[PATCH v10 0/3] Documentation: add description of enable multi-cpus support for kdump

[PATCH v10 0/3] Documentation: add description of enable multi-cpus support for kdump

[Baoquan He]

This is v10 post. In this patchset patch 1/3 is added to give more details
about nr_cpus and maxcpus in kernel-parameters.txt. This is suggested by
Jonathan since the description of them is unclear so that people can't see
what's the exact difference between them. Otherwise no big further change
for 2/3 and 3/3 which comprise the old post.

Please access below link to check the previous changelog.
https://lkml.org/lkml/2016/8/17/646

Baoquan He (1):
  docs: kernel-parameter : Improve the description of nr_cpus and
    maxcpus

Zhou Wenjian (2):
  Documentation: kdump: remind user of nr_cpus
  Documentation: kdump: add description of enable multi-cpus support

 Documentation/kdump/kdump.txt       |  9 +++++++++
 Documentation/kernel-parameters.txt | 20 +++++++++++++-------
 2 files changed, 22 insertions(+), 7 deletions(-)

-- 
2.5.5

[PATCH v10 0/3] Documentation: add description of enable multi-cpus support for kdump

[Baoquan He]

This is v10 post. In this patchset patch 1/3 is added to give more details
about nr_cpus and maxcpus in kernel-parameters.txt. This is suggested by
Jonathan since the description of them is unclear so that people can't see
what's the exact difference between them. Otherwise no big further change
for 2/3 and 3/3 which comprise the old post.

Please access below link to check the previous changelog.
https://lkml.org/lkml/2016/8/17/646

Baoquan He (1):
  docs: kernel-parameter : Improve the description of nr_cpus and
    maxcpus

Zhou Wenjian (2):
  Documentation: kdump: remind user of nr_cpus
  Documentation: kdump: add description of enable multi-cpus support

 Documentation/kdump/kdump.txt       |  9 +++++++++
 Documentation/kernel-parameters.txt | 20 +++++++++++++-------
 2 files changed, 22 insertions(+), 7 deletions(-)

-- 
2.5.5


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[PATCH v10 1/3] docs: kernel-parameter : Improve the description of nr_cpus and maxcpus

[Baoquan He]

>From the old description people still can't get what's the exact
difference between nr_cpus and maxcpus. Especially in kdump kernel
nr_cpus is always suggested if it's implemented in ARCH. The reason
is nr_cpus is used to limit the max number of possible cpu in system,
the sum of already plugged cpus and hot plug cpus can't exceed its
value. However maxcpus is used to limit how many cpus are allowed to
be brought up during bootup.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kernel-parameters.txt | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a4f4d69..98d6406 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			than or equal to this physical address is ignored.
 
 	maxcpus=	[SMP] Maximum number of processors that	an SMP kernel
-			should make use of.  maxcpus=n : n >= 0 limits the
-			kernel to using 'n' processors.  n=0 is a special case,
-			it is equivalent to "nosmp", which also disables
-			the IO APIC.
+			will bring up during bootup.  maxcpus=n : n >= 0 limits
+			the kernel to bring up 'n' processors. Surely after
+			bootup you can bring up the other plugged cpu by executing
+			"echo 1 > /sys/devices/system/cpu/cpuX/online". So maxcpus
+			only takes effect during system bootup.
+			While n=0 is a special case, it is equivalent to "nosmp",
+			which also disables the IO APIC.
 
 	max_loop=	[LOOP] The number of loop block devices that get
 	(loop.max_loop)	unconditionally pre-created at init time. The default
@@ -2773,9 +2776,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	nr_cpus=	[SMP] Maximum number of processors that	an SMP kernel
 			could support.  nr_cpus=n : n >= 1 limits the kernel to
-			supporting 'n' processors. Later in runtime you can not
-			use hotplug cpu feature to put more cpu back to online.
-			just like you compile the kernel NR_CPUS=n
+			support 'n' processors. It could be larger than the
+			number of already plugged CPU during bootup, later in
+			runtime you can physically add extra cpu until it reaches
+			n. So during boot up some boot time memory for per-cpu
+			variables need be pre-allocated for later physical cpu
+			hot plugging.
 
 	nr_uarts=	[SERIAL] maximum number of UARTs to be registered.
 
