NUMA Hotplug Emulator (v7)

* PATCHSET INTRODUCTION

patch 1: Documentation.
patch 2: Adds a numa=possible=<N> command line option to set an additional N nodes
as being possible for memory hotplug.

patch 3: Add node hotplug emulation, introduce debugfs node/add_node interface

patch 4: Abstract cpu register functions, make these interface friend for cpu
hotplug emulation
patch 5: Support cpu probe/release in x86, it provide a software method to hot
add/remove cpu with sysfs interface.
patch 6: Fake CPU socket with logical CPU on x86, to prevent the scheduling
domain to build the incorrect hierarchy.
patch 7: Implement mem_hotplug/add_memory debugfs interface

* FEEDBACKDS & RESPONSES

v7:

David: We don't need two different interfaces, one in sysfs and one in debugfs,
to hotplug memory.
Response: We use the debugfs for memory hotplug emulation only, for sysfs memory probe
interface, we did not do any modifications, so we remove original patch 7
from patchset.
David: Suggest new probe files in debugfs for each online node:
/sys/kernel/debug/mem_hotplug/add_node (already exists) /sys/kernel/debug/mem_hotplug/node0/add_memory /sys/kernel/debug/mem_hotplug/node1/add_memory

Response: We need not make a simple thing such complicated, We'd prefer to
rename the mem_hotplug/probe interface as mem_hotplug/add_memory.
/sys/kernel/debug/mem_hotplug/add_node (already exists) /sys/kernel/debug/mem_hotplug/add_memory (rename probe as add_memory)

v6:

Greg KH:  Suggest to use interface mem_hotplug/add_node
David:    Agree with Greg's suggestion
Response: We move the interface from node/add_node to mem_hotplug/add_node, and we also move 

memory/probe interface to mem_hotplug/probe since both are related to memory hotplug.

Kletnieks Valdis: suggest to renumber the patch serie, and move patch 8/8 to patch 1/8.
Response: Move patch 8/8 to patch 1/8, and we will include the full description in 0/8 when
we send patches in future.

v5:

David: Suggests to use a flexible method to to do node hotplug emulation. After
review our 2 versions emulator implemetations, David provides a better solution
to solve both the flexibility and memory wasting issue.

Add numa=possible=<N> command line option, provide sysfs inteface /sys/devices/system/node/add_node interface, and move the inteface to debugfs /sys/kernel/debug/hotplug/add_node after hearing the voice from community.

Greg KH: move the interface from hotplug/add_node to node/add_node

Response: Accept David's node=possible=<n> command line options. After talking
with David, he agree to add his patch to our patchset, thanks David's solution(patch 1).

David's original interface /sys/kernel/debug/hotplug/add_node is not so clear for node hotplug emulation, we accept Greg's suggestion, move the interface to ndoe/add_node (patch 2)

Dave Hansen: For memory hotplug, Dave reminds Greg KH's advice, suggest us to use configfs replace
sysfs. After Dave knows that it is just for test purpose, Dave thinks debugfs should
be the best.

Response: memory probe sysfs interface already exists, I'd like to still keep it, and extend it
to support memory add on a specified node(patch 6).

We accepts Dave's suggestion, implement memory probe interface with debugfs(patch 7).

Randy Dunlap: Correct many grammatical errors in our documentation(patch 8).

Response: Thanks for Randy's careful review, we already correct them.

v4:

Split CPU hotplug emulation code since David has send a patchset for node hotplug emulation.

v3 & v2:

1) Patch 0
Balbir & Greg: Suggest to use tool git/quilt to manage/send the patchset.
Response: Thanks for the recommendation, With help from Fengguang, I get quilt
working, it is a great tool.

2) Patch 2
Jaswinder Singh: if (hidden_num) is not required in patch 2
Response: good catching, it is removed in v2.

3) Patch 3
Dave Hansen: Suggest to create a dedicated sysfs file for each possible node.
Greg: How big would this "list" be? What will it look like exactly?
Haicheng: It should follow "one value per file". It intends to show acceptable
parameters.

For example, if we have 4 fake offlined nodes, like node 2-5, then:
$ cat /sys/devices/system/node/probe
2-5

Then user hotadds node3 to system:
$ echo 3 > /sys/devices/system/node/probe $ cat /sys/devices/system/node/probe
2,4-5

Greg: As you are trying to add a new sysfs file, please create the matching
Documentation/ABI/ file as well.
Response: We miss it, and we already add it in v2.

Patch 4 & 5:
Paul Mundt: This looks like an incredibly painful interface. How about scrapping all of this _emu() mess and just reworking the register_cpu() interface?
Response: accept Paul's suggestion, and remove the cpu _emu functions.

