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Subject[PATCH v5 net-next 08/11] skbuff: introduce {,__}napi_build_skb() which reuses NAPI cache heads
Instead of just bulk-flushing skbuff_heads queued up through
napi_consume_skb() or __kfree_skb_defer(), try to reuse them
on allocation path.
If the cache is empty on allocation, bulk-allocate the first
16 elements, which is more efficient than per-skb allocation.
If the cache is full on freeing, bulk-wipe the second half of
the cache (32 elements).
This also includes custom KASAN poisoning/unpoisoning to be
double sure there are no use-after-free cases.

To not change current behaviour, introduce a new function,
napi_build_skb(), to optionally use a new approach later
in drivers.

Note on selected bulk size, 16:
- this equals to XDP_BULK_QUEUE_SIZE, DEV_MAP_BULK_SIZE
and especially VETH_XDP_BATCH, which is also used to
bulk-allocate skbuff_heads and was tested on powerful
setups;
- this also showed the best performance in the actual
test series (from the array of {8, 16, 32}).

Suggested-by: Edward Cree <ecree.xilinx@gmail.com> # Divide on two halves
Suggested-by: Eric Dumazet <edumazet@google.com> # KASAN poisoning
Cc: Dmitry Vyukov <dvyukov@google.com> # Help with KASAN
Cc: Paolo Abeni <pabeni@redhat.com> # Reduced batch size
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
include/linux/skbuff.h | 2 +
net/core/skbuff.c | 94 ++++++++++++++++++++++++++++++++++++------
2 files changed, 83 insertions(+), 13 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0e0707296098..906122eac82a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1087,6 +1087,8 @@ struct sk_buff *build_skb(void *data, unsigned int frag_size);
struct sk_buff *build_skb_around(struct sk_buff *skb,
void *data, unsigned int frag_size);

+struct sk_buff *napi_build_skb(void *data, unsigned int frag_size);
+
/**
* alloc_skb - allocate a network buffer
* @size: size to allocate
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 860a9d4f752f..9e1a8ded4acc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -120,6 +120,8 @@ static void skb_under_panic(struct sk_buff *skb, unsigned int sz, void *addr)
}

#define NAPI_SKB_CACHE_SIZE 64
+#define NAPI_SKB_CACHE_BULK 16
+#define NAPI_SKB_CACHE_HALF (NAPI_SKB_CACHE_SIZE / 2)

struct napi_alloc_cache {
struct page_frag_cache page;
@@ -164,6 +166,25 @@ void *__netdev_alloc_frag_align(unsigned int fragsz, unsigned int align_mask)
}
EXPORT_SYMBOL(__netdev_alloc_frag_align);

+static struct sk_buff *napi_skb_cache_get(void)
+{
+ struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+ struct sk_buff *skb;
+
+ if (unlikely(!nc->skb_count))
+ nc->skb_count = kmem_cache_alloc_bulk(skbuff_head_cache,
+ GFP_ATOMIC,
+ NAPI_SKB_CACHE_BULK,
+ nc->skb_cache);
+ if (unlikely(!nc->skb_count))
+ return NULL;
+
+ skb = nc->skb_cache[--nc->skb_count];
+ kasan_unpoison_object_data(skbuff_head_cache, skb);
+
+ return skb;
+}
+
/* Caller must provide SKB that is memset cleared */
static void __build_skb_around(struct sk_buff *skb, void *data,
unsigned int frag_size)
@@ -265,6 +286,53 @@ struct sk_buff *build_skb_around(struct sk_buff *skb,
}
EXPORT_SYMBOL(build_skb_around);

+/**
+ * __napi_build_skb - build a network buffer
+ * @data: data buffer provided by caller
+ * @frag_size: size of data, or 0 if head was kmalloced
+ *
+ * Version of __build_skb() that uses NAPI percpu caches to obtain
+ * skbuff_head instead of inplace allocation.
+ *
+ * Returns a new &sk_buff on success, %NULL on allocation failure.
+ */
+static struct sk_buff *__napi_build_skb(void *data, unsigned int frag_size)
+{
+ struct sk_buff *skb;
+
+ skb = napi_skb_cache_get();
+ if (unlikely(!skb))
+ return NULL;
+
+ memset(skb, 0, offsetof(struct sk_buff, tail));
+ __build_skb_around(skb, data, frag_size);
+
+ return skb;
+}
+
+/**
+ * napi_build_skb - build a network buffer
+ * @data: data buffer provided by caller
+ * @frag_size: size of data, or 0 if head was kmalloced
+ *
+ * Version of __napi_build_skb() that takes care of skb->head_frag
+ * and skb->pfmemalloc when the data is a page or page fragment.
+ *
+ * Returns a new &sk_buff on success, %NULL on allocation failure.
+ */
+struct sk_buff *napi_build_skb(void *data, unsigned int frag_size)
+{
+ struct sk_buff *skb = __napi_build_skb(data, frag_size);
+
+ if (likely(skb) && frag_size) {
+ skb->head_frag = 1;
+ skb_propagate_pfmemalloc(virt_to_head_page(data), skb);
+ }
+
+ return skb;
+}
+EXPORT_SYMBOL(napi_build_skb);
+
/*
* kmalloc_reserve is a wrapper around kmalloc_node_track_caller that tells
* the caller if emergency pfmemalloc reserves are being used. If it is and
@@ -838,31 +906,31 @@ void __consume_stateless_skb(struct sk_buff *skb)
kfree_skbmem(skb);
}

-static inline void _kfree_skb_defer(struct sk_buff *skb)
+static void napi_skb_cache_put(struct sk_buff *skb)
{
struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
+ u32 i;

/* drop skb->head and call any destructors for packet */
skb_release_all(skb);

- /* record skb to CPU local list */
+ kasan_poison_object_data(skbuff_head_cache, skb);
nc->skb_cache[nc->skb_count++] = skb;

-#ifdef CONFIG_SLUB
- /* SLUB writes into objects when freeing */
- prefetchw(skb);
-#endif
-
- /* flush skb_cache if it is filled */
if (unlikely(nc->skb_count == NAPI_SKB_CACHE_SIZE)) {
- kmem_cache_free_bulk(skbuff_head_cache, NAPI_SKB_CACHE_SIZE,
- nc->skb_cache);
- nc->skb_count = 0;
+ for (i = NAPI_SKB_CACHE_HALF; i < NAPI_SKB_CACHE_SIZE; i++)
+ kasan_unpoison_object_data(skbuff_head_cache,
+ nc->skb_cache[i]);
+
+ kmem_cache_free_bulk(skbuff_head_cache, NAPI_SKB_CACHE_HALF,
+ nc->skb_cache + NAPI_SKB_CACHE_HALF);
+ nc->skb_count = NAPI_SKB_CACHE_HALF;
}
}
+
void __kfree_skb_defer(struct sk_buff *skb)
{
- _kfree_skb_defer(skb);
+ napi_skb_cache_put(skb);
}

void napi_consume_skb(struct sk_buff *skb, int budget)
@@ -887,7 +955,7 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
return;
}

- _kfree_skb_defer(skb);
+ napi_skb_cache_put(skb);
}
EXPORT_SYMBOL(napi_consume_skb);

--
2.30.1

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