netdev - Re: [PATCH net-next] net: Remove likely from l3mdev_master_ifindex_by

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Message-ID: <cea1458c-6445-45d0-bfa1-3c093384cc90@kernel.org>
Date: Tue, 8 Oct 2024 12:25:28 -0600
From: David Ahern <dsahern@...nel.org>
To: Breno Leitao <leitao@...ian.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, "open list:L3MDEV" <netdev@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: Remove likely from
 l3mdev_master_ifindex_by_index

On 10/8/24 10:32 AM, Breno Leitao wrote:
> 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(-)
> 

Reviewed-by: David Ahern <dsahern@...nel.org>