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

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Date: Wed, 5 Jun 2024 13:55:07 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
 linux-kernel@...r.kernel.org, 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@...r.kernel.org
Subject: Re: [PATCH v3 net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On 05/06/2024 12.41, Sebastian Andrzej Siewior wrote:
> On 2024-06-05 12:28:08 [+0200], Jesper Dangaard Brouer wrote:
>>
>> Hmm, but how will this affect performance?
> 
> As I wrote in the changelog for v4, I haven't notice a difference. I
> tried to move bpf_net_ctx_set() from cpu_map_bpf_prog_run() to
> cpu_map_kthread_run() to have this assignment only once and I didn't see
> a difference/ I couldn't tell the two kernels apart.
> 

This would be my preferred solution.
See below, your benchmark wasn't testing/measuring this changed code on
remote CPU running kthread.

> This is what I have been using for testing
> 
> | xdp-bench redirect-cpu --cpu 3 --remote-action drop eth1 -e
> 
> in case I was changing the wrong part…

As we saw earlier (with your hardware setup) this test is benchmarking
the RX-NAPI XDP-redirect code.  As the cpumap "remote" CPUs kthread had
idle cycles.

The extra clearing bpf_net_ctx_set() for each packet in the kthread on
the remote CPU will not change the benchmark numbers (as it have idle
cycles).

Looking closer at kernel code + your patch, I see that this clearing
isn't done for each packet, but per bulk (up-to CPUMAP_BATCH 8).  Given
that, I'm more okay with this change.

--Jesper