linux-kernel - Re: <PING> Re: [patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time

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Date:	Sun, 5 Oct 2008 16:52:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Chuck Ebbert <cebbert@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: <PING> Re: [patch x86/core] x86: allow number of additional
 hotplug CPUs to be set at compile time

On Sun, 5 Oct 2008, Ingo Molnar wrote:
> * Chuck Ebbert <cebbert@...hat.com> wrote:
> 
> > Yes, it works and I don't see how it could cause any problems.
> > 
> > Ingo, can we get this in 2.6.27? You can drop my original patch.
> > 
> > Tested-by: Chuck Ebbert <cebbert@...hat.com>
> 
> looks good, applied to tip/x86/core, thanks!

No, this patch is horrible. 

The correct check is num_present_cpus(). There is no need to make the
weird additional_cpus hackery globally available.

Btw, additional_cpus has interesting properties. Providing a negative
number < -1 on the kernel command line - happened due to a typo -
explodes in early boot, which is not really surprising, but should be
sanity checked.

Thanks,

	tglx
---------------->
Subject: x86: make UP alternatives switch depend on present cpus
From: Thomas Gleixner <tglx@...utronix.de>
Date: Sun, 05 Oct 2008 16:45:22 +0200

num_possible_cpus() can be > 1 when disabled CPUs have been accounted.

Disabled CPUs are not in the cpu_present_map, so we can use
num_present_cpus() as a safe indicator to switch to UP alternatives.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index fb04e49..a84ac7b 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -444,7 +444,7 @@ void __init alternative_instructions(void)
 					    _text, _etext);
 
 		/* Only switch to UP mode if we don't immediately boot others */
-		if (num_possible_cpus() == 1 || setup_max_cpus <= 1)
+		if (num_present_cpus() == 1 || setup_max_cpus <= 1)
 			alternatives_smp_switch(0);
 	}
 #endif
--
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