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

Paul E. McKenney
2014-07-23T20:23:27Z

On Wed, Jul 23, 2014 at 04:16:11PM -0400, Pranith Kumar wrote:
> 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 :-)

If you got this kind of savings in __rcu_read_lock() or
__rcu_read_unlock(), I might be interested. Hard to get excited about
__call_rcu_core(), especially given that a smarter compiler might be
able to make this transformation on its own.

Thanx, Paul

> 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
>