skbuff: micro-optimize {,__}skb_header_pointer()

{,__}skb_header_pointer() helpers exist mainly for preventing
accesses-beyond-end of the linear data.
In the vast majorify of cases, they bail out on the first condition.
All code going after is mostly a fallback.
Mark the most common branch as 'likely' one to move it in-line.
Also, skb_copy_bits() can return negative values only when the input
arguments are invalid, e.g. offset is greater than skb->len. It can
be safely marked as 'unlikely' branch, assuming that hotpath code
provides sane input to not fail here.

These two bump the throughput with a single Flow Dissector pass on
every packet (e.g. with RPS or driver that uses eth_get_headlen())
on 20 Mbps per flow/core.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 46c61e1..ecc0296 100644 (file)
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3680,11 +3680,10 @@ static inline void * __must_check
 __skb_header_pointer(const struct sk_buff *skb, int offset, int len,
                     const void *data, int hlen, void *buffer)
 {
-       if (hlen - offset >= len)
+       if (likely(hlen - offset >= len))
                return (void *)data + offset;
 
-       if (!skb ||
-           skb_copy_bits(skb, offset, buffer, len) < 0)
+       if (!skb || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
                return NULL;
 
        return buffer;