linux-kernel - Re: [PATCH] Resurect proper handling of maxcpus= kernel option

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Message-ID: <20080811181642.GS4524@elte.hu>
Date:	Mon, 11 Aug 2008 20:16:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Max.Krasnyansky@...lcomm.com
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Max Krasnyansky <maxk@...lcomm.com>, lizf@...fujitsu.com,
	jeff.chua.linux@...il.com, Glauber Costa <gcosta@...hat.com>
Subject: Re: [PATCH] Resurect proper handling of maxcpus= kernel option


* Max.Krasnyansky@...lcomm.com <Max.Krasnyansky@...lcomm.com> wrote:

> From: Max Krasnyansky <maxk@...lcomm.com>
> 
> For some reason we had redundant parsers registered for maxcpus=. One 
> in init/main.c and another in arch/x86/smpboot.c So I nuked the one in 
> arch/x86.
> 
> Also 64-bit kernels used to handle maxcpus= as documented in 
> Documentation/cpu-hotplug.txt. CPUs with 'id > maxcpus' are 
> initialized but not booted. 32-bit version for some reason ignored 
> them even though all the infrastructure for booting them later is 
> there.
> 
> In the current mainline both 64 and 32 bit versions are broken. I'm 
> too lazy to look through git history but I'm guessing it happened as 
> part of the i386 and x86_64 unification.

yes in essence. 32-bit always had maxcpus as a hard restriction in the 
number of CPUs. This got extended to 64-bit as well, via commit 
89b08200ad:

    x86: make x86_64 accept the max_cpus parameter

in v2.6.25. Two major kernel releases and nobody noticed - it's a rarely 
used option.

> This patch restores the correct behaviour. I've tested x86_64 version 
> on 4- and 8- way Core2 and 2-way Opteron based machines. Various 
> config combinations SMP, !SMP, CPU_HOTPLUG, !CPU_HOTPLUG. Booted with 
> maxcpus=1 and maxcpus=4, etc. Everything is working as expected.
> 
> I cannot test 32-bit version (no 32-bit machines here).

This will need some test time on 32-bit as that is where this represents 
a material change. ( albeit what matters most is the maxcpus=1 
distinction - and for that nosmp can be used as well to turn off 
multi-cpu support altogether. So we could do this in v2.6.27 as well. )

Also, a quick grep shows that your patch will very likely break the 
visws build:

 arch/x86/kernel/visws_quirks.c:extern unsigned int __cpuinitdata maxcpus;
 arch/x86/kernel/visws_quirks.c:        if (ncpus > maxcpus)
 arch/x86/kernel/visws_quirks.c:                ncpus = maxcpus;

could you please fix that?

	Ingo
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