linux-kernel - Re: [PATCH v2] kthread: Export kthread functions

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Date:	Mon, 3 Aug 2015 10:23:56 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Neil Horman <nhorman@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Kershner <david.kershner@...sys.com>, tj@...nel.org,
	laijs@...fujitsu.com, nacc@...ux.vnet.ibm.com, mingo@...hat.com,
	open list <linux-kernel@...r.kernel.org>,
	jes.sorensen@...hat.com, sparmaintainer@...sys.com
Subject: Re: [PATCH v2] kthread: Export kthread functions


> On Aug 1, 2015, at 21:32, Neil Horman <nhorman@...hat.com> wrote:
> 
> On Sat, Aug 01, 2015 at 03:12:42PM +0800, yalin wang wrote:
>> 
>>> 在 2015年7月31日，22:14，Thomas Gleixner <tglx@...utronix.de> 写道：
>>> 
>>> 
>>> On Fri, 31 Jul 2015, Thomas Gleixner wrote:
>>> 
>>>> On Fri, 31 Jul 2015, yalin wang wrote:
>>>>> it is optimised to 2 instructions ,
>>>>> 
>>>>> this is my patch, hope can be merged :
>>>> 
>>>> We are not exposing the internals of kthread management. Period.
>>> 
>>> And your 'optimization' is completely bogus:
>>> 
>>> Before your modification:
>>> 
>>> size kernel/built-in.o
>>> 
>>>  text	   data	    bss	    dec	    hex	filename
>>> 1091514	 141498  341928 1574940  18081c	../build/kernel/built-in.o
>>> 
>>> After:
>>> 
>>>  text	   data	    bss	    dec	    hex	filename
>>> 1091664  141498  341928 1575090  1808b2	../build/kernel/built-in.o
>>> 
>>> That's an increase of text size by 150 byte. Interesting optimization.
>>> 
>>> Thanks,
>>> 
>>> 	tglx
>>> 
>>> 
>> strange,  this is my test result:
>> 
>> size   built-in.o*
>>  text	   data	    bss	    dec	    hex	filename
>> 743937	  50786	  56008	 850731	  cfb2b	built-in.o        // with the patch
>> 744069	  50786	  56008	 850863	  cfbaf	built-in.o_old  // with out the patch
>> 
> So you're willing to expose the internals of kthread_park in exchange for the
> hope of saving 132 bytes of text.
> 
> Thats just dumb.  I agree with tglx, this shouldn't change.
> 
> Neil
not just size, mainly for performance,
without inline:

ffffffc0000d26b0:       97fff4aa        bl      ffffffc0000cf958 <kthread_should_park>
ffffffc0000d26b4:       53001c00        uxtb    w0, w0

if kthread_should_park() inline:
ffffffc0000d1a44:       f85c8020        ldr     x0, [x1,#-56]    									// kthread_should_park line
ffffffc0000d1a48:       36100300        tbz     w0, #2, ffffffc0000d1aa8 <smpboot_thread_fn+0xbc>   // kthread_should_park  line

still use 2 instructions, but don’t need a function call,
maybe can do more optimisation by gcc sometimes .
Anyway, this is just a suggest,
it is up to you apply it or not. :) 

Thanks











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