netdev - Re: xfrm in RT

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Message-ID: <Z4oOldW33zFbYQ6/@gauss3.secunet.de>
Date: Fri, 17 Jan 2025 09:02:29 +0100
From: Steffen Klassert <steffen.klassert@...unet.com>
To: Sebastian Sewior <bigeasy@...utronix.de>
CC: Alexei Starovoitov <alexei.starovoitov@...il.com>, Network Development
	<netdev@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>
Subject: Re: xfrm in RT

On Wed, Dec 18, 2024 at 04:44:26PM +0100, Sebastian Sewior wrote:
> On 2024-12-18 09:32:26 [+0100], Steffen Klassert wrote:
> > On Tue, Dec 17, 2024 at 04:07:16PM -0800, Alexei Starovoitov wrote:
> > > Hi,
> > > 
> > > Looks like xfrm isn't friendly to PREEMPT_RT.
> Thank you for the report.

Sorry for the delay.

> > > xfrm_input_state_lookup() is doing:
> > > 
> > > int cpu = get_cpu();
> > > ...
> > > spin_lock_bh(&net->xfrm.xfrm_state_lock);
> > 
> > We just need the cpu as a lookup key, no need to
> > hold on the cpu. So we just can do put_cpu()
> > directly after we fetched the value.
> 
> I would assume that the espX_gro_receive() caller is within NAPI. Can't
> tell what xfrm_input() is.
> However if you don't care about staying on the current CPU for the whole
> time (your current get_cpu() -> put_cpu() span) you could do something
> like
> 
> diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
> index 67ca7ac955a37..66b108a5b87d4 100644
> --- a/net/xfrm/xfrm_state.c
> +++ b/net/xfrm/xfrm_state.c
> @@ -1116,9 +1116,8 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
>  {
>  	struct hlist_head *state_cache_input;
>  	struct xfrm_state *x = NULL;
> -	int cpu = get_cpu();
>  
> -	state_cache_input =  per_cpu_ptr(net->xfrm.state_cache_input, cpu);
> +	state_cache_input = raw_cpu_ptr(net->xfrm.state_cache_input);
>  
>  	rcu_read_lock();
>  	hlist_for_each_entry_rcu(x, state_cache_input, state_cache_input) {
> @@ -1150,7 +1149,6 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
>  
>  out:
>  	rcu_read_unlock();
> -	put_cpu();
>  	return x;
>  }

This looks like the correct fix. Do you want to submit this pach?