linux-kernel - Re: [PATCH 0/3] smp: reduce stack requirements for smp_call_function

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Date:	Sat, 06 Sep 2008 11:12:32 -0700
From:	Mike Travis <travis@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, Jes Sorensen <jes@....com>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] smp: reduce stack requirements for	smp_call_function_mask

Ingo Molnar wrote:
> * Mike Travis <travis@....com> wrote:
> 
>>   * Cleanup cpumask_t usages in smp_call_function_mask function chain
>>     to prevent stack overflow problem when NR_CPUS=4096.
>>
>>   * Reduce the number of passed cpumask_t variables in the following
>>     call chain for x86_64:
>>
>> 	smp_call_function_mask -->
>> 	    arch_send_call_function_ipi->
>> 		    smp_ops.send_call_func_ipi -->
>> 			    genapic->send_IPI_mask
>>
>>     Since the smp_call_function_mask() is an EXPORTED function, we
>>     cannot change it's calling interface for a patch to 2.6.27.
>>
>>     The smp_ops.send_call_func_ipi interface is internal only and
>>     has two arch provided functions:
>>
>> 	arch/x86/kernel/smp.c:  .send_call_func_ipi = native_send_call_func_ipi
>> 	arch/x86/xen/smp.c:     .send_call_func_ipi = xen_smp_send_call_function_ipi
>> 	arch/x86/mach-voyager/voyager_smp.c:    (uses native_send_call_func_ipi)
>>
>>     Therefore modifying the internal interface to use a cpumask_t pointer
>>     is straight-forward.
>>
>>     The changes to genapic are much more extensive and are affected by the
>>     recent additions of the x2apic modes, so they will be done for 2.6.28 only.
>>
>> Based on 2.6.27-rc5-git6.
>>
>> Applies to linux-2.6.tip/master (with FUZZ).
> 
> applied to tip/cpus4096, thanks Mike.

Thanks Ingo!  Could you send me the git id for the merge?

> 
> I'm still wondering whether we should get rid of non-reference based 
> cpumask_t altogether ...

I've got a whole slew of "get-ready-to-remove-cpumask_t's" coming soon.
There are two phases, one completely within the x86 arch and the 2nd hits
the generic smp_call_function_mask ABI (won't be doable as a back-ported
patch to 2.6.27.)

> 
> Did you have a chance to look at the ftrace/stacktrace tracer in latest 
> tip/master, which will show the maximum stack footprint that can occur?

Hmm, no.  I'm using a default config right now as I can boot that pretty
easily.  I'll turn on the ftrace thing and check it out.

> 
> Also, i've applied the patch below as well to restore MAXSMP in a muted 
> form - with big warning signs added as well.

The main thing is to allow the distros to set it manually for their QA
testing of 2.6.27.  I'm sure I'll get back bugs because of just that.

(Is there a way to have them know to assign bugzilla's to me if NR_CPUS=4k
is the root of the problem?  This is an extremely serious issue for SGI
and I'd like to avoid any delays in me finding out about problems.)

Thanks again,
Mike

> 
> 	Ingo
> 
> -------------->
>>>From 363a5e3d7b4b69371f21bcafd7fc76e68c73733a Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@...e.hu>
> Date: Sat, 6 Sep 2008 15:24:52 +0200
> Subject: [PATCH] x86: add MAXSMP
> 
> restore MAXSMP, it's a nice debugging helper to trigger various crashes
> and problems with maximum sized x86 systems.
> 
> Make it depend on EXPERIMENTAL and DEBUG_KERNEL, and inform the user
> about the effects (stacksize, overhead, memory usage) of this flag.
> 
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/Kconfig |   11 ++++++++---
>  1 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ed97f2b..91212c1 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -580,10 +580,15 @@ config IOMMU_HELPER
>  
>  config MAXSMP
>  	bool "Configure Maximum number of SMP Processors and NUMA Nodes"
> -	depends on X86_64 && SMP && BROKEN
> -	default n
> +	depends on X86_64 && SMP && DEBUG_KERNEL && EXPERIMENTAL
>  	help
> -	  Configure maximum number of CPUS and NUMA Nodes for this architecture.
> +	  Configure maximum number of CPUS and NUMA Nodes for this
> +	  architecture (up to 4096!).
> +
> +	  This can increase memory usage, bigger stack footprint and can
> +	  add some runtime overhead as well so unless you want a generic
> +	  distro kernel you likely want to say N.
> +
>  	  If unsure, say N.
>  
>  config NR_CPUS

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