netdev - Re: [PATCH net] net: lwtunnel: disable preemption when required

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Message-ID: <CAADnVQLiM5MA3Xyrkqmubku6751ZPrDk6v-HmC1jnOaL47=t+g@mail.gmail.com>
Date: Thu, 3 Apr 2025 13:35:10 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Justin Iurman <justin.iurman@...ege.be>, Sebastian Sewior <bigeasy@...utronix.de>
Cc: Stanislav Fomichev <stfomichev@...il.com>, Network Development <netdev@...r.kernel.org>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH net] net: lwtunnel: disable preemption when required

On Thu, Apr 3, 2025 at 12:08 PM Justin Iurman <justin.iurman@...ege.be> wrote:
>
> On 4/3/25 18:24, Stanislav Fomichev wrote:
> > On 04/03, Justin Iurman wrote:
> >> In lwtunnel_{input|output|xmit}(), dev_xmit_recursion() may be called in
> >> preemptible scope for PREEMPT kernels. This patch disables preemption
> >> before calling dev_xmit_recursion(). Preemption is re-enabled only at
> >> the end, since we must ensure the same CPU is used for both
> >> dev_xmit_recursion_inc() and dev_xmit_recursion_dec() (and any other
> >> recursion levels in some cases) in order to maintain valid per-cpu
> >> counters.
> >
> > Dummy question: CONFIG_PREEMPT_RT uses current->net_xmit.recursion to
> > track the recursion. Any reason not to do it in the generic PREEMPT case?
>
> I'd say PREEMPT_RT is a different beast. IMO, softirqs can be
> preempted/migrated in RT kernels, which is not true for non-RT kernels.
> Maybe RT kernels could use __this_cpu_* instead of "current" though, but
> it would be less trivial. For example, see commit ecefbc09e8ee ("net:
> softnet_data: Make xmit per task.") on why it makes sense to use
> "current" in RT kernels. I guess the opposite as you suggest (i.e.,
> non-RT kernels using "current") would be technically possible, but there
> must be a reason it is defined the way it is... so probably incorrect or
> inefficient?

Stating the obvious...
Sebastian did a lot of work removing preempt_disable from the networking
stack.
We're certainly not adding them back.
This patch is no go.