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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Mon, 9 May 2022 17:34:19 -0700
Message-ID: <CAEf4BzZD5q2j229_gL_nDFse2v=k2Ea0nfguH+sOA2O1Nm5sQw@mail.gmail.com>
Subject: Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu_elem for percpu map
To:     Feng zhou <zhoufeng.zf@bytedance.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        john fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@kernel.org>,
        Dave Marchevsky <davemarchevsky@fb.com>,
        Joanne Koong <joannekoong@fb.com>,
        Geliang Tang <geliang.tang@suse.com>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        duanxiongchun@bytedance.com,
        Muchun Song <songmuchun@bytedance.com>,
        Dongdong Wang <wangdongdong.6@bytedance.com>,
        Cong Wang <cong.wang@bytedance.com>,
        zhouchengming@bytedance.com
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On Fri, May 6, 2022 at 7:49 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>
> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>
> Trace some functions, such as enqueue_task_fair, need to access the
> corresponding cpu, not the current cpu, and bpf_map_lookup_elem percpu map
> cannot do it. So add bpf_map_lookup_percpu_elem to accomplish it for
> percpu_array_map, percpu_hash_map, lru_percpu_hash_map.
>
> The implementation method is relatively simple, refer to the implementation
> method of map_lookup_elem of percpu map, increase the parameters of cpu, and
> obtain it according to the specified cpu.
>

I don't think it's safe in general to access per-cpu data from another
CPU. I'd suggest just having either a ARRAY_OF_MAPS or adding CPU ID
as part of the key, if you need such a custom access pattern.

> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
> ---
>  include/linux/bpf.h            |  2 ++
>  include/uapi/linux/bpf.h       |  9 +++++++++
>  kernel/bpf/arraymap.c          | 15 +++++++++++++++
>  kernel/bpf/core.c              |  1 +
>  kernel/bpf/hashtab.c           | 32 ++++++++++++++++++++++++++++++++
>  kernel/bpf/helpers.c           | 18 ++++++++++++++++++
>  kernel/bpf/verifier.c          | 17 +++++++++++++++--
>  kernel/trace/bpf_trace.c       |  2 ++
>  tools/include/uapi/linux/bpf.h |  9 +++++++++
>  9 files changed, 103 insertions(+), 2 deletions(-)
>

[...]