linux-kernel - Re: [PATCH net-next v2 5/9] tun: use bulk NAPI cache allocation in tun_xdp

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Message-ID: <20251205075805.vW4ShQvN@linutronix.de>
Date: Fri, 5 Dec 2025 08:58:05 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jon Kohler <jon@...anix.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>,
	Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Stanislav Fomichev <sdf@...ichev.me>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:XDP (eXpress Data Path):Keyword:(?:b|_)xdp(?:b|_)" <bpf@...r.kernel.org>,
	Alexander Lobakin <aleksander.lobakin@...el.com>
Subject: Re: [PATCH net-next v2 5/9] tun: use bulk NAPI cache allocation in
 tun_xdp_one

On 2025-12-03 15:35:24 [+0000], Jon Kohler wrote:
> Thanks, Sebastian - so if I’m reading this correct, it *is* fine to do
> the two following patterns, outside of NAPI:
> 
>    local_bh_disable();
>    skb = napi_build_skb(buf, len);
>    local_bh_enable();
> 
>    local_bh_disable();
>    napi_consume_skb(skb, 1);
>    local_bh_enable();
> 
> If so, I wonder if it would be cleaner to have something like
>    build_skb_bh(buf, len);
> 
>    consume_skb_bh(skb, 1);
> 
> Then have those methods handle the local_bh enable/disable, so that
> the toggle was a property of a call, not a requirement of the call? 

Having budget = 0 would be for non-NAPI users. So passing the 1 is
superfluous. You goal seems to be to re-use napi_alloc_cache. Right? And
this is better than skb_pool?

There is already napi_alloc_skb() which expects BH to be disabled and
netdev_alloc_skb() (and friends) which do disable BH if needed. I don't
see an equivalent for non-NAPI users. Haven't checked if any of these
could replace your napi_build_skb().

Historically non-NAPI users would be IRQ users and those can't do
local_bh_disable(). Therefore there is dev_kfree_skb_irq_reason() for
them. You need to delay the free for two reasons.
It seems pure software implementations didn't bother so far.

It might make sense to do napi_consume_skb() similar to
__netdev_alloc_skb() so that also budget=0 users fill the pool if this
is really a benefit.

Sebastian