linux-kernel - Re: [PATCH bpf-next v2 1/5] bpf: make MAX_BPF_FUNC

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Message-ID: <CAADnVQL8F23zxfYBacD9mFt_2uWRXN8Cno3tZGce4W3QC8iSew@mail.gmail.com>
Date:   Fri, 2 Jun 2023 11:17:29 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        David Ahern <dsahern@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        benbjiang@...cent.com, Ilya Leoshkevich <iii@...ux.ibm.com>,
        Menglong Dong <imagedong@...cent.com>,
        Xu Kuohai <xukuohai@...wei.com>,
        Manu Bretelle <chantr4@...il.com>,
        Ross Zwisler <zwisler@...gle.com>, Eddy Z <eddyz87@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 1/5] bpf: make MAX_BPF_FUNC_ARGS 14

On Fri, Jun 2, 2023 at 12:01 AM <menglong8.dong@...il.com> wrote:
>
> From: Menglong Dong <imagedong@...cent.com>
>
> According to the current kernel version, below is a statistics of the
> function arguments count:
>
> argument count | FUNC_PROTO count
> 7              | 367
> 8              | 196
> 9              | 71
> 10             | 43
> 11             | 22
> 12             | 10
> 13             | 15
> 14             | 4
> 15             | 0
> 16             | 1
>
> It's hard to statisics the function count, so I use FUNC_PROTO in the btf
> of vmlinux instead. The function with 16 arguments is ZSTD_buildCTable(),
> which I think can be ignored.
>
> Therefore, let's make the maximum of function arguments count 14. It used
> to be 12, but it seems that there is no harm to make it big enough.

I think we're just fine at 12.
People need to fix their code. ZSTD_buildCTable should be first in line.
Passing arguments on the stack is not efficient from performance pov.