linux-kernel - Re: [RFT][patch 17/18] sched: use jump labels to reduce overhead when bandwidth control is inactive

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Message-ID: <20110805152411.GC2522@redhat.com>
Date:	Fri, 5 Aug 2011 11:24:12 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Richard Henderson <rth@...hat.com>, a.p.zijlstra@...llo.nl
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <bsingharora@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [RFT][patch 17/18] sched: use jump labels to reduce overhead
 when bandwidth control is inactive

On Fri, Aug 05, 2011 at 08:11:15AM -0700, Richard Henderson wrote:
> On 08/05/2011 01:30 AM, Peter Zijlstra wrote:
> > OK, so _WHY_ does that make a difference and will a next version of
> > gnu-binutils not mess that up?
> 
> The Why is micro-architectual, and I can't answer that.

In tracking this down, I eventually found that just having the
jump_label.o file compiled into the kernel, but not actually using the
static_branch(), or 'asm goto' anywhere, led to a performance hit.
Thus, the compiler or the 'asm goto' itself wasn't actually causing any
degradation. 

Since the jump_label.o file is only slow-path code, it can be moved away
from core or heavily called kernel routines. I suspect this is probably
an icache issue, but I can't say for sure.

Thanks,

-Jason

> 
> But ld will never re-order the files as given on the command-line.
> There are too many functions and tables that are constructed 
> piece-wise from input sections; re-ordering them would change
> the semantics of the program.
> 
> 
> r~
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