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

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Date: Mon, 20 May 2024 18:52:30 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, 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 Wed, May 15, 2024 at 6:43 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> 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…

I think 112 bytes or whatever the size of bpf_net_context is a bit
too much to consume in task_struct.
Yes, it's big, but there are systems with 1m threads. 112Mbyte is not
that small.
bpf_net_ctx_set/get are not in critical path and get_ri will be
inlined without any conditionals, so performance should be the same.