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

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Date:	Wed, 23 Jul 2014 13:23:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

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@...ux.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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