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

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Message-ID: <87y18mohhp.fsf@toke.dk>
Date: Mon, 06 May 2024 21:41:38 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc: "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>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, 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@...r.kernel.org
Subject: Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> 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.

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?

(not just here, but in the code below as well).

I'm a little concerned that we are introducing a bunch of new branches
in the XDP hot path. Which is also why I'm asking for benchmarks :)

[...]

> +	/* ri->map is assigned in __bpf_xdp_redirect_map() from within a eBPF
> +	 * program/ during NAPI callback. It is used during
> +	 * xdp_do_generic_redirect_map()/ __xdp_do_redirect_frame() from the
> +	 * redirect callback afterwards. ri->map is cleared after usage.
> +	 * The path has no explicit RCU read section but the local_bh_disable()
> +	 * is also a RCU read section which makes the complete softirq callback
> +	 * RCU protected. This in turn makes ri->map RCU protocted and it is

s/protocted/protected/

> +	 * sufficient to wait a grace period to ensure that no "ri->map == map"
> +	 * exist.  dev_map_free() removes the map from the list and then

s/exist/exists/

-Toke