linux-kernel - Re: [PATCH -v2 5/5] x86: use dmi check to treat disabled cpus as hotplug cpus.

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Date:	Tue, 12 Jan 2010 18:21:08 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2 5/5] x86: use dmi check to treat disabled cpus as
 hotplug cpus.

On Tue, 2010-01-12 at 18:13 -0800, Yinghai Lu wrote:
> On 01/12/2010 06:06 PM, Suresh Siddha wrote:
> > On Tue, 2010-01-12 at 17:55 -0800, Yinghai Lu wrote:
> >> you mean this blacklist could be too long?
> > 
> > yes.
> > 
> >>
> >> switch to whitelist for the box can treat disabled cpus as hotplug
> >> cpus?
> > 
> > Do we really need all this? Systems which support physical hotplug
> > typically have lot's of cpu's (mostly will have atleast > 8). Even if we
> > really need to identify, then perhaps we can check if ACPI has any hints
> > regarding the physical hotplug support at boot time (we should check
> > with Andi Kleen or Len Brown on this).
> > 
> > For now, I think we can keep it simple by using the detected enabled
> > cpu's.
> 
> sth like this ?

Well, I was referring more to the apic model selection than the prefill
case. About this possible cpu's, will the wastage be more than few kb
for few disabled cpu's.

thanks,
suresh

> ---
>  arch/x86/kernel/smpboot.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-2.6/arch/x86/kernel/smpboot.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/smpboot.c
> +++ linux-2.6/arch/x86/kernel/smpboot.c
> @@ -1190,7 +1190,6 @@ early_param("possible_cpus", _setup_poss
>   * - Ashok Raj
>   *
>   * Three ways to find out the number of additional hotplug CPUs:
> - * - If the BIOS specified disabled CPUs in ACPI/mptables use that.
>   * - The user can overwrite it with possible_cpus=NUM
>   * - Otherwise don't reserve additional CPUs.
>   * We do this because additional CPUs waste a lot of memory.
> @@ -1205,7 +1204,7 @@ __init void prefill_possible_map(void)
>  		num_processors = 1;
>  
>  	if (setup_possible_cpus == -1)
> -		possible = num_processors + disabled_cpus;
> +		possible = num_processors;
>  	else
>  		possible = setup_possible_cpus;
>  

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