linux-kernel - RE: [PATCH 8/8] kernel/cpu.c: Define bsp

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Date:	Wed, 5 Oct 2011 13:25:11 -0700
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Ingo Molnar <mingo@...e.hu>, H Peter Anvin <hpa@...or.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Brown, Len" <len.brown@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 8/8] kernel/cpu.c: Define bsp_hotpluggable variable

> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Wednesday, October 05, 2011 12:25 PM
> To: Yu, Fenghua
> Cc: Ingo Molnar; H Peter Anvin; Zwane Mwaikambo; Luck, Tony; Mallick,
> Asit K; Siddha, Suresh B; Len Brown; linux-kernel
> Subject: Re: [PATCH 8/8] kernel/cpu.c: Define bsp_hotpluggable variable
> 
> On Wed, 5 Oct 2011, Fenghua Yu wrote:
> 
> > From: Fenghua Yu <fenghua.yu@...el.com>
> >
> > This gloable variable controls BSP (aka CPU0) hotplug. If set, BSP is
> > hotpluggable. By default, it's 0. On X86, kernel option
> bsp_hotpluggable sets
> > the variable as 1.
> 
> .... and on !x86 its just pointless.

I'll change the variable to an inline function which returns bsp_hotpluggable on x86 and is empty function on !x86.

> 
> I have yet to see a justification for that whole cpu0 unplugging
> business.
> 
> If there is a real reason that this is desireable, then ALL cpu0
> assumptions in arch/x86 need to be cleaned up and fixed.

I can think of two reasons for bsp offline/online:

1. RAS needs the feature. If socket0 needs to be hotplugged for any reason (any thread on socket0 is bad, shared cache issue, uncore issue, etc), CPU0 is required to be offlined.
2. CPU0 is symmetrical to other CPU's. There is no specific reason why it shouldn't be offlined except BIOS requirements.

Thanks.

-Fenghua
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