linux-kernel - Re: [PATCH net-next 1/4] tun: rcu_deference xdp

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Message-ID: <69E5FB29-E3BF-46C6-B6AD-48559DFD2A22@nutanix.com>
Date: Thu, 8 May 2025 13:40:34 +0000
From: Jon Kohler <jon@...anix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC: "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net"
	<daniel@...earbox.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org"
	<bpf@...r.kernel.org>,
        "aleksander.lobakin@...el.com"
	<aleksander.lobakin@...el.com>,
        Jason Wang <jasowang@...hat.com>, Andrew Lunn
	<andrew+netdev@...n.ch>,
        Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
	<pabeni@...hat.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/4] tun: rcu_deference xdp_prog only once per
 batch



> On May 8, 2025, at 9:31 AM, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> Jon Kohler wrote:
>> 
>> 
>>> On May 7, 2025, at 4:43 PM, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>>> 
>>> !-------------------------------------------------------------------|
>>> CAUTION: External Email
>>> 
>>> |-------------------------------------------------------------------!
>>> 
>>> Jon Kohler wrote:
>>>> Hoist rcu_dereference(tun->xdp_prog) out of tun_xdp_one, so that
>>>> rcu_deference is called once during batch processing.
>>> 
>>> I'm skeptical that this does anything.
>>> 
>>> The compiler can inline tun_xdp_one and indeed seems to do so. And
>>> then it can cache the read in a register if that is the best use of
>>> a register.
>> 
>> The thought here is that if a compiler decided to not-inline tun_xdp_one
>> (perhaps it grew to big, or the compiler was being sassy), that the intent
>> would simply be that this wants to be called once-and-only-once. This
>> change just makes that intent more clear, and is a nice little cleanup.
>> 
>> I’ve got a series that stacks on top of this that enables multi-buffer support
>> and I can keep an eye on if that gets inlined or not.
> 
> That will only give you one outcome with a specific compiler, platform
> and build configuration.
> 
> I would just drop this and let the compiler handle such optimizations.

Ok, thanks for the feedback, will do