linux-kernel - Re: [RFC PATCH 2/4] x86: use dmi check to treat disabled cpus as hotplug cpus.

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Date:	Tue, 12 Jan 2010 12:17:21 -0500
From:	Valdis.Kletnieks@...edu
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	linux-kbuild@...r.kernel.org
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"ananth@...ibm.com" <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/4] x86: use dmi check to treat disabled cpus as hotplug cpus.

On Tue, 12 Jan 2010 08:26:22 PST, Linus Torvalds said:
> 
> 
> On Tue, 12 Jan 2010, Valdis.Kletnieks@...edu wrote:
> > 
> > OK, I'll bite - how do you build an X86-64 kernel that doesn't have
> > CONFIG_HOTPLUG_CPU selected?  Try as I might, even if I have PM_SLEEP=n,
> > PM_SLEEP_SMP insists on being set, and then selecting HOTPLUG_SMP.
> 
> If that is true, then there is some bug in the kconfig parser. 
> PM_SLEEP_SMP depends on PM_SLEEP, so with PM_SLEEP=n it should never be 
> set.

So now I go back and check, and it *was* possible to get a PM_SLEEP_SMP=n.
Apparently in my previous attempts, I tried turning stuff off and PM_SLEEP_SMP
stayed on just like this time as long as I was puttering around in menuconfig.
But turning it off, *exiting menuconfig*, and then re-starting menuconfig made
it work.  Weird.  It seems like if something does a 'select' on something
that isn't a visible symbol, and the symbol gets toggled, the selects
aren't redriven - and since it's not a visible symbol, you can't toggle it
yourself. But saving and restarting menuconfig forces a refresh and things
start acting right. Adding Roman and the kbuild list to the cc:

And turning off PM_SLEEP and HOTPLUG_CPU ended up saving a chunk of memory:

Before:
% size vmlinux
   text    data     bss     dec     hex filename
8964445 1377200 6094320 16435965         facafd vmlinux

After:
% size vmlinux
   text    data     bss     dec     hex filename
8889523 1378768 6089648 16357939         f99a33 vmlinux

Content of type "application/pgp-signature" skipped