linux-kernel - Re: [PATCH] acpi, cpuidle: Register with cpuidle even if cpu is onlined after boot (beyond maxcpus)

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Message-ID: <4FE987BB.4020508@linux.vnet.ibm.com>
Date:	Tue, 26 Jun 2012 15:28:19 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	Thomas Renninger <trenn@...e.de>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	linux-acpi@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	lenb@...nel.org, "Rafael J. Wysocki" <rjw@...k.pl>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] acpi, cpuidle: Register with cpuidle even if cpu is onlined
 after boot (beyond maxcpus)

On 06/26/2012 03:11 PM, Daniel Lezcano wrote:
> On 06/26/2012 11:29 AM, Thomas Renninger wrote:
>> On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote:
>>> On 06/25/2012 07:23 PM, Thomas Renninger wrote:
>>>
>>>> On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote:
>>>>>
>>>>> Daniel Lezcano noticed that after booting with maxcpus=X, if we online the
>>>>> remaining cpus by writing: echo 1 > /sys/devices/system/cpu/cpuY/online, then
>>>>> for the newly onlined cpus, the cpuidle directory is not found under
>>>>> /sys/devices/system/cpu/cpuY.
>>>>>
>>>>> Partly, the reason for this is that acpi restricts the initialization to cpus
>>>>> within the maxcpus limit. (See commit 75cbfb9 "ACPI: Do not try to set up acpi
>>>>> processor stuff on cores exceeding maxcpus="). The maxcpus= kernel parameter is
>>>>> used to restrict the number of cpus brought up during boot. That doesn't mean
>>>>> that we should hard restrict the bring up of the remaining cpus later on.
>>>>
>>>> Sorry, but IMO it exaclty does mean that (adding more general lists for
>>>> further comments).
>>>>
>>>> If you can online more cores than maxcpus= via sysfs, this sounds like a bug.
>>>> Not the other way around.
>>>>
>>>> Compare with Documentation/kernel-parameters.txt:
>>>>         maxcpus=        [SMP] Maximum number of processors that an SMP kernel
>>>>                         should make use of.  maxcpus=n : n >= 0 limits the
>>>>                         kernel to using 'n' processors.  n=0 is a special case,
>>>>                         it is equivalent to "nosmp", which also disables
>>>>                         the IO APIC.
>>>>
>>>> Chances that you run into more problems are high.
>>>
>>>
>>> Right, I agree on that. So, IMHO, maxcpus=X doesn't mean that the kernel must and
>>> should forbid any new cpus from coming online, but in the interest of avoiding
>>> problems/complications in some obscure paths, I guess it makes sense to avoid
>>> onlining new cpus beyond maxcpus.
>>
>> Yep, for such reasons:
>>    - That nobody realizes this to be useful and makes use of it in a productive
>>      environment
>>    - If I see maxcpus=X in a bugreport's dmesg command line,
>>      I want to be sure that's true.
>>    - To enforce that things work as documented
>>
>>
>> Wow, after looking a bit into this I found (Documentation/cpu-hotplug.txt):
>>
>> maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
>>              maxcpus=2 will only boot 2. You can choose to bring the
>>              other cpus later online, read FAQ's for more info.
>>
>> Looks like someone already documented this (IMO broken) behavior.
>> I didn't find further info in the FAQs.
>>
>>> In any case, I was just trying to see why the simple removal of the setup_max_cpus
>>> check in acpi_processor_add() wasn't enough to expose the cpuidle directories under
>>> the new cpus.. and while debugging that, I came up with this patch. I don't mind
>>> if this doesn't get picked up.
>>
>>> Right, the usecase of why somebody would like to online new cpus beyond maxcpus
>>> doesn't look all that solid anyway. So I am OK with leaving the code as it is now.
>>
>> In the end this is a debug option, I expect everybody is aware of that.
>> Yep, let's just leave it...
> 
> In this case, let's remove the intel_idle_cpu_init stuff in
> acpi_cpu_soft_notify, no ?
> 

Why? And how would that help? The intel_idle_cpu_init() call is essential if intel_idle
driver is being used instead of acpi idle.

Regards,
Srivatsa S. Bhat

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