netdev - Re: [RFC net-next] tcp: allow larger TSO to be built under overload

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Date:   Tue, 8 Mar 2022 14:42:34 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [RFC net-next] tcp: allow larger TSO to be built under overload

On Tue, Mar 8, 2022 at 2:26 PM Eric Dumazet <edumazet@...gle.com> wrote:
>

>
> Thanks, I think I will make sure that we use the 32bit divide then,
> because compiler might not be smart enough to detect both operands are < ~0U

BTW, it seems the compiler (clang for me) is smart enough.

bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);
return max_t(u32, bytes / mss_now, min_tso_segs);

Compiler is using the divide by 32bit operation (div %ecx)

If you remove the min_t() clamping, and only keep:

return max_t(u32, bytes / mss_now, min_tso_segs);

Then clang makes a special case if bytes >= (1UL<<32)

    790d: 48 89 c2              mov    %rax,%rdx
    7910: 48 c1 ea 20          shr    $0x20,%rdx
    7914: 74 07                je     791d <tcp_tso_autosize+0x4d>
    7916: 31 d2                xor    %edx,%edx
    7918: 48 f7 f1              div    %rcx
  # More expensive divide
    791b: eb 04                jmp    7921 <tcp_tso_autosize+0x51>
    791d: 31 d2                xor    %edx,%edx
    791f: f7 f1                div    %ecx

    7921: 44 39 c0              cmp    %r8d,%eax
    7924: 44 0f 47 c0          cmova  %eax,%r8d
    7928: 44 89 c0              mov    %r8d,%eax
    792b: 5d                    pop    %rbp
    792c: c3                    ret