linux-kernel - Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline

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Message-ID: <4EDE2388.7080806@linux.vnet.ibm.com>
Date:	Tue, 06 Dec 2011 19:45:36 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Fenghua Yu <fenghua.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Len Brown <len.brown@...el.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline

On 12/06/2011 02:12 PM, Ingo Molnar wrote:

> 
> * Fenghua Yu <fenghua.yu@...el.com> wrote:
> 
>> From: Fenghua Yu <fenghua.yu@...el.com>
>>
>> BSP or CPU0 has been the last obstacle to CPU hotplug on x86. 
>> This patch set implements BSP online and offline and removes 
>> this obstacle to CPU hotplug.

[snip]
> 

> Also, could you please enumerate all limitations that could 
> possibly happen? The documentation has this list right now:
> 
> +1. Resume from hibernate/suspend depends on BSP. Hibernate/suspend will fail if
> +BSP is offline and you need to online BSP before hibernate/suspend can continue.
> 
> This needs to be fixed on some other fashion than warning people 
> in documentation that it would break.
> 


Actually, this patchset does more than just warn people. It has
checks to see if the CPU0 is offline, and if so, it fails the
suspend/hibernate attempt. See patch 7/7
(x86/power/cpu.c: Don't hibernate/suspend if CPU0 is offline)

> Firstly, at minimum a suspend/hibernate attempt should fail in 
> some deterministic fashion.


It does, as mentioned above. In fact, this patchset does it
proactively: whether the hardware/firmware supports suspend/resume
with BSP offlined or not, it just prevents anybody from doing
suspend/hibernate when the boot CPU is offline.

I am not saying that this is the *right* way to do it; I am just
pointing out that this patchset _does_ handle it.

> 
> Secondly, and more importantly, is there *any* hardware in 
> existence that has a BIOS that can suspend/resume successfully 
> with BSP offlined? If such hardware exists then we need to 
> support it properly - initially perhaps by whitelisting such 
> systems.
> 
> Then if demand for this picks up some more intelligent method of 
> cooperating with the firmware could be added: the firmware could 
> actually signal to us whether it supports suspend/resume from 
> other than the boot CPU.
> 

 
Regards,
Srivatsa S. Bhat

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