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

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Message-ID: <wrfjwpxdf5yk.fsf@carbonite.lan.trained-monkey.org>
Date:	Sun, 02 Aug 2015 22:42:27 -0400
From:	Jes Sorensen <Jes.Sorensen@...hat.com>
To:	yalin wang <yalin.wang2010@...il.com>
Cc:	Neil Horman <nhorman@...hat.com>,
	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>,
	sparmaintainer@...sys.com
Subject: Re: [PATCH v2] kthread: Export kthread functions

yalin wang <yalin.wang2010@...il.com> writes:
>> On Aug 1, 2015, at 21:32, Neil Horman <nhorman@...hat.com> wrote:
>>> 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. :) 

kthread_park() isn't exactly a performance critical function call.
Saving two instructions does not outway the cost of exposing the
internals of the kthread API.

Jes
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