linux-kernel - Re: [PATCH v3 net-next 14/15] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Date: Wed, 29 May 2024 14:09:52 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: 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>, Toke Høiland-Jørgensen <toke@...hat.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v3 net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On Wed, May 29, 2024 at 9:29 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> The XDP redirect process is two staged:
> - bpf_prog_run_xdp() is invoked to run a eBPF program which inspects the
>   packet and makes decisions. While doing that, the per-CPU variable
>   bpf_redirect_info is used.
>
> - Afterwards xdp_do_redirect() is invoked and accesses bpf_redirect_info
>   and it may also access other per-CPU variables like xskmap_flush_list.
>
> At the very end of the NAPI callback, xdp_do_flush() is invoked which
> does not access bpf_redirect_info but will touch the individual per-CPU
> lists.
>
> The per-CPU variables are only used in the NAPI callback hence disabling
> bottom halves is the only protection mechanism. Users from preemptible
> context (like cpu_map_kthread_run()) explicitly disable bottom halves
> for protections reasons.
> Without locking in local_bh_disable() on PREEMPT_RT this data structure
> requires explicit locking.
>
> PREEMPT_RT has forced-threaded interrupts enabled and every
> NAPI-callback runs in a thread. If each thread has its own data
> structure then locking can be avoided.
>
> Create a struct bpf_net_context which contains struct bpf_redirect_info.
> Define the variable on stack, use bpf_net_ctx_set() to save a pointer to
> it. Use the __free() annotation to automatically reset the pointer once
> function returns.
> The bpf_net_ctx_set() may nest. For instance a function can be used from
> within NET_RX_SOFTIRQ/ net_rx_action which uses bpf_net_ctx_set() and
> NET_TX_SOFTIRQ which does not. Therefore only the first invocations
> updates the pointer.
> Use bpf_net_ctx_get_ri() as a wrapper to retrieve the current struct
> bpf_redirect_info.
>
> 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.
>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Eduard Zingerman <eddyz87@...il.com>
> Cc: Hao Luo <haoluo@...gle.com>
> Cc: Jesper Dangaard Brouer <hawk@...nel.org>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: KP Singh <kpsingh@...nel.org>
> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> Cc: Song Liu <song@...nel.org>
> Cc: Stanislav Fomichev <sdf@...gle.com>
> Cc: Toke Høiland-Jørgensen <toke@...hat.com>
> Cc: Yonghong Song <yonghong.song@...ux.dev>
> Cc: bpf@...r.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

lgtm
Acked-by: Alexei Starovoitov <ast@...nel.org>