linux-kernel - Re: [Xen-devel] [PATCH 05/27] xen, cpu hotplug: Don't call cpu_bringup() in xen_play

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Date:	Tue, 05 Jun 2012 23:18:26 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jan Beulich <JBeulich@...e.com>,
	Russell King <linux@....linux.org.uk>, yong.zhang0@...il.com,
	Jeremy Fitzhardinge <jeremy@...p.org>, peterz@...radead.org,
	mingo@...nel.org, x86@...nel.org, akpm@...ux-foundation.org,
	nikunj@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	vatsa@...ux.vnet.ibm.com,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Ingo Molnar <mingo@...hat.com>, rusty@...tcorp.com.au,
	rjw@...k.pl, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, Keir Fraser <keir@....org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 05/27] xen, cpu hotplug: Don't call cpu_bringup()
 in xen_play_dead()

On 06/05/2012 11:10 PM, Thomas Gleixner wrote:

> On Sat, 2 Jun 2012, Srivatsa S. Bhat wrote:
>> Ok.. So, I would love to hear a confirmation about whether this patch (which
>> removes cpu_bringup() in xen_play_dead()) will break things or it is good as is.
>>
>> If its not correct, then we can probably make __cpu_post_online() return an int,
>> with the meaning:
>>
>> 0 => success, go ahead and call cpu_idle()
>> non-zero => stop here, thanks for your services so far.. now leave the rest to me.
>>
>> So all other archs will return 0, Xen will return non-zero, and it will handle
>> when to call cpu_idle() and when not to do so.
>>
>> Might sound a bit ugly, but I don't see much other option. Suggestions are
>> appreciated!
> 
> Yes, it's butt ugly. 
> 
> You are tripping over the main misconception of the current hotplug
> code: It's asymetric.
> 
> So people added warts and workarounds like the xen one. What you are
> proposing is another wart and workaround.
> 
> The real way to avoid it, is to have the symetric state machine in
> place first and then convert everything to that instead of introducing
> an intermediate state which resembles the existing state.
> 
> One of the main things we need to do to make it symetric is to kill
> the play_dead() thing in the idle loop and make idle a function which
> returns on cpu_should_die().
> 
> Give me a day or two and I get you a working version of that. (Up is
> functional, just down refuses to play along)
> 


Oh great! So, then I'll wait for your patches and then adapt this patchset
to your model then. Let me know if I can help out with something..

Regards,
Srivatsa S. Bhat

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