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

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Date: Tue, 7 May 2024 15:27:44 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Toke Høiland-Jørgensen <toke@...hat.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] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On 07/05/2024 14.36, Sebastian Andrzej Siewior wrote:
> On 2024-05-06 16:09:47 [-0700], Alexei Starovoitov wrote:
>>>> 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...
>>>

When asking for change anyhow...

XDP redirect is an extreme fast-path.

Adding an WARN macro cause adding an 'ud2' instruction that cause CPU
instruction-cache to stop pre-fetching.

For this reason we in include/net/xdp.h have #define XDP_WARN and
function xdp_warn() that lives in net/core/xdp.c.
See how it is used in xdp_update_frame_from_buff().

Described in https://git.kernel.org/torvalds/c/34cc0b338a61
  - 34cc0b338a61 ("xdp: Xdp_frame add member frame_sz and handle in 
convert_to_xdp_frame")

>>>> +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?
>>
>> Indeed.
>> Let's drop all NULL checks, since they definitely add overhead.
>> I'd also remove ifdef CONFIG_PREEMPT_RT and converge on single implementation:
>> static inline struct bpf_net_context * bpf_net_ctx_get(void)
>> {
>>   return current->bpf_net_context;
>> }
> 
> Okay, let me do that then.

I need/want to echo Toke's request to benchmark these changes.

--Jesper