Patch 7:
Dave Hansen: If we're going to put multiple values into the file now and

		 add to the ABI, can we be more explicit about it?
		echo "physical_address=0x40000000 numa_node=3" > memory/probe

Response: Dave's new interface was accpeted, and more we still keep the old
format for compatibility. We documented the these interfaces into
Documentation/ABI in v2.
Greg: suggest to use configfs replace for the memory probe interface
Andi: This is a debugging interface. It doesn't need to have the
most pretty interface in the world, because it will be only used for QA by a few people. it's just a QA interface, not the next generation of POSIX.
Response: We still keep it as sysfs interface since node/cpu/memory probe interface
are all in sysfs, we can create another group of patches to support configfs if we have this strong requirement in future.

v1:

the RFC version for NUMA Hotplug Emulator.

* WHAT IS HOTPLUG EMULATOR

NUMA hotplug emulator is collectively named for the hotplug emulation it is able to emulate NUMA Node Hotplug thru a pure software way. It intends to help people easily debug and test node/cpu/memory hotplug related stuff on a none-NUMA-hotplug-support machine, even an UMA machine.

The emulator provides mechanism to emulate the process of physcial cpu/mem hotadd, it provides possibility to debug CPU and memory hotplug on the machines without NUMA support for kenrel developers. It offers an interface for cpu and memory hotplug test purpose.

* WHY DO WE USE HOTPLUG EMULATOR

We are focusing on the hotplug emualation for a few months. The emualor helps
team to reproduce all the major hotplug bugs. It plays an important role to
the hotplug code quality assuirance. Because of the hotplug emulator, we already move most of the debug working to virtual evironment.

* Principles & Usages

NUMA hotplug emulator include 3 different parts: node/CPU/memory hotplug emulation.

1) Node hotplug emulation:

Adds a numa=possible=<N> command line option to set an additional N nodes as being possible for memory hotplug. This set of possible nodes control nr_node_ids and the sizes of several dynamically allocated node arrays.

This allows memory hotplug to create new nodes for newly added memory rather than binding it to existing nodes.

For emulation on x86, it would be possible to set aside memory for hotplugged nodes (say, anything above 2G) and to add an additional four nodes as being possible on boot with

mem=2G numa=possible=4

and then creating a new 128M node at runtime:

	# echo 128M@0x80000000 > /sys/kernel/debug/node/add_node
	On node 1 totalpages: 0
	init_memory_mapping: 0000000080000000-0000000088000000
	 0080000000 - 0088000000 page 2M

Once the new node has been added, its memory can be onlined. If this memory represents memory section 16, for example:

	# echo online > /sys/devices/system/memory/memory16/state
	Built 2 zonelists in Node order, mobility grouping on.  Total pages: 514846
	Policy zone: Normal
 [ The memory section(s) mapped to a particular node are visible via
   /sys/devices/system/node/node1, in this example. ]

2) CPU hotplug emulation:

The emulator reserve CPUs throu grub parameter, the reserved CPUs can be hot-add/hot-remove in software method.

When hotplug a CPU with emulator, we are using a logical CPU to emulate the CPU hotplug process. For the CPU supported SMT, some logical CPUs are in the same socket, but it may located in different NUMA node after we have emulator. We put the logical CPU into a fake CPU socket, and assign it an unique phys_proc_id. For the fake socket, we put one logical CPU in only.

- to hide CPUs
- Using boot option "maxcpus=N" hide CPUs
N is the number of initialize CPUs
- Using boot option "cpu_hpe=on" to enable cpu hotplug emulation
when cpu_hpe is enabled, the rest CPUs will not be initialized

- to hot-add CPU to node

	# echo nid > cpu/probe

- to hot-remove CPU

	# echo nid > cpu/release

3) Memory hotplug emulation:

The emulator reserves memory before OS boots, the reserved memory region is removed from e820 table, and they can be hot-added via the mem_hoplug/add_memory debugfs interface. This interface supports to add memory to the specified node.

The difficulty of Memory Release is well-known, we have no plan for it until now.

- reserve memory thru a kernel boot paramter
mem=1024m

- add a memory section to node 3

    # echo 0x40000000,3 > mem_hotplug/add_memory
	OR
    # echo 1024m,3 > mem_hotplug/add_memory
	OR
    # echo "physical_address=0x40000000 numa_node=3" > mem_hotplug/add_memory

* ACKNOWLEDGMENT

NUMA Hotplug Emulator includes a team's efforts, thanks all of them. They are:
Andi Kleen, Haicheng Li, Shaohui Zheng, Fengguang Wu, David Rientjes and Yongkang You

Thanks & Regards,
Shaohui

From: Shaohui Zheng <[email protected]>

add a text file Documentation/x86/x86_64/numa_hotplug_emulator.txt to explain the usage for the hotplug emulator.

