net: more accurate skb truesize

skb truesize currently accounts for sk_buff struct and part of skb head.
kmalloc() roundings are also ignored.

Considering that skb_shared_info is larger than sk_buff, its time to
take it into account for better memory accounting.

This patch introduces SKB_TRUESIZE(X) macro to centralize various
assumptions into a single place.

At skb alloc phase, we put skb_shared_info struct at the exact end of
skb head, to allow a better use of memory (lowering number of
reallocations), since kmalloc() gives us power-of-two memory blocks.

Unless SLUB/SLUB debug is active, both skb->head and skb_shared_info are
aligned to cache lines, as before.

Note: This patch might trigger performance regressions because of
misconfigured protocol stacks, hitting per socket or global memory
limits that were previously not reached. But its a necessary step for a
more accurate memory accounting.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Andi Kleen <ak@linux.intel.com>
CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 4 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5b2c5f1d4dba..a7f855dca922 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -184,11 +184,20 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 		goto out;
 	prefetchw(skb);
 
-	size = SKB_DATA_ALIGN(size);
-	data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),
-			gfp_mask, node);
+	/* We do our best to align skb_shared_info on a separate cache
+	 * line. It usually works because kmalloc(X > SMP_CACHE_BYTES) gives
+	 * aligned memory blocks, unless SLUB/SLAB debug is enabled.
+	 * Both skb->head and skb_shared_info are cache line aligned.
+	 */
+	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	data = kmalloc_node_track_caller(size, gfp_mask, node);
 	if (!data)
 		goto nodata;
+	/* kmalloc(size) might give us more room than requested.
+	 * Put skb_shared_info exactly at the end of allocated zone,
+	 * to allow max possible filling before reallocation.
+	 */
+	size = SKB_WITH_OVERHEAD(ksize(data));
 	prefetchw(data + size);
 
 	/*
@@ -197,7 +206,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	 * the tail pointer in struct sk_buff!
 	 */
 	memset(skb, 0, offsetof(struct sk_buff, tail));
-	skb->truesize = size + sizeof(struct sk_buff);
+	/* Account for allocated memory : skb + skb->head */
+	skb->truesize = SKB_TRUESIZE(size);
 	atomic_set(&skb->users, 1);
 	skb->head = data;
 	skb->data = data;