linux-kernel - [PATCH net-next] net: Remove likely from l3mdev_master_ifindex_by

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Message-ID: <20241008163205.3939629-1-leitao@debian.org>
Date: Tue,  8 Oct 2024 09:32:04 -0700
From: Breno Leitao <leitao@...ian.org>
To: David Ahern <dsahern@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>
Cc: kernel-team@...a.com,
	netdev@...r.kernel.org (open list:L3MDEV),
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH net-next] net: Remove likely from l3mdev_master_ifindex_by_index

The likely() annotation in l3mdev_master_ifindex_by_index() has been
found to be incorrect 100% of the time in real-world workloads (e.g.,
web servers).

Annotated branches shows the following in these servers:

	correct incorrect  %        Function                  File              Line
	      0 169053813 100 l3mdev_master_ifindex_by_index l3mdev.h             81

This is happening because l3mdev_master_ifindex_by_index() is called
from __inet_check_established(), which calls
l3mdev_master_ifindex_by_index() passing the socked bounded interface.

	l3mdev_master_ifindex_by_index(net, sk->sk_bound_dev_if);

Since most sockets are not going to be bound to a network device,
the likely() is giving the wrong assumption.

Remove the likely() annotation to ensure more accurate branch
prediction.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 include/net/l3mdev.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/l3mdev.h b/include/net/l3mdev.h
index 031c661aa14d..2d6141f28b53 100644
--- a/include/net/l3mdev.h
+++ b/include/net/l3mdev.h
@@ -78,7 +78,7 @@ static inline int l3mdev_master_ifindex_by_index(struct net *net, int ifindex)
 	struct net_device *dev;
 	int rc = 0;

-	if (likely(ifindex)) {
+	if (ifindex) {
 		rcu_read_lock();

 		dev = dev_get_by_index_rcu(net, ifindex);
-- 
2.43.5