---

Index: linux-hpe4/Documentation/x86/x86_64/numa_hotplug_emulator.txt
===================================================================
@@ -0,0 +1,104 @@
+NUMA Hotplug Emulator for x86_64
+---------------------------------------------------
+
+NUMA hotplug emulator is able to emulate NUMA Node Hotplug
+thru a pure software way. It intends to help people easily debug
+and test node/CPU/memory hotplug related stuff on a
+none-NUMA-hotplug-support machine, even a UMA machine and virtual
+environment.
+
+1) Node hotplug emulation:
+
+Adds a numa=possible=<N> command line option to set an additional N nodes
+as being possible for memory hotplug.  This set of possible nodes
+control nr_node_ids and the sizes of several dynamically allocated node
+arrays.
+
+This allows memory hotplug to create new nodes for newly added memory
+rather than binding it to existing nodes.
+
+For emulation on x86, it would be possible to set aside memory for hotplugged
+nodes (say, anything above 2G) and to add an additional four nodes as being
+possible on boot with
+
+	mem=2G numa=possible=4
+
+and then creating a new 128M node at runtime:
+
+	# echo 128M@0x80000000 > /sys/kernel/debug/node/add_node
+	On node 1 totalpages: 0
+	init_memory_mapping: 0000000080000000-0000000088000000
+	 0080000000 - 0088000000 page 2M
+
+Once the new node has been added, its memory can be onlined.  If this
+memory represents memory section 16, for example:
+
+	# echo online > /sys/devices/system/memory/memory16/state
+	Built 2 zonelists in Node order, mobility grouping on.  Total pages: 514846
+	Policy zone: Normal
+ [ The memory section(s) mapped to a particular node are visible via
+   /sys/devices/system/node/node1, in this example. ]
+
+2) CPU hotplug emulation:
+
+The emulator reserve CPUs throu grub parameter, the reserved CPUs can be
+hot-add/hot-remove in software method, it emulates the process of physical
+cpu hotplug.
+
+When hotplugging a CPU with emulator, we are using a logical CPU to emulate the CPU
+socket hotplug process. For the CPU supported SMT, some logical CPUs are in the
+same socket, but it may located in different NUMA node after we have emulator.
+We put the logical CPU into a fake CPU socket, and assign it a unique
+phys_proc_id. For the fake socket, we put one logical CPU in only.
+
+ - to hide CPUs
+	- Using boot option "maxcpus=N" hide CPUs
+	  N is the number of CPUs to initialize; the reset will be hidden.
+	- Using boot option "cpu_hpe=on" to enable CPU hotplug emulation
+      when cpu_hpe is enabled, the rest CPUs will not be initialized
+
+ - to hot-add CPU to node
+	# echo nid > cpu/probe
+
+ - to hot-remove CPU
+	# echo nid > cpu/release
+
+3) Memory hotplug emulation:
+
+The emulator reserves memory before OS boots, the reserved memory region is
+removed from e820 table, and they can be hot-added via the
+mem_hoplug/add_memory debugfs interface. This interface supports to add memory
+to the specified node.
+
+The difficulty of Memory Release is well-known, we have no plan for it until now.
+
+ - reserve memory thru a kernel boot paramter
+ 	mem=1024m
+
+ - add a memory section to node 3
+    # echo 0x40000000,3 > mem_hotplug/add_memory
+	OR
+    # echo 1024m,3 > mem_hotplug/add_memory
+	OR
+    # echo "physical_address=0x40000000 numa_node=3" > mem_hotplug/add_memory
+
+4) Script for hotplug testing
+
+These scripts provides convenience when we hot-add memory/cpu in batch.
+
+- Online all memory sections:
+for m in /sys/devices/system/memory/memory*;
+do
+	echo online > $m/state;
+done
+
+- CPU Online:
+for c in /sys/devices/system/cpu/cpu*;
+do
+	echo 1 > $c/online;
+done
+
+- David Rientjes <[email protected]>
+- Haicheng Li <[email protected]>
+- Shaohui Zheng <[email protected]>
+  Nov 2010

Thanks & Regards,
Shaohui


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From: David Rientjes <[email protected]>

Adds a numa=possible=<N> command line option to set an additional N nodes as being possible for memory hotplug. This set of possible nodes controls nr_node_ids and the sizes of several dynamically allocated node arrays.

This allows memory hotplug to create new nodes for newly added memory rather than binding it to existing nodes.

The first use-case for this will be node hotplug emulation which will use these possible nodes to create new nodes to test the memory hotplug callbacks and surrounding memory hotplug code.

---
 Documentation/x86/x86_64/boot-options.txt |    4 ++++
 arch/x86/mm/numa_64.c                     |   18 +++++++++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

@@ -174,6 +174,10 @@ NUMA
 		If given as an integer, fills all system RAM with N fake nodes
 		interleaved over physical nodes.
 
