linux-kernel - Re: [PATCH V2] mm: add a new command-line kmemcheck value

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Date:	Tue, 18 Feb 2014 11:28:06 +0100
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
Cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Vegard Nossum <vegardno@....uio.no>,
	Pekka Enberg <penberg@...nel.org>,
	Mel Gorman <mgorman@...e.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH V2] mm: add a new command-line kmemcheck value

On 17 February 2014 03:34, Xishi Qiu <qiuxishi@...wei.com> wrote:
> If we want to debug the kernel memory, we should turn on CONFIG_KMEMCHECK
> and rebuild the kernel. This always takes a long time and sometimes
> impossible, e.g. users don't have the kernel source code or the code
> is different from "www.kernel.org" (private features may be added to the
> kernel, and usually users can not get the whole code).
>
> This patch adds a new command-line "kmemcheck=3", then the kernel will run
> as the same as CONFIG_KMEMCHECK=off even CONFIG_KMEMCHECK is turn on.
> "kmemcheck=0/1/2" is the same as originally. This means we can always turn
> on CONFIG_KMEMCHECK, and use "kmemcheck=3" to control it on/off with out
> rebuild the kernel.
>
> In another word, "kmemcheck=3" is equivalent:
> 1) turn off CONFIG_KMEMCHECK
> 2) rebuild the kernel
> 3) reboot
>
> The different between kmemcheck=0 and 3 is the used memory and nr_cpus.
> Also kmemcheck=0 can used in runtime, and kmemcheck=3 is only used in boot.
> boottime: kmemcheck=0/1/2/3 (command-line)
> runtime: kmemcheck=0/1/2 (/proc/sys/kernel/kmemcheck)

This is not the right way to do what you want.

The behaviour that we want is:

 - CONFIG_KMEMCHECK=y + kmemcheck=0 (boot parameter) should have a
minimal runtime impact and not limit the number of CPUs
 - CONFIG_KMEMCHECK=y + kmemcheck=1 should limit the number of CPUs during boot
 - setting kmemcheck to 1 via /proc/sys/kernel/kmemcheck should
probably return an error if more than 1 CPU is online


Vegard
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