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

Pranith Kumar
2014-07-23T20:16:17Z

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.
>

<resending as the assembly was garbled>

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

Size:
text data bss dec hex filename
before 30664 7844 32 38540 968c kernel/rcu/tree.o
after 30648 7844 32 38524 967c kernel/rcu/tree.o

Assembly:

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