+  numa=possible=<N>
+		Sets an additional N nodes as being possible for memory
+		hotplug.
+
 ACPI
 
   acpi=off	Don't enable ACPI

@@ -33,6 +33,7 @@ s16 apicid_to_node[MAX_LOCAL_APIC] __cpuinitdata = {
 int numa_off __initdata;
 static unsigned long __initdata nodemap_addr;
 static unsigned long __initdata nodemap_size;
+static unsigned long __initdata numa_possible_nodes;
 
 /*
  * Map cpu index to node index

@@ -611,7 +612,7 @@ void __init initmem_init(unsigned long start_pfn, unsigned long last_pfn,
 
 #ifdef CONFIG_NUMA_EMU
 	if (cmdline && !numa_emulation(start_pfn, last_pfn, acpi, k8))
-		return;
+		goto out;
 	nodes_clear(node_possible_map);
 	nodes_clear(node_online_map);
 #endif

@@ -619,14 +620,14 @@ void __init initmem_init(unsigned long start_pfn, unsigned long last_pfn,
 #ifdef CONFIG_ACPI_NUMA
 	if (!numa_off && acpi && !acpi_scan_nodes(start_pfn << PAGE_SHIFT,
 						  last_pfn << PAGE_SHIFT))
-		return;
+		goto out;
 	nodes_clear(node_possible_map);
 	nodes_clear(node_online_map);
 #endif
 
 #ifdef CONFIG_K8_NUMA
 	if (!numa_off && k8 && !k8_scan_nodes())
-		return;
+		goto out;
 	nodes_clear(node_possible_map);
 	nodes_clear(node_online_map);
 #endif

@@ -646,6 +647,15 @@ void __init initmem_init(unsigned long start_pfn, unsigned long last_pfn,
 		numa_set_node(i, 0);
 	memblock_x86_register_active_regions(0, start_pfn, last_pfn);
 	setup_node_bootmem(0, start_pfn << PAGE_SHIFT, last_pfn << PAGE_SHIFT);
+out: __maybe_unused
+	for (i = 0; i < numa_possible_nodes; i++) {
+		int nid;
+
+		nid = first_unset_node(node_possible_map);
+		if (nid == MAX_NUMNODES)
+			break;
+		node_set(nid, node_possible_map);
+	}
 }
 
 unsigned long __init numa_free_all_bootmem(void)

@@ -675,6 +685,8 @@ static __init int numa_setup(char *opt)
 	if (!strncmp(opt, "noacpi", 6))
 		acpi_numa = -1;
 #endif
+	if (!strncmp(opt, "possible=", 9))
+		numa_possible_nodes = simple_strtoul(opt + 9, NULL, 0);
 	return 0;
 }
 early_param("numa", numa_setup);

Thanks & Regards,
Shaohui


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From: David Rientjes <[email protected]>

Add an interface to allow new nodes to be added when performing memory hot-add. This provides a convenient interface to test memory hotplug notifier callbacks and surrounding hotplug code when new nodes are onlined without actually having a machine with such hotpluggable SRAT entries.

This adds a new debugfs interface at /sys/kernel/debug/mem_hotplug/add_node that behaves in a similar way to the memory hot-add "probe" interface. Its format is size@start, where "size" is the size of the new node to be added and "start" is the physical address of the new memory.

The new node id is a currently offline, but possible, node. The bit must be set in node_possible_map so that nr_node_ids is sized appropriately.

For emulation on x86, for example, it would be possible to set aside memory for hotplugged nodes (say, anything above 2G) and to add an additional four nodes as being possible on boot with

mem=2G numa=possible=4

and then creating a new 128M node at runtime:

	# echo 128M@0x80000000 > /sys/kernel/debug/mem_hotplug/add_node
	On node 1 totalpages: 0
	init_memory_mapping: 0000000080000000-0000000088000000
	 0080000000 - 0088000000 page 2M
Once the new node has been added, its memory can be onlined.  If this
memory represents memory section 16, for example:

	# echo online > /sys/devices/system/memory/memory16/state
	Built 2 zonelists in Node order, mobility grouping on.  Total pages: 514846
	Policy zone: Normal
 [ The memory section(s) mapped to a particular node are visible via
   /sys/devices/system/node/node1, in this example. ]

The new node is now hotplugged and ready for testing.

---
 Documentation/memory-hotplug.txt |   24 +++++++++++++++
 mm/memory_hotplug.c              |   59 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+), 0 deletions(-)

Index: linux-hpe4/Documentation/memory-hotplug.txt
===================================================================
@@ -18,6 +18,7 @@
 4. Physical memory hot-add phase
   4.1 Hardware(Firmware) Support
   4.2 Notify memory hot-add event by hand
+  4.3 Node hotplug emulation
 5. Logical Memory hot-add phase
   5.1. State of memory
   5.2. How to online memory

@@ -215,6 +216,29 @@
 Please see "How to online memory" in this text.
 
