Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

[Pranith Kumar <bobby.prani@gmail.com>]

On Wed, Jul 23, 2014 at 3:15 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>> > If you change the "awake" to something like "am_online", I could get
>> > behind this one.
>>
>> OK! I will submit that in the next series(with the zalloc check).
>
> You caught me at a weak moment...  This change just adds an extra
> line of code and doesn't really help anything.
>
> So please leave this one out.
>

It adds an extra line of code and generates better assembly code. Last
try to convince you before I give up :-)

Before:

    if (!rcu_is_watching() && cpu_online(smp_processor_id()))
    26d3:    83 e2 01                 and    $0x1,%edx
    26d6:    75 1f                    jne    26f7 <__call_rcu+0x1c7>
    26d8:    65 8b 14 25 00 00 00     mov    %gs:0x0,%edx
    26df:    00
            26dc: R_X86_64_32S    cpu_number
    26e0:    48 8b 0d 00 00 00 00     mov    0x0(%rip),%rcx        #
26e7 <__call_rcu+0x1b7>
            26e3: R_X86_64_PC32    cpu_online_mask-0x4
    26e7:    89 d2                    mov    %edx,%edx
    26e9:    48 0f a3 11              bt     %rdx,(%rcx)
    26ed:    19 d2                    sbb    %edx,%edx
    26ef:    85 d2                    test   %edx,%edx
    26f1:    0f 85 29 02 00 00        jne    2920 <__call_rcu+0x3f0>
        invoke_rcu_core();

    /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
    if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
    26f7:    48 f7 45 d0 00 02 00     testq  $0x200,-0x30(%rbp)
    26fe:    00
    26ff:    0f 84 e6 fe ff ff        je     25eb <__call_rcu+0xbb>
    2705:    65 8b 14 25 00 00 00     mov    %gs:0x0,%edx
    270c:    00
            2709: R_X86_64_32S    cpu_number
    270d:    48 8b 0d 00 00 00 00     mov    0x0(%rip),%rcx        #
2714 <__call_rcu+0x1e4>
            2710: R_X86_64_PC32    cpu_online_mask-0x4
    2714:    89 d2                    mov    %edx,%edx
    2716:    48 0f a3 11              bt     %rdx,(%rcx)
    271a:    19 d2                    sbb    %edx,%edx
    271c:    85 d2                    test   %edx,%edx
    271e:    0f 84 c7 fe ff ff        je     25eb <__call_rcu+0xbb>

After:

    bool cpu_up = cpu_online(smp_processor_id());
    26c1:    65 8b 14 25 00 00 00     mov    %gs:0x0,%edx
    26c8:    00
            26c5: R_X86_64_32S    cpu_number
    26c9:    48 8b 0d 00 00 00 00     mov    0x0(%rip),%rcx        #
26d0 <__call_rcu+0x1a0>
            26cc: R_X86_64_PC32    cpu_online_mask-0x4
    26d0:    89 d2                    mov    %edx,%edx
    26d2:    48 0f a3 11              bt     %rdx,(%rcx)
    26d6:    19 d2                    sbb    %edx,%edx
    26d8:    85 d2                    test   %edx,%edx
    26da:    41 0f 95 c4              setne  %r12b

    if (!rcu_is_watching() && cpu_up)
    26f0:    83 e2 01                 and    $0x1,%edx
    26f3:    75 09                    jne    26fe <__call_rcu+0x1ce>
    26f5:    45 84 e4                 test   %r12b,%r12b
    26f8:    0f 85 12 02 00 00        jne    2910 <__call_rcu+0x3e0>
        invoke_rcu_core();

    /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
    if (irqs_disabled_flags(flags) || !cpu_up)
    26fe:    48 f7 45 d0 00 02 00     testq  $0x200,-0x30(%rbp)
    2705:    00
    2706:    0f 84 df fe ff ff        je     25eb <__call_rcu+0xbb>
    270c:    45 84 e4                 test   %r12b,%r12b
    270f:    0f 84 d6 fe ff ff        je     25eb <__call_rcu+0xbb>


-- 
Pranith