linux-kernel - Re: [PATCH v2] Documentation: kdump: add description of bringing up SMP dump-capture kernel

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Message-ID: <57A05E7D.90101@cn.fujitsu.com>
Date:	Tue, 2 Aug 2016 16:49:01 +0800
From:	"Zhou, Wenjian/周文剑" 
	<zhouwj-fnst@...fujitsu.com>
To:	Baoquan He <bhe@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <kexec@...ts.infradead.org>,
	<dyoung@...hat.com>, <d.hatayama@...fujitsu.com>
Subject: Re: [PATCH v2] Documentation: kdump: add description of bringing
 up SMP dump-capture kernel

Hi Baoquan,

On 08/02/2016 03:46 PM, Baoquan He wrote:
> Hi Wenjian,
>
> On 08/01/16 at 08:23am, Zhou Wenjian wrote:
>> v1->v2: change nr_cpus to maxcpus
>>
>> SMP dump-capture kernel is useful to improve the performance of kdump in
>> some cases. So add the description of bringing up SMP dump-capture kernel.
>>
>> Signed-off-by: Zhou Wenjian <zhouwj-fnst@...fujitsu.com>
>
> Discussed with people, it could be better to adjust the
> description about nr_cpus and maxcpus part. I think you can still
> describe nr_cpus/maxcpus in patch 1/2, and keep parallel dumping part in
> 2/2.
>
> Originally maxcpus=1 is used for all ARCHes. Later people found
> nr_cpus=1 is better since nr_cpus decides the number of possible cpu
> while maxcpus decides the max working cpu after system boot. So nr_cpus
> can save memory because percpu will pre-allocate memory for each
> possible cpu for hotplug. So on x86 nr_cpus is used because much memory
> can be saved if possible cpu number is very large.
>
> So you can mention that both maxcpus and nr_cpus can be used but nr_cpus
> has advantage if it has been implemented in some ARCHes like x86_64. And
> I guess you mush have tested parallel dumping feature with nr_cpus
> specified, it makes sense to tell people with the real situation.
>

I think it is better to describe the difference in somewhere else.
Maybe, it's a good choice which just replace maxcpus by maxcpus/nr_cpus.
Then user can choose maxcpus or nr_cpus.
What do you think about it?

-- 
Thanks
Zhou

> Thanks
> Baoquan
>
>> ---
>>   Documentation/kdump/kdump.txt | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
>> index 88ff63d..c5762b7 100644
>> --- a/Documentation/kdump/kdump.txt
>> +++ b/Documentation/kdump/kdump.txt
>> @@ -394,6 +394,12 @@ Notes on loading the dump-capture kernel:
>>     dump. Hence generally it is useful either to build a UP dump-capture
>>     kernel or specify maxcpus=1 option while loading dump-capture kernel.
>>
>> +* We should bring up a SMP dump-capture kernel if we intend to use multi-threads
>> +  programs with it, such as parallel dump feature of makedumpfile. Otherwise,
>> +  the multi-threads program may have a great performance degradation.
>> +  To bring up a SMP dump-capture kernel, we should specify maxcpus=[X] and
>> +  disable_cpu_apic=[Y] options while loading it.
>> +
>>   * For s390x there are two kdump modes: If a ELF header is specified with
>>     the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
>>     is done on all other architectures. If no elfcorehdr= kernel parameter is
>> --
>> 1.8.3.1
>>
>>
>>
>
>