 
+4.3 Node hotplug emulation
+------------
+With debugfs, it is possible to test node hotplug by assigning the newly
+added memory to a new node id when using a different interface with a similar
+behavior to "probe" described in section 4.2.  If a node id is possible
+(there are bits in /sys/devices/system/memory/possible that are not online),
+then it may be used to emulate a newly added node as the result of memory
+hotplug by using the debugfs "add_node" interface.
+
+The add_node interface is located at "mem_hotplug/add_node" at the debugfs
+mount point.
+
+You can create a new node of a specified size starting at the physical
+address of new memory by
+
+% echo size@start_address_of_new_memory > /sys/kernel/debug/mem_hotplug/add_node
+
+Where "size" can be represented in megabytes or gigabytes (for example,
+"128M" or "1G").  The minumum size is that of a memory section.
+
+Once the new node has been added, it is possible to online the memory by
+toggling the "state" of its memory section(s) as described in section 5.1.
+
 
 ------------------------------
 5. Logical Memory hot-add phase

Thanks & Regards,
Shaohui


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From: Shaohui Zheng <[email protected]>

Abstract cpu register functions, provide a more flexible interface register_cpu_node, the new interface provides convenience to add cpu to a specified node, we can use it to add a cpu to a fake node.

---

Index: linux-hpe4/arch/x86/include/asm/cpu.h
===================================================================
@@ -27,6 +27,7 @@
 
 #ifdef CONFIG_HOTPLUG_CPU
 extern int arch_register_cpu(int num);
+extern int arch_register_cpu_node(int num, int nid);
 extern void arch_unregister_cpu(int);
 #endif
 

Index: linux-hpe4/arch/x86/kernel/topology.c
===================================================================
@@ -52,6 +52,15 @@
 }
 EXPORT_SYMBOL(arch_register_cpu);
 
+int __ref arch_register_cpu_node(int num, int nid)
+{
+	if (num)
+		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
+
+	return register_cpu_node(&per_cpu(cpu_devices, num).cpu, num, nid);
+}
+EXPORT_SYMBOL(arch_register_cpu_node);
+
 void arch_unregister_cpu(int num)
 {
 	unregister_cpu(&per_cpu(cpu_devices, num).cpu);

Index: linux-hpe4/drivers/base/cpu.c
===================================================================
@@ -208,17 +208,18 @@
 static SYSDEV_CLASS_ATTR(offline, 0444, print_cpus_offline, NULL);
 
 /*
- * register_cpu - Setup a sysfs device for a CPU.
+ * register_cpu_node - Setup a sysfs device for a CPU.
  * @cpu - cpu->hotpluggable field set to 1 will generate a control file in
  *	  sysfs for this CPU.
  * @num - CPU number to use when creating the device.
+ * @nid - Node ID to use, if any.
  *
  * Initialize and register the CPU device.
  */
-int __cpuinit register_cpu(struct cpu *cpu, int num)
+int __cpuinit register_cpu_node(struct cpu *cpu, int num, int nid)
 {
 	int error;
-	cpu->node_id = cpu_to_node(num);
+	cpu->node_id = nid;
 	cpu->sysdev.id = num;
 	cpu->sysdev.cls = &cpu_sysdev_class;
 

@@ -229,7 +230,7 @@
 	if (!error)
 		per_cpu(cpu_sys_devices, num) = &cpu->sysdev;
 	if (!error)
-		register_cpu_under_node(num, cpu_to_node(num));
+		register_cpu_under_node(num, nid);
 
 #ifdef CONFIG_KEXEC
 	if (!error)

Index: linux-hpe4/include/linux/cpu.h
===================================================================
@@ -30,7 +30,13 @@
 	struct sys_device sysdev;
 };
 
-extern int register_cpu(struct cpu *cpu, int num);
+extern int register_cpu_node(struct cpu *cpu, int num, int nid);
+
+static inline int register_cpu(struct cpu *cpu, int num)
+{
+	return register_cpu_node(cpu, num, cpu_to_node(num));
+}
+
 extern struct sys_device *get_cpu_sysdev(unsigned cpu);
 
 extern int cpu_add_sysdev_attr(struct sysdev_attribute *attr);

Thanks & Regards,
Shaohui


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From: Shaohui Zheng <[email protected]>

CPU physical hot-add/hot-remove are supported on some hardwares, and it was already supported in current linux kernel. NUMA Hotplug Emulator provides a mechanism to emulate the process with software method. It can be used for testing or debuging purpose.

CPU physical hotplug is different with logical CPU online/offline. Logical online/offline is controled by interface /sys/device/cpu/cpuX/online. CPU hotplug emulator uses probe/release interface. It becomes possible to do cpu hotplug automation and stress

Add cpu interface probe/release under sysfs for x86_64. User can use this interface to emulate the cpu hot-add and hot-remove process.

Directive:
*) Reserve CPU thru grub parameter like:
maxcpus=4

the rest CPUs will not be initiliazed.

*) Probe CPU
we can use the probe interface to hot-add new CPUs:
echo nid > /sys/devices/system/cpu/probe

*) Release a CPU
echo cpu > /sys/devices/system/cpu/release

A reserved CPU will be hot-added to the specified node.
1) nid == 0, the CPU will be added to the real node which the CPU should be in
2) nid != 0, add the CPU to node nid even through it is a fake node.

