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

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Date:	Sat, 02 Jun 2012 23:36:00 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Russell King <linux@....linux.org.uk>, yong.zhang0@...il.com,
	Jeremy Fitzhardinge <jeremy@...p.org>, peterz@...radead.org,
	mingo@...nel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	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 06/01/2012 09:06 PM, Jan Beulich wrote:

>>>> On 01.06.12 at 17:13, "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
> wrote:
>> On 06/01/2012 06:29 PM, Jan Beulich wrote:
>>
>>>>>> On 01.06.12 at 11:11, "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
>>> wrote:
>>>> xen_play_dead calls cpu_bringup() which looks weird, because xen_play_dead()
>>>> is invoked in the cpu down path, whereas cpu_bringup() (as the name 
>>>> suggests) is useful in the cpu bringup path.
>>>
>>> This might not be correct - the code as it is without this change is
>>> safe even when the vCPU gets onlined back later by an external
>>> entity (e.g. the Xen tool stack), and it would in that case resume
>>> at the return point of the VCPUOP_down hypercall. That might
>>> be a heritage from the original XenoLinux tree though, and be
>>> meaningless in pv-ops context - Jeremy, Konrad?
>>>
>>> Possibly it was bogus/unused even in that original tree - Keir?
>>>
>>
>>
>> Thanks for your comments Jan!
>>
>> In case this change is wrong, the other method I had in mind was to call
>> cpu_bringup_and_idle() in xen_play_dead(). (Even ARM does something similar,
>> in the sense that it runs the cpu bringup code including cpu_idle(), in the
>> cpu offline path, namely the cpu_die() function). Would that approach work
>> for xen as well? If yes, then we wouldn't have any issues to convert xen to
>> generic code.
> 
> No, that wouldn't work either afaict - the function is expected
> to return.
> 


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!

Regards,
Srivatsa S. Bhat

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