linux-kernel - Re: [PATCH net-next 14/15 v2] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Date: Wed, 15 May 2024 15:43:26 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>,
	Alexei Starovoitov <alexei.starovoitov@...il.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>,
	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 v2] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On 2024-05-14 13:54:43 [+0200], Toke Høiland-Jørgensen wrote:
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1504,6 +1505,8 @@ struct task_struct {
> >  	/* Used for BPF run context */
> >  	struct bpf_run_ctx		*bpf_ctx;
> >  #endif
> > +	/* Used by BPF for per-TASK xdp storage */
> > +	struct bpf_net_context		*bpf_net_context;
> 
> Okay, so if we are going the route of always putting this in 'current',
> why not just embed the whole struct bpf_net_context inside task_struct,
> instead of mucking about with the stack-allocated structures and
> setting/clearing of pointers?

The whole struct bpf_net_context has 112 bytes. task_struct has 12352
bytes in my debug-config or 7296 bytes with defconfig on x86-64. Adding
it unconditionally would grow task_struct by ~1% but it would make
things way easier: The NULL case goes away, the assignment and cleanup
goes away, the INIT_LIST_HEAD can be moved to fork(). If the size
increase is not an issue then why not. Let me prepare…

> -Toke

Sebastian