---

Index: linux-hpe4/arch/x86/kernel/acpi/boot.c
===================================================================
@@ -647,8 +647,44 @@
 }
 EXPORT_SYMBOL(acpi_map_lsapic);
 
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+static void acpi_map_cpu2node_emu(int cpu, int physid, int nid)
+{
+#ifdef CONFIG_ACPI_NUMA
+#ifdef CONFIG_X86_64
+	apicid_to_node[physid] = nid;
+	numa_set_node(cpu, nid);
+#else /* CONFIG_X86_32 */
+	apicid_2_node[physid] = nid;
+	cpu_to_node_map[cpu] = nid;
+#endif
+#endif
+}
+
+static u16 cpu_to_apicid_saved[CONFIG_NR_CPUS];
+int __ref acpi_map_lsapic_emu(int pcpu, int nid)
+{
+	/* backup cpu apicid to array cpu_to_apicid_saved */
+	if (cpu_to_apicid_saved[pcpu] == 0 &&
+		per_cpu(x86_cpu_to_apicid, pcpu) != BAD_APICID)
+		cpu_to_apicid_saved[pcpu] = per_cpu(x86_cpu_to_apicid, pcpu);
+
+	per_cpu(x86_cpu_to_apicid, pcpu) = cpu_to_apicid_saved[pcpu];
+	acpi_map_cpu2node_emu(pcpu, per_cpu(x86_cpu_to_apicid, pcpu), nid);
+
+	return pcpu;
+}
+EXPORT_SYMBOL(acpi_map_lsapic_emu);
+#endif
+
 int acpi_unmap_lsapic(int cpu)
 {
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+	/* backup cpu apicid to array cpu_to_apicid_saved */
+	if (cpu_to_apicid_saved[cpu] == 0 &&
+		per_cpu(x86_cpu_to_apicid, cpu) != BAD_APICID)
+		cpu_to_apicid_saved[cpu] = per_cpu(x86_cpu_to_apicid, cpu);
+#endif
 	per_cpu(x86_cpu_to_apicid, cpu) = -1;
 	set_cpu_present(cpu, false);
 	num_processors--;

Index: linux-hpe4/arch/x86/kernel/smpboot.c
===================================================================
@@ -107,8 +107,6 @@
         mutex_unlock(&x86_cpu_hotplug_driver_mutex);
 }
 
-ssize_t arch_cpu_probe(const char *buf, size_t count) { return -1; }
-ssize_t arch_cpu_release(const char *buf, size_t count) { return -1; }
 #else
 static struct task_struct *idle_thread_array[NR_CPUS] __cpuinitdata ;
 #define get_idle_for_cpu(x)      (idle_thread_array[(x)])

Index: linux-hpe4/arch/x86/mm/numa_64.c
===================================================================
@@ -12,6 +12,7 @@
 #include <linux/module.h>
 #include <linux/nodemask.h>
 #include <linux/sched.h>
+#include <linux/cpu.h>
 
 #include <asm/e820.h>
 #include <asm/proto.h>

@@ -785,6 +786,19 @@
 }
 #endif
 
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+static __init int cpu_hpe_setup(char *opt)
+{
+	if (!opt)
+		return -EINVAL;
+
+	if (!strncmp(opt, "on", 2) || !strncmp(opt, "1", 1))
+		cpu_hpe_on = 1;
+
+	return 0;
+}
+early_param("cpu_hpe", cpu_hpe_setup);
+#endif  /* CONFIG_ARCH_CPU_PROBE_RELEASE */
 
 void __cpuinit numa_set_node(int cpu, int node)
 {

Index: linux-hpe4/drivers/acpi/processor_driver.c
===================================================================
@@ -530,6 +530,14 @@
 		goto err_free_cpumask;
 
 	sysdev = get_cpu_sysdev(pr->id);
+	/*
+	 * Reserve cpu for hotplug emulation, the reserved cpu can be hot-added
+	 * throu the cpu probe interface. Return directly.
+	 */
+	if (sysdev == NULL) {
+		goto out;
+	}
+
 	if (sysfs_create_link(&device->dev.kobj, &sysdev->kobj, "sysdev")) {
 		result = -EFAULT;
 		goto err_remove_fs;

@@ -570,6 +578,7 @@
 		goto err_remove_sysfs;
 	}
 
+out:
 	return 0;
 
 err_remove_sysfs:

Index: linux-hpe4/drivers/base/cpu.c
===================================================================
@@ -22,9 +22,15 @@
 };
 EXPORT_SYMBOL(cpu_sysdev_class);
 
-static DEFINE_PER_CPU(struct sys_device *, cpu_sys_devices);
+DEFINE_PER_CPU(struct sys_device *, cpu_sys_devices);
 
 #ifdef CONFIG_HOTPLUG_CPU
+/*
+ * cpu_hpe_on is a switch to enable/disable cpu hotplug emulation. it is
+ * disabled in default, we can enable it throu grub parameter cpu_hpe=on
+ */
+int cpu_hpe_on;
+
 static ssize_t show_online(struct sys_device *dev, struct sysdev_attribute *attr,
 			   char *buf)
 {

