From: Jes Sorensen <Jes.Sorensen@redhat.com>
To: yalin wang <yalin.wang2010@gmail.com>
Cc: Neil Horman <nhorman@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Kershner <david.kershner@unisys.com>, tj@kernel.org,
        laijs@cn.fujitsu.com, nacc@linux.vnet.ibm.com, mingo@redhat.com,
        open list <linux-kernel@vger.kernel.org>, sparmaintainer@unisys.com
Subject: Re: [PATCH v2] kthread: Export kthread functions
References: <1437777920-31156-1-git-send-email-david.kershner@unisys.com>
	<1438099141-8614-1-git-send-email-david.kershner@unisys.com>
	<20150728142748.a756d7540ad5cdaf4c9efc9e@linux-foundation.org>
	<alpine.DEB.2.11.1507291233470.3825@nanos>
	<BC1DBE23-DF3E-49C1-BE59-A81EC98F5B1A@gmail.com>
	<20150730120223.GA27430@hmsreliant.think-freely.org>
	<FC536F44-E3A4-45E0-9B21-4C1CC256F94C@gmail.com>
	<alpine.DEB.2.11.1507311019040.3825@nanos>
	<alpine.DEB.2.11.1507311610180.3825@nanos>
	<6440C9D8-37D8-4E30-89D7-04CB96BC8114@gmail.com>
	<20150801133221.GA6603@neilslaptop.think-freely.org>
	<D33E7A95-05FC-423E-94FE-012949566988@gmail.com>
Date: Sun, 02 Aug 2015 22:42:27 -0400
In-Reply-To: <D33E7A95-05FC-423E-94FE-012949566988@gmail.com> (yalin wang's
	message of "Mon, 3 Aug 2015 10:23:56 +0800")
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yalin wang <yalin.wang2010@gmail.com> writes:
>> On Aug 1, 2015, at 21:32, Neil Horman <nhorman@redhat.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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