linux-kernel - [RFC 1/4] net/ipv4/fib: Remove run-time check in tnode

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Date:   Tue, 26 Mar 2019 15:30:22 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <dima@...sta.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        David Ahern <dsahern@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Ido Schimmel <idosch@...lanox.com>, netdev@...r.kernel.org
Subject: [RFC 1/4] net/ipv4/fib: Remove run-time check in tnode_alloc()

TNODE_KMALLOC_MAX is not used anywhere, while TNODE_VMALLOC_MAX check in
tnode_alloc() only adds additional cmp/jmp instructions to tnode
allocation. During rebalancing of the trie the function can be called
thousands of times. Runtime check takes cache line and predictor entry.
Futhermore, this check is always false on 64-bit platfroms and ipv4 has
only 4 byte address range and bits are limited by KEYLENGTH (32).

Move the check under unlikely() and change comparison to BITS_PER_LONG,
optimizing allocation of tnode during rebalancing (and removing it
complitely for platforms with BITS_PER_LONG > KEYLENGTH).

Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 net/ipv4/fib_trie.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index a573e37e0615..ad7d56c421cb 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -312,11 +312,6 @@ static inline void alias_free_mem_rcu(struct fib_alias *fa)
 	call_rcu(&fa->rcu, __alias_free_mem);
 }
 
-#define TNODE_KMALLOC_MAX \
-	ilog2((PAGE_SIZE - TNODE_SIZE(0)) / sizeof(struct key_vector *))
-#define TNODE_VMALLOC_MAX \
-	ilog2((SIZE_MAX - TNODE_SIZE(0)) / sizeof(struct key_vector *))
-
 static void __node_free_rcu(struct rcu_head *head)
 {
 	struct tnode *n = container_of(head, struct tnode, rcu);
@@ -333,8 +328,7 @@ static struct tnode *tnode_alloc(int bits)
 {
 	size_t size;
 
-	/* verify bits is within bounds */
-	if (bits > TNODE_VMALLOC_MAX)
+	if ((BITS_PER_LONG <= KEYLENGTH) && unlikely(bits >= BITS_PER_LONG))
 		return NULL;
 
 	/* determine size and verify it is non-zero and didn't overflow */
-- 
2.21.0