Index: linux-hpe4/include/linux/acpi.h
===================================================================
@@ -102,6 +102,7 @@
 #ifdef CONFIG_ACPI_HOTPLUG_CPU
 /* Arch dependent functions for cpu hotplug support */
 int acpi_map_lsapic(acpi_handle handle, int *pcpu);
+int acpi_map_lsapic_emu(int pcpu, int nid);
 int acpi_unmap_lsapic(int cpu);
 #endif /* CONFIG_ACPI_HOTPLUG_CPU */
 

Index: linux-hpe4/include/linux/cpu.h
===================================================================
@@ -30,6 +30,8 @@
 	struct sys_device sysdev;
 };
 
+DECLARE_PER_CPU(struct sys_device *, cpu_sys_devices);
+
 extern int register_cpu_node(struct cpu *cpu, int num, int nid);
 
 static inline int register_cpu(struct cpu *cpu, int num)

@@ -149,6 +151,7 @@
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
 int cpu_down(unsigned int cpu);
+extern int cpu_hpe_on;
 
 #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
 extern void cpu_hotplug_driver_lock(void);

@@ -171,6 +174,7 @@
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
 #define unregister_hotcpu_notifier(nb)	({ (void)(nb); })
+static int cpu_hpe_on;
 #endif		/* CONFIG_HOTPLUG_CPU */
 
 #ifdef CONFIG_PM_SLEEP_SMP

Index: linux-hpe4/Documentation/x86/x86_64/boot-options.txt
===================================================================
@@ -316,3 +316,9 @@
 		Do not use GB pages for kernel direct mappings.
 	gbpages
 		Use GB pages for kernel direct mappings.
+	cpu_hpe=on/off
+		Enable/disable CPU hotplug emulation with software method. When cpu_hpe=on,
+		sysfs provides probe/release interface to hot add/remove CPUs dynamically.
+		We can use maxcpus=<N> to reserve CPUs.
+		This option is disabled by default.
+			

Thanks & Regards,
Shaohui


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From: Shaohui Zheng <[email protected]>

When hotplug a CPU with emulator, we are using a logical CPU to emulate the CPU hotplug process. For the CPU supported SMT, some logical CPUs are in the same socket, but it may located in different NUMA node after we have emulator. it misleads the scheduling domain to build the incorrect hierarchy, and it causes the following call trace when rebalance the scheduling domain:

divide error: 0000 [#1] SMP
last sysfs file: /sys/devices/system/cpu/cpu8/online CPU 0

Modules linked in: fbcon tileblit font bitblit softcursor radeon ttm drm_kms_helper e1000e usbhid via_rhine mii drm i2c_algo_bit igb dca
Pid: 0, comm: swapper Not tainted 2.6.32hpe #78 X8DTN
RIP: 0010:[<ffffffff81051da5>]  [<ffffffff81051da5>] find_busiest_group+0x6c5/0xa10
RSP: 0018:ffff880028203c30  EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000015ac0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff880277e8cfa0 RDI: 0000000000000000
RBP: ffff880028203dc0 R08: ffff880277e8cfa0 R09: 0000000000000040
R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff880028200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 00007f16cfc85770 CR3: 0000000001001000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 0, threadinfo ffffffff81822000, task ffffffff8184a600)
Stack:
 ffff880028203d60 ffff880028203cd0 ffff8801c204ff08 ffff880028203e38
<0> 0101ffff81018c59 ffff880028203e44 00000001810806bd ffff8801c204fe00
<0> 0000000528200000 ffffffff00000000 0000000000000018 0000000000015ac0
Call Trace:
 <IRQ> 
 [<ffffffff81088ee0>] ? tick_dev_program_event+0x40/0xd0
 [<ffffffff81053b2c>] rebalance_domains+0x17c/0x570
 [<ffffffff81018c89>] ? read_tsc+0x9/0x20
 [<ffffffff81088ee0>] ? tick_dev_program_event+0x40/0xd0
 [<ffffffff810569ed>] run_rebalance_domains+0xbd/0xf0
 [<ffffffff8106471f>] __do_softirq+0xaf/0x1e0
 [<ffffffff810b7d18>] ? handle_IRQ_event+0x58/0x160
 [<ffffffff810130ac>] call_softirq+0x1c/0x30
 [<ffffffff81014a85>] do_softirq+0x65/0xa0
 [<ffffffff810645cd>] irq_exit+0x7d/0x90
 [<ffffffff81013ff0>] do_IRQ+0x70/0xe0
 [<ffffffff810128d3>] ret_from_intr+0x0/0x11
 <EOI> 
 [<ffffffff8133387f>] ? acpi_idle_enter_bm+0x281/0x2b5
 [<ffffffff81333878>] ? acpi_idle_enter_bm+0x27a/0x2b5
 [<ffffffff8145dc8f>] ? cpuidle_idle_call+0x9f/0x130
 [<ffffffff81010e2b>] ? cpu_idle+0xab/0x100
 [<ffffffff8158aee6>] ? rest_init+0x66/0x70
 [<ffffffff81905d90>] ? start_kernel+0x3e3/0x3ef
 [<ffffffff8190533a>] ? x86_64_start_reservations+0x125/0x129
 [<ffffffff81905438>] ? x86_64_start_kernel+0xfa/0x109
Code: 00 00 e9 4c fb ff ff 0f 1f 80 00 00 00 00 48 8b b5 d8 fe ff ff 48 8b 45 a8 4d 29 ef 8b 56 08 48 c1 e0 0a 49 89 f0 48 89 d7 31 d2 <48> f7 f7 31 d2 48 89 45 a0 8b 76 08 4c 89 f0 48 c1 e0 0a 48 f7 
RIP  [<ffffffff81051da5>] find_busiest_group+0x6c5/0xa10
 RSP <ffff880028203c30>

