[PATCH v4 4/9] 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>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/slub.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 724266fdd996c091f1f0b34012c5179f17dfa422..cc273cc45f632e16644355831132cdc391219cec 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2758,9 +2758,12 @@ 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);
@@ -2807,6 +2810,9 @@ 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) {
@@ -2837,9 +2843,12 @@ 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);
@@ -2862,19 +2871,23 @@ 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);

-- 
2.49.0