list_del(&empty->barn_list);
 		list_add(&full->barn_list, &barn->sheaves_full);
 		barn->nr_empty--;
 		barn->nr_full++;
+	} else {
+		empty = ERR_PTR(-ENOMEM);
 	}
 
 	spin_unlock_irqrestore(&barn->lock, flags);

-- 
2.48.1