linux-kernel - Re: [PATCH 1/5] x86/smpboot: Add the missing description of possible

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Message-ID: <1dfcdb3d-6a92-8654-7cc4-ef262ddb0915@cn.fujitsu.com>
Date:   Wed, 21 Mar 2018 17:41:45 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <linux-acpi@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <corbet@....net>,
        <rjw@...ysocki.net>, <lenb@...nel.org>, <hpa@...or.com>
Subject: Re: [PATCH 1/5] x86/smpboot: Add the missing description of
 possible_cpus



At 03/21/2018 05:34 PM, Dou Liyang wrote:
> Hi Peter,
> 
> At 03/21/2018 05:08 PM, Peter Zijlstra wrote:
>> On Wed, Mar 21, 2018 at 01:33:24PM +0800, Dou Liyang wrote:
>>> How about:
>>>
>>> possible_cpus=    [s390,x86_64] Set the number of possible CPUs which
>>>         are determined by the ACPI tables MADT or mptables by
>>>         default. possible_cpus=n : n >= 1 enforces the possible
>>>         number to be 'n'.
>>>         While nr_cpus is also be set: nr_cpus=m, choice the
>>>         minimum one for the number of possible CPUs.
>>
>> So what is the exact difference between possible_cpus and nr_cpus ? I
> 
> the possible_cpus= can reset the number of possible CPUs, even bigger
> than 'num_processors + disabled_cpus', But nr_cpus= can't.
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       the maximum number kernel gets from ACPI/mptables, no matter what
number nr_cpus= is, the number of possible CPUs will not bigger than it.

>
>> konw maxcpus= limits the number of CPUs we bring up, and possible_cpus
>> limits the possible_map, but I'm not entirely sure what nr_cpus does
>> here.
>>
> 
> nr_cpus can limited the maximum CPUs that the kernel could support.
> 
> Here is a double check in case of using them at the same time, even if I
> think just using possible_cpus= is enough. :-)
> 
> Thanks,
>      dou.
>>
>>