linux-kernel - Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Message-ID: <20240507123636.cTnT7TvU@linutronix.de>
Date: Tue, 7 May 2024 14:36:36 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Yonghong Song <yonghong.song@...ux.dev>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On 2024-05-06 16:09:47 [-0700], Alexei Starovoitov wrote:
> > > On PREEMPT_RT the pointer to bpf_net_context is saved task's
> > > task_struct. On non-PREEMPT_RT builds the pointer saved in a per-CPU
> > > variable (which is always NODE-local memory). Using always the
> > > bpf_net_context approach has the advantage that there is almost zero
> > > differences between PREEMPT_RT and non-PREEMPT_RT builds.
> >
> > Did you ever manage to get any performance data to see if this has an
> > impact?
> >
> > [...]
> >
> > > +static inline struct bpf_net_context *bpf_net_ctx_get(void)
> > > +{
> > > +     struct bpf_net_context *bpf_net_ctx = this_cpu_read(bpf_net_context);
> > > +
> > > +     WARN_ON_ONCE(!bpf_net_ctx);
> >
> > If we have this WARN...
> >
> > > +static inline struct bpf_redirect_info *bpf_net_ctx_get_ri(void)
> > > +{
> > > +     struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
> > > +
> > > +     if (!bpf_net_ctx)
> > > +             return NULL;
> >
> > ... do we really need all the NULL checks?
> 
> Indeed.
> Let's drop all NULL checks, since they definitely add overhead.
> I'd also remove ifdef CONFIG_PREEMPT_RT and converge on single implementation:
> static inline struct bpf_net_context * bpf_net_ctx_get(void)
> {
>  return current->bpf_net_context;
> }

Okay, let me do that then.

Sebastian