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

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Date:	Mon, 02 Mar 2009 16:08:26 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	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 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.

we may need to revisit apic_is_clustered_box()

/*
 * apic_is_clustered_box() -- Check if we can expect good TSC
 *
 * Thus far, the major user of this is IBM's Summit2 series:
 *
 * Clustered boxes may have unsynced TSC problems if they are
 * multi-chassis. Use available data to take a good guess.
 * If in doubt, go HPET.
 */
__cpuinit int apic_is_clustered_box(void)

....

        /*
         * If clusters > 2, then should be multi-chassis.
         * May have to revisit this when multi-core + hyperthreaded CPUs come
         * out, but AFAIK this will work even for them.
         */


with intel new cpus with 8 cores and HT enabled, even 2 sockets system will get 3 (?) so called "apic cluster"

we really need to use dmi etc way to detect multi-chassis system from now on

YH
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