Messages in this thread | | | Date | Wed, 23 Jul 2014 13:23:20 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 09/16] rcu: Remove redundant check for online cpu |
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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 >
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