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

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  [PATCH v2] Documentation: kdump: add description of bringing up SMP dump-capture kernel Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com> - 2016-08-01 02:30 +0200
    Re: [PATCH v2] Documentation: kdump: add description of bringing up  SMP dump-capture kernel Baoquan He <bhe@redhat.com> - 2016-08-02 10:00 +0200
      Re: [PATCH v2] Documentation: kdump: add description of bringing  up SMP dump-capture kernel "Zhou, Wenjian/周文剑"   <zhouwj-fnst@cn.fujitsu.com> - 2016-08-02 11:10 +0200
        Re: [PATCH v2] Documentation: kdump: add description of bringing up  SMP dump-capture kernel Baoquan He <bhe@redhat.com> - 2016-08-02 16:30 +0200

#1452921 — [PATCH v2] Documentation: kdump: add description of bringing up SMP dump-capture kernel

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@cn.fujitsu.com>
---
 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

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#1453649 — 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@cn.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
> 
> 
> 

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#1453678 — 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@cn.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
>>
>>
>>
>
>

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#1454556 — 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@cn.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

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