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

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Message-ID: <20160802074602.GA3663@x1.redhat.com>
Date:	Tue, 2 Aug 2016 15:46:02 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Zhou Wenjian <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

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.

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
> 
> 
>