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

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Message-ID: <20160802140955.GB3663@x1.redhat.com>
Date:	Tue, 2 Aug 2016 22:09:55 +0800
From:	Baoquan He <bhe@...hat.com>
To:	"Zhou, Wenjian/周文剑" 
	<zhouwj-fnst@...fujitsu.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

On 08/02/16 at 04:49pm, "Zhou, Wenjian/周文剑" wrote:
> 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?

Putting it in kdump.txt could be better. nr_cpus/maxcpus are normal
kernel options, the reason we mentioned them here is crashkernel memory
is usually limited, we have to try our best to save memory. So if
nr_cpus is available on some ARCHes, we should use it. On x86 nr_cpus=1
has been taken for kdump kernel for a very long time. I think letting
people know below things would be good:

~~~~~~~~~~
Firslty both maxcpus and nr_cpus can be used for kdump kernel to specify
number of allowed cpu.

maxcpus only specify the number of available cpu, while nr_cpus will
specify the number of possible cpu which is necessary for hotplug.
nr_cpus can be used to limit the amount of percpu memory pre-allocation.
And now you just test parallel dumping feature on x86, or more
specifically on x86_64.
~~~~~~~~~~~

Thanks
Baoquan