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Message-ID: <93062ce7-8dfa-48a9-a4ad-24c5a3993b41@kernel.org>
Date: Tue, 7 May 2024 15:27:44 +0200
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Subject: Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
 Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: =?UTF-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>,
 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>
References: <20240503182957.1042122-1-bigeasy@linutronix.de>
 <20240503182957.1042122-15-bigeasy@linutronix.de> <87y18mohhp.fsf@toke.dk>
 <CAADnVQJkiwaYXUo+LyKoV96VFFCFL0VY5Jgpuv_0oypksrnciA@mail.gmail.com>
 <20240507123636.cTnT7TvU@linutronix.de>
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From: Jesper Dangaard Brouer <hawk@kernel.org>
In-Reply-To: <20240507123636.cTnT7TvU@linutronix.de>
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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