net: add dedicated kmem_cache for typical/small skb->head

Recent removal of ksize() in alloc_skb() increased
performance because we no longer read
the associated struct page.

We have an equivalent cost at kfree_skb() time.

kfree(skb->head) has to access a struct page,
often cold in cpu caches to get the owning
struct kmem_cache.

Considering that many allocations are small (at least for TCP ones)
we can have our own kmem_cache to avoid the cache line miss.

This also saves memory because these small heads
are no longer padded to 1024 bytes.

CONFIG_SLUB=y
$ grep skbuff_small_head /proc/slabinfo
skbuff_small_head   2907   2907    640   51    8 : tunables    0    0    0 : slabdata     57     57      0

CONFIG_SLAB=y
$ grep skbuff_small_head /proc/slabinfo
skbuff_small_head    607    624    640    6    1 : tunables   54   27    8 : slabdata    104    104      5

Notes:

- After Kees Cook patches and this one, we might
  be able to revert commit
  dbae2b062824 ("net: skb: introduce and use a single page frag cache")
  because GRO_MAX_HEAD is also small.

- This patch is a NOP for CONFIG_SLOB=y builds.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c123283..bdb1e01 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -89,6 +89,34 @@ static struct kmem_cache *skbuff_fclone_cache __ro_after_init;
 #ifdef CONFIG_SKB_EXTENSIONS
 static struct kmem_cache *skbuff_ext_cache __ro_after_init;
 #endif
+
+/* skb_small_head_cache and related code is only supported
+ * for CONFIG_SLAB and CONFIG_SLUB.
+ * As soon as SLOB is removed from the kernel, we can clean up this.
+ */
+#if !defined(CONFIG_SLOB)
+# define HAVE_SKB_SMALL_HEAD_CACHE 1
+#endif
+
+#ifdef HAVE_SKB_SMALL_HEAD_CACHE
+static struct kmem_cache *skb_small_head_cache __ro_after_init;
+
+#define SKB_SMALL_HEAD_SIZE SKB_HEAD_ALIGN(MAX_TCP_HEADER)
+
+/* We want SKB_SMALL_HEAD_CACHE_SIZE to not be a power of two.
+ * This should ensure that SKB_SMALL_HEAD_HEADROOM is a unique
+ * size, and we can differentiate heads from skb_small_head_cache
+ * vs system slabs by looking at their size (skb_end_offset()).
+ */
+#define SKB_SMALL_HEAD_CACHE_SIZE                                      \
+       (is_power_of_2(SKB_SMALL_HEAD_SIZE) ?                   \
+               (SKB_SMALL_HEAD_SIZE + L1_CACHE_BYTES) :        \
+               SKB_SMALL_HEAD_SIZE)
+
+#define SKB_SMALL_HEAD_HEADROOM                                                \
+       SKB_WITH_OVERHEAD(SKB_SMALL_HEAD_CACHE_SIZE)
+#endif /* HAVE_SKB_SMALL_HEAD_CACHE */
+
 int sysctl_max_skb_frags __read_mostly = MAX_SKB_FRAGS;
 EXPORT_SYMBOL(sysctl_max_skb_frags);
 
@@ -486,6 +514,23 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
        void *obj;
 
        obj_size = SKB_HEAD_ALIGN(*size);
+#ifdef HAVE_SKB_SMALL_HEAD_CACHE
+       if (obj_size <= SKB_SMALL_HEAD_CACHE_SIZE &&
+           !(flags & KMALLOC_NOT_NORMAL_BITS)) {
+
+               /* skb_small_head_cache has non power of two size,
+                * likely forcing SLUB to use order-3 pages.
+                * We deliberately attempt a NOMEMALLOC allocation only.
+                */
+               obj = kmem_cache_alloc_node(skb_small_head_cache,
+                               flags | __GFP_NOMEMALLOC | __GFP_NOWARN,
+                               node);
+               if (obj) {
+                       *size = SKB_SMALL_HEAD_CACHE_SIZE;
+                       goto out;
+               }
+       }
+#endif
        *size = obj_size = kmalloc_size_roundup(obj_size);
        /*
         * Try a regular allocation, when that fails and we're not entitled
@@ -805,6 +850,16 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data)
        return page_pool_return_skb_page(virt_to_page(data));
 }
 
+static void skb_kfree_head(void *head, unsigned int end_offset)
+{
+#ifdef HAVE_SKB_SMALL_HEAD_CACHE
+       if (end_offset == SKB_SMALL_HEAD_HEADROOM)
+               kmem_cache_free(skb_small_head_cache, head);
+       else
+#endif
+               kfree(head);
+}
+
 static void skb_free_head(struct sk_buff *skb)
 {
        unsigned char *head = skb->head;
@@ -814,7 +869,7 @@ static void skb_free_head(struct sk_buff *skb)
                        return;
                skb_free_frag(head);
        } else {
-               kfree(head);
+               skb_kfree_head(head, skb_end_offset(skb));
        }
 }
 
@@ -1997,7 +2052,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
        return 0;
 
 nofrags:
-       kfree(data);
+       skb_kfree_head(data, size);
 nodata:
        return -ENOMEM;
 }
@@ -4634,6 +4689,13 @@ void __init skb_init(void)
                                                0,
                                                SLAB_HWCACHE_ALIGN|SLAB_PANIC,
                                                NULL);
+#ifdef HAVE_SKB_SMALL_HEAD_CACHE
+       skb_small_head_cache = kmem_cache_create("skbuff_small_head",
+                                               SKB_SMALL_HEAD_CACHE_SIZE,
+                                               0,
+                                               SLAB_HWCACHE_ALIGN | SLAB_PANIC,
+                                               NULL);
+#endif
        skb_extensions_init();
 }
 
@@ -6298,7 +6360,7 @@ static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
        if (skb_cloned(skb)) {
                /* drop the old head gracefully */
                if (skb_orphan_frags(skb, gfp_mask)) {
-                       kfree(data);
+                       skb_kfree_head(data, size);
                        return -ENOMEM;
                }
                for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
@@ -6406,7 +6468,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
        memcpy((struct skb_shared_info *)(data + size),
               skb_shinfo(skb), offsetof(struct skb_shared_info, frags[0]));
        if (skb_orphan_frags(skb, gfp_mask)) {
-               kfree(data);
+               skb_kfree_head(data, size);
                return -ENOMEM;
        }
        shinfo = (struct skb_shared_info *)(data + size);
@@ -6442,7 +6504,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
                /* skb_frag_unref() is not needed here as shinfo->nr_frags = 0. */
                if (skb_has_frag_list(skb))
                        kfree_skb_list(skb_shinfo(skb)->frag_list);
-               kfree(data);
+               skb_kfree_head(data, size);
                return -ENOMEM;
        }
        skb_release_data(skb, SKB_CONSUMED);