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>
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5b2c5f1..a7f855d 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -184,11 +184,20 @@
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 @@
* 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;