Subject: [RFC PATCH 1/3] net: introduce kfree_skb_bulk() user of kmem_cache_free_bulk()

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 04 Sep 2015 19:00:53 +0200

Introduce the first user of SLAB bulk free API kmem_cache_free_bulk(),
in the network stack in form of function kfree_skb_bulk() which bulk
free SKBs (not skb clones or skb->head, yet).

As this is the third user of SKB reference decrementing, split out
refcnt decrement into helper function and use this in all call points.

Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
---
 include/linux/skbuff.h |    1 +
 net/core/skbuff.c      |   87 +++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 71 insertions(+), 17 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b97597970ce7..e5f1e007723b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -762,6 +762,7 @@ static inline struct rtable *skb_rtable(const struct sk_buff *skb)
 }
 
 void kfree_skb(struct sk_buff *skb);
+void kfree_skb_bulk(struct sk_buff **skbs, unsigned int size);
 void kfree_skb_list(struct sk_buff *segs);
 void skb_tx_error(struct sk_buff *skb);
 void consume_skb(struct sk_buff *skb);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 429b407b4fe6..034545934158 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -661,26 +661,83 @@ void __kfree_skb(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(__kfree_skb);
 
+/*
+ *	skb_dec_and_test - Helper to drop ref to SKB and see is ready to free
+ *	@skb: buffer to decrement reference
+ *
+ *	Drop a reference to the buffer, and return true if it is ready
+ *	to free. Which is if the usage count has hit zero or is equal to 1.
+ *
+ *	This is performance critical code that should be inlined.
+ */
+static inline bool skb_dec_and_test(struct sk_buff *skb)
+{
+	if (unlikely(!skb))
+		return false;
+	if (likely(atomic_read(&skb->users) == 1))
+		smp_rmb();
+	else if (likely(!atomic_dec_and_test(&skb->users)))
+		return false;
+	/* If reaching here SKB is ready to free */
+	return true;
+}
+
 /**
  *	kfree_skb - free an sk_buff
  *	@skb: buffer to free
  *
  *	Drop a reference to the buffer and free it if the usage count has
- *	hit zero.
+ *	hit zero or is equal to 1.
  */
 void kfree_skb(struct sk_buff *skb)
 {
-	if (unlikely(!skb))
-		return;
-	if (likely(atomic_read(&skb->users) == 1))
-		smp_rmb();
-	else if (likely(!atomic_dec_and_test(&skb->users)))
-		return;
-	trace_kfree_skb(skb, __builtin_return_address(0));
-	__kfree_skb(skb);
+	if (skb_dec_and_test(skb)) {
+		trace_kfree_skb(skb, __builtin_return_address(0));
+		__kfree_skb(skb);
+	}
 }
 EXPORT_SYMBOL(kfree_skb);
 
+/**
+ *	kfree_skb_bulk - bulk free SKBs when refcnt allows to
+ *	@skbs: array of SKBs to free
+ *	@size: number of SKBs in array
+ *
+ *	If SKB refcnt allows for free, then release any auxiliary data
+ *	and then bulk free SKBs to the SLAB allocator.
+ *
+ *	Note that interrupts must be enabled when calling this function.
+ */
+void kfree_skb_bulk(struct sk_buff **skbs, unsigned int size)
+{
+	int i;
+	size_t cnt = 0;
+
+	for (i = 0; i < size; i++) {
+		struct sk_buff *skb = skbs[i];
+
+		if (!skb_dec_and_test(skb))
+			continue; /* skip skb, not ready to free */
+
+		/* Construct an array of SKBs, ready to be free'ed and
+		 * cleanup all auxiliary, before bulk free to SLAB.
+		 * For now, only handle non-cloned SKBs, related to
+		 * SLAB skbuff_head_cache
+		 */
+		if (skb->fclone == SKB_FCLONE_UNAVAILABLE) {
+			skb_release_all(skb);
+			skbs[cnt++] = skb;
+		} else {
+			/* SKB was a clone, don't handle this case */
+			__kfree_skb(skb);
+		}
+	}
+	if (likely(cnt)) {
+		kmem_cache_free_bulk(skbuff_head_cache, cnt, (void **) skbs);
+	}
+}
+EXPORT_SYMBOL(kfree_skb_bulk);
+
 void kfree_skb_list(struct sk_buff *segs)
 {
 	while (segs) {
@@ -722,14 +779,10 @@ EXPORT_SYMBOL(skb_tx_error);
  */
 void consume_skb(struct sk_buff *skb)
 {
-	if (unlikely(!skb))
-		return;
-	if (likely(atomic_read(&skb->users) == 1))
-		smp_rmb();
-	else if (likely(!atomic_dec_and_test(&skb->users)))
-		return;
-	trace_consume_skb(skb);
-	__kfree_skb(skb);
+	if (skb_dec_and_test(skb)) {
+		trace_consume_skb(skb);
+		__kfree_skb(skb);
+	}
 }
 EXPORT_SYMBOL(consume_skb);

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