Merge branch 'net-use-kmem_cache_free_bulk-in-kfree_skb_list'

Jesper Dangaard Brouer says:

====================
net: use kmem_cache_free_bulk in kfree_skb_list

The kfree_skb_list function walks SKB (via skb->next) and frees them
individually to the SLUB/SLAB allocator (kmem_cache). It is more
efficient to bulk free them via the kmem_cache_free_bulk API.

Netstack NAPI fastpath already uses kmem_cache bulk alloc and free
APIs for SKBs.

The kfree_skb_list call got an interesting optimization in commit
520ac30f ("net_sched: drop packets after root qdisc lock is
released") that can create a list of SKBs "to_free" e.g. when qdisc
enqueue fails or deliberately chooses to drop . It isn't a normal data
fastpath, but the situation will likely occur when system/qdisc are
under heavy workloads, thus it makes sense to use a faster API for
freeing the SKBs.

E.g. the (often distro default) qdisc fq_codel will drop batches of
packets from fattest elephant flow, default capped at 64 packets (but
adjustable via tc argument drop_batch).

Performance measurements done in [1]:
 [1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/kfree_skb_list01.org
====================

Link: https://lore.kernel.org/r/167361788585.531803.686364041841425360.stgit@firesoulSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/skbuff.c

@@ -930,6 +930,21 @@ void __kfree_skb(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(__kfree_skb);
 
+static __always_inline
+bool __kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
+{
+	if (unlikely(!skb_unref(skb)))
+		return false;
+
+	DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
+
+	if (reason == SKB_CONSUMED)
+		trace_consume_skb(skb);
+	else
+		trace_kfree_skb(skb, __builtin_return_address(0), reason);
+	return true;
+}
+
 /**
  *	kfree_skb_reason - free an sk_buff with special reason
  *	@skb: buffer to free
@@ -942,28 +957,59 @@ EXPORT_SYMBOL(__kfree_skb);
 void __fix_address
 kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
 {
-	if (unlikely(!skb_unref(skb)))
-		return;
-
-	DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
-
-	if (reason == SKB_CONSUMED)
-		trace_consume_skb(skb);
-	else
-		trace_kfree_skb(skb, __builtin_return_address(0), reason);
+	if (__kfree_skb_reason(skb, reason))
 		__kfree_skb(skb);
 }
 EXPORT_SYMBOL(kfree_skb_reason);
 
-void kfree_skb_list_reason(struct sk_buff *segs,
+#define KFREE_SKB_BULK_SIZE	16
+
+struct skb_free_array {
+	unsigned int skb_count;
+	void *skb_array[KFREE_SKB_BULK_SIZE];
+};
+
+static void kfree_skb_add_bulk(struct sk_buff *skb,
+			       struct skb_free_array *sa,
 			       enum skb_drop_reason reason)
 {
+	/* if SKB is a clone, don't handle this case */
+	if (unlikely(skb->fclone != SKB_FCLONE_UNAVAILABLE)) {
+		__kfree_skb(skb);
+		return;
+	}
+
+	skb_release_all(skb, reason);
+	sa->skb_array[sa->skb_count++] = skb;
+
+	if (unlikely(sa->skb_count == KFREE_SKB_BULK_SIZE)) {
+		kmem_cache_free_bulk(skbuff_head_cache, KFREE_SKB_BULK_SIZE,
+				     sa->skb_array);
+		sa->skb_count = 0;
+	}
+}
+
+void __fix_address
+kfree_skb_list_reason(struct sk_buff *segs, enum skb_drop_reason reason)
+{
+	struct skb_free_array sa;
+
+	sa.skb_count = 0;
+
 	while (segs) {
 		struct sk_buff *next = segs->next;
 
-		kfree_skb_reason(segs, reason);
+		skb_mark_not_on_list(segs);
+
+		if (__kfree_skb_reason(segs, reason))
+			kfree_skb_add_bulk(segs, &sa, reason);
+
 		segs = next;
 	}
+
+	if (sa.skb_count)
+		kmem_cache_free_bulk(skbuff_head_cache, sa.skb_count,
+				     sa.skb_array);
 }
 EXPORT_SYMBOL(kfree_skb_list_reason);