netdev - Re: [PATCH net-next 06/10] udp: update sk_rmem

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Message-ID: <willemdebruijn.kernel.111bed09b8999@gmail.com>
Date: Wed, 17 Sep 2025 11:01:30 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Kuniyuki Iwashima <kuniyu@...gle.com>, 
 David Ahern <dsahern@...nel.org>, 
 netdev@...r.kernel.org, 
 eric.dumazet@...il.com, 
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 06/10] udp: update sk_rmem_alloc before busylock
 acquisition

Eric Dumazet wrote:
> Avoid piling too many producers on the busylock
> by updating sk_rmem_alloc before busylock acquisition.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/udp.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index edd846fee90ff7850356a5cb3400ce96856e5429..658ae87827991a78c25c2172d52e772c94ea217f 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1753,13 +1753,16 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
>  	if (rmem > (rcvbuf >> 1)) {
>  		skb_condense(skb);
>  		size = skb->truesize;
> +		rmem = atomic_add_return(size, &sk->sk_rmem_alloc);
> +		if (rmem > rcvbuf)
> +			goto uncharge_drop;

This does more than just reorganize code. Can you share some context
on the behavioral change?

>  		busy = busylock_acquire(sk);
> +	} else {
> +		atomic_add(size, &sk->sk_rmem_alloc);
>  	}
>  
>  	udp_set_dev_scratch(skb);
>  
> -	atomic_add(size, &sk->sk_rmem_alloc);
> -
>  	spin_lock(&list->lock);
>  	err = udp_rmem_schedule(sk, size);
>  	if (err) {
> -- 
> 2.51.0.384.g4c02a37b29-goog
>