linux-kernel - Re: [PATCH] x86: don't compile vsmp

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Date:	Sun, 22 Mar 2009 13:48:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Ravikiran G Thirumalai <kiran@...lex86.org>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	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>,
	shai@...lex86.org
Subject: Re: [PATCH] x86: don't compile vsmp_64 for 32bit


* Ravikiran G Thirumalai <kiran@...lex86.org> wrote:

> On Sat, Feb 28, 2009 at 10:44:30AM +0100, Ingo Molnar wrote:
> >
> >* Ravikiran G Thirumalai <kiran@...lex86.org> wrote:
> >
> >> 
> >> True, but by how much? 212 bytes, out of 7285943 bytes which 
> >> is very very small percentage wise.
> >
> >How does this eliminate the validity of the patch?
> >
> 
> It costs 212 bytes to leave is_vsmp_box() to not just be a dummy 
> no-op. Having is_vsmp_box() detect if the hardware is indeed vSMP, 
> is meaningful even when CONFIG_VSMP is not turned on.  This is 
> because is_vsmp_box() is used to tell the kernel, that although 
> the cpus being used are supposed to have TSCs in sync, they are 
> not really in sync.  This is because you cannot ensure TSCs won't 
> drift between multiple boards being aggregated on vSMP systems. 
> Take the case of distro kernels.  Distro kernels typically do not 
> have CONFIG_X86_VSMP on.  Due to the large internode cacheline 
> setting, CONFIG_VSMP would not be on on the generic distro 
> installer kernels. If is_vsmp_box() is a no-op, the generic distro 
> installer kernels will assume TSCs to be synched, which is bad.  
> Hence, it will be nice if, for the cost of 212 bytes, vsmp64.o be 
> compiled either unconditionally, OR conditionally for 64bit 
> architectures only.  The question is, is 212 bytes out of 7285943 
> bytes too expensive for the generic kernels?  I hope not.

Sorry - got distracted and forgot about this thread. The TSC quirk 
indeed looks required for your systems - you dont have a reliable 
TSC due to virtualization, right?

Mind sending a patch (partial revert or so) against latest -tip that 
fixes that?

	Ingo
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