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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Mon, 20 May 2024 18:52:30 -0700
Message-ID: <CAADnVQK53MOZiLnB-qnQG+ADWGcEMzT0Y0DDdxMKFz9t5n0U1A@mail.gmail.com>
Subject: Re: [PATCH net-next 14/15 v2] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>, 
	Jesper Dangaard Brouer <hawk@kernel.org>, LKML <linux-kernel@vger.kernel.org>, 
	Network Development <netdev@vger.kernel.org>, "David S. Miller" <davem@davemloft.net>, 
	Boqun Feng <boqun.feng@gmail.com>, Daniel Borkmann <daniel@iogearbox.net>, 
	Eric Dumazet <edumazet@google.com>, Frederic Weisbecker <frederic@kernel.org>, 
	Ingo Molnar <mingo@redhat.com>, Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, 
	Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>, 
	Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>, 
	Alexei Starovoitov <ast@kernel.org>, Andrii Nakryiko <andrii@kernel.org>, Eduard Zingerman <eddyz87@gmail.com>, 
	Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>, 
	John Fastabend <john.fastabend@gmail.com>, KP Singh <kpsingh@kernel.org>, 
	Martin KaFai Lau <martin.lau@linux.dev>, Song Liu <song@kernel.org>, 
	Stanislav Fomichev <sdf@google.com>, Yonghong Song <yonghong.song@linux.dev>, bpf <bpf@vger.kernel.org>
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On Wed, May 15, 2024 at 6:43=E2=80=AFAM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2024-05-14 13:54:43 [+0200], Toke H=C3=B8iland-J=C3=B8rgensen 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=E2=80=A6

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.