-- 
2.5.5

[PATCH v10 1/3] docs: kernel-parameter : Improve the description of nr_cpus and maxcpus

[Baoquan He]

From the old description people still can't get what's the exact
difference between nr_cpus and maxcpus. Especially in kdump kernel
nr_cpus is always suggested if it's implemented in ARCH. The reason
is nr_cpus is used to limit the max number of possible cpu in system,
the sum of already plugged cpus and hot plug cpus can't exceed its
value. However maxcpus is used to limit how many cpus are allowed to
be brought up during bootup.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kernel-parameters.txt | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a4f4d69..98d6406 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			than or equal to this physical address is ignored.
 
 	maxcpus=	[SMP] Maximum number of processors that	an SMP kernel
-			should make use of.  maxcpus=n : n >= 0 limits the
-			kernel to using 'n' processors.  n=0 is a special case,
-			it is equivalent to "nosmp", which also disables
-			the IO APIC.
+			will bring up during bootup.  maxcpus=n : n >= 0 limits
+			the kernel to bring up 'n' processors. Surely after
+			bootup you can bring up the other plugged cpu by executing
+			"echo 1 > /sys/devices/system/cpu/cpuX/online". So maxcpus
+			only takes effect during system bootup.
+			While n=0 is a special case, it is equivalent to "nosmp",
+			which also disables the IO APIC.
 
 	max_loop=	[LOOP] The number of loop block devices that get
 	(loop.max_loop)	unconditionally pre-created at init time. The default
@@ -2773,9 +2776,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	nr_cpus=	[SMP] Maximum number of processors that	an SMP kernel
 			could support.  nr_cpus=n : n >= 1 limits the kernel to
-			supporting 'n' processors. Later in runtime you can not
-			use hotplug cpu feature to put more cpu back to online.
-			just like you compile the kernel NR_CPUS=n
+			support 'n' processors. It could be larger than the
+			number of already plugged CPU during bootup, later in
+			runtime you can physically add extra cpu until it reaches
+			n. So during boot up some boot time memory for per-cpu
+			variables need be pre-allocated for later physical cpu
+			hot plugging.
 
 	nr_uarts=	[SERIAL] maximum number of UARTs to be registered.
 
-- 
2.5.5


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[PATCH v10 2/3] Documentation: kdump: remind user of nr_cpus

[Baoquan He]

From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>

nr_cpus can help to save memory. So we should remind user of it.

Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Xunlei Pang <xpang@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kdump/kdump.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index 88ff63d..f7ef340 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -393,6 +393,8 @@ Notes on loading the dump-capture kernel:
 * We generally don' have to bring up a SMP kernel just to capture the
   dump. Hence generally it is useful either to build a UP dump-capture
   kernel or specify maxcpus=1 option while loading dump-capture kernel.
+  Note, though maxcpus always works, you had better replace it with
+  nr_cpus to save memory if supported by the current ARCH, such as x86.
 
 * 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
-- 
2.5.5

[PATCH v10 2/3] Documentation: kdump: remind user of nr_cpus

[Baoquan He]

From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>

nr_cpus can help to save memory. So we should remind user of it.

Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Xunlei Pang <xpang@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kdump/kdump.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index 88ff63d..f7ef340 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -393,6 +393,8 @@ Notes on loading the dump-capture kernel:
 * We generally don' have to bring up a SMP kernel just to capture the
   dump. Hence generally it is useful either to build a UP dump-capture
   kernel or specify maxcpus=1 option while loading dump-capture kernel.
+  Note, though maxcpus always works, you had better replace it with
+  nr_cpus to save memory if supported by the current ARCH, such as x86.
 
 * 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
-- 
2.5.5


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[PATCH v10 3/3] Documentation: kdump: add description of enable multi-cpus support

[Baoquan He]

From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>

Multi-cpu support is useful to improve the performance of kdump in
some cases. So add the description of enable multi-cpu support in
dump-capture kernel.

Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Xunlei Pang <xpang@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kdump/kdump.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index f7ef340..b0eb27b 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -396,6 +396,13 @@ Notes on loading the dump-capture kernel:
   Note, though maxcpus always works, you had better replace it with
   nr_cpus to save memory if supported by the current ARCH, such as x86.
 
