Re: [PATCH net-next 1/4] tun: rcu_deference xdp_prog only once per batch

[Willem de Bruijn]

Jon Kohler wrote:
> 
> 
> > On May 7, 2025, at 4:43 PM, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
> > 
> > !-------------------------------------------------------------------|
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> > 
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> > 
> > 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.