+* You should enable multi-cpu support in dump-capture kernel if you intend
+  to use multi-thread programs with it, such as parallel dump feature of
+  makedumpfile. Otherwise, the multi-thread program may have a great
+  performance degradation. To enable multi-cpu support, you should bring up an
+  SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
+  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
-- 
2.5.5

[PATCH v10 3/3] Documentation: kdump: add description of enable multi-cpus support

[Baoquan He]

From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>

Multi-cpu support is useful to improve the performance of kdump in
some cases. So add the description of enable multi-cpu support in
dump-capture kernel.

Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Xunlei Pang <xpang@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
---
 Documentation/kdump/kdump.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index f7ef340..b0eb27b 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -396,6 +396,13 @@ Notes on loading the dump-capture kernel:
   Note, though maxcpus always works, you had better replace it with
   nr_cpus to save memory if supported by the current ARCH, such as x86.
 
+* You should enable multi-cpu support in dump-capture kernel if you intend
+  to use multi-thread programs with it, such as parallel dump feature of
+  makedumpfile. Otherwise, the multi-thread program may have a great
+  performance degradation. To enable multi-cpu support, you should bring up an
+  SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
+  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
-- 
2.5.5


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Re: [PATCH v10 3/3] Documentation: kdump: add description of enable multi-cpus support

[Baoquan He]

On 09/19/16 at 12:01pm, Baoquan He wrote:
> From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
> 
> Multi-cpu support is useful to improve the performance of kdump in
> some cases. So add the description of enable multi-cpu support in
> dump-capture kernel.
> 
> Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
> Acked-by: Baoquan He <bhe@redhat.com>
> Acked-by: Xunlei Pang <xpang@redhat.com>
> Signed-off-by: Baoquan He <bhe@redhat.com>

Oh, sorry. My git config caused this Signed-off-by adding automatically.
Will resend. Nack this patchset.

> ---
>  Documentation/kdump/kdump.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
> index f7ef340..b0eb27b 100644
> --- a/Documentation/kdump/kdump.txt
> +++ b/Documentation/kdump/kdump.txt
> @@ -396,6 +396,13 @@ Notes on loading the dump-capture kernel:
>    Note, though maxcpus always works, you had better replace it with
>    nr_cpus to save memory if supported by the current ARCH, such as x86.
>  
> +* You should enable multi-cpu support in dump-capture kernel if you intend
> +  to use multi-thread programs with it, such as parallel dump feature of
> +  makedumpfile. Otherwise, the multi-thread program may have a great
> +  performance degradation. To enable multi-cpu support, you should bring up an
> +  SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
> +  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
> -- 
> 2.5.5
> 

Re: [PATCH v10 3/3] Documentation: kdump: add description of enable multi-cpus support

[Baoquan He]

On 09/19/16 at 12:01pm, Baoquan He wrote:
> From: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
> 
> Multi-cpu support is useful to improve the performance of kdump in
> some cases. So add the description of enable multi-cpu support in
> dump-capture kernel.
> 
> Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
> Acked-by: Baoquan He <bhe@redhat.com>
> Acked-by: Xunlei Pang <xpang@redhat.com>
> Signed-off-by: Baoquan He <bhe@redhat.com>

Oh, sorry. My git config caused this Signed-off-by adding automatically.
Will resend. Nack this patchset.

> ---
>  Documentation/kdump/kdump.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
> index f7ef340..b0eb27b 100644
> --- a/Documentation/kdump/kdump.txt
> +++ b/Documentation/kdump/kdump.txt
> @@ -396,6 +396,13 @@ Notes on loading the dump-capture kernel:
>    Note, though maxcpus always works, you had better replace it with
>    nr_cpus to save memory if supported by the current ARCH, such as x86.
>  
> +* You should enable multi-cpu support in dump-capture kernel if you intend
> +  to use multi-thread programs with it, such as parallel dump feature of
> +  makedumpfile. Otherwise, the multi-thread program may have a great
> +  performance degradation. To enable multi-cpu support, you should bring up an
> +  SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
> +  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
> -- 
> 2.5.5
> 

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