Solution:

We put the logical CPU into a fake CPU socket, and assign it an unique
phys_proc_id. For the fake socket, we put one logical CPU in only. This
method fixes the above bug.

---

Index: linux-hpe4/arch/x86/include/asm/processor.h
===================================================================
@@ -113,6 +113,15 @@
 	/* Index into per_cpu list: */
 	u16			cpu_index;
 #endif
+
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+	/*
+	 * Use a logic cpu to emulate a physical cpu's hotplug. We put the
+	 * logical cpu into a fake socket, assign a fake physical id to it,
+	 * and create a fake core.
+	 */
+	__u8		cpu_probe_on; /* A flag to enable cpu probe/release */
+#endif
 } __attribute__((__aligned__(SMP_CACHE_BYTES)));
 
 #define X86_VENDOR_INTEL	0

Index: linux-hpe4/arch/x86/kernel/smpboot.c
===================================================================
@@ -97,6 +97,7 @@
  */
 static DEFINE_MUTEX(x86_cpu_hotplug_driver_mutex);
 
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
 void cpu_hotplug_driver_lock()
 {
         mutex_lock(&x86_cpu_hotplug_driver_mutex);

@@ -106,6 +107,7 @@
 {
         mutex_unlock(&x86_cpu_hotplug_driver_mutex);
 }
+#endif
 
 #else
 static struct task_struct *idle_thread_array[NR_CPUS] __cpuinitdata ;

@@ -198,6 +200,8 @@
 {
 	int cpuid, phys_id;
 	unsigned long timeout;
+	u8 cpu_probe_on = 0;
+	struct cpuinfo_x86 *c;
 
 	/*
 	 * If waken up by an INIT in an 82489DX configuration

@@ -277,7 +281,20 @@
 	/*
 	 * Save our processor parameters
 	 */
+	c = &cpu_data(cpuid);
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+	cpu_probe_on = c->cpu_probe_on;
+	phys_id = c->phys_proc_id;
+#endif
+
 	smp_store_cpu_info(cpuid);
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+	if (cpu_probe_on) {
+		c->phys_proc_id = phys_id; /* restore the fake phys_proc_id */
+		c->cpu_core_id = 0; /* force the logical cpu to core 0 */
+		c->cpu_probe_on = cpu_probe_on;
+	}
+#endif
 
 	notify_cpu_starting(cpuid);
 

@@ -400,6 +417,11 @@
 {
 	int i;
 	struct cpuinfo_x86 *c = &cpu_data(cpu);
+	int cpu_probe_on = 0;
+
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+	cpu_probe_on = c->cpu_probe_on;
+#endif
 
 	cpumask_set_cpu(cpu, cpu_sibling_setup_mask);
 

@@ -431,7 +453,8 @@
 
 	for_each_cpu(i, cpu_sibling_setup_mask) {
 		if (per_cpu(cpu_llc_id, cpu) != BAD_APICID &&
-		    per_cpu(cpu_llc_id, cpu) == per_cpu(cpu_llc_id, i)) {
+		    per_cpu(cpu_llc_id, cpu) == per_cpu(cpu_llc_id, i) &&
+			cpu_probe_on == 0) {
 			cpumask_set_cpu(i, c->llc_shared_map);
 			cpumask_set_cpu(cpu, cpu_data(i).llc_shared_map);
 		}

Index: linux-hpe4/arch/x86/kernel/topology.c
===================================================================
@@ -70,6 +70,36 @@
 }
 EXPORT_SYMBOL(arch_unregister_cpu);
 
+#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
+/*
+ * Put the logical cpu into a new sokect, and encapsule it into core 0.
+ */
+static void fake_cpu_socket_info(int cpu)
+{
+	struct cpuinfo_x86 *c = &cpu_data(cpu);
+	int i, phys_id = 0;
+
+	/* calculate the max phys_id */
+	for_each_present_cpu(i) {
+		struct cpuinfo_x86 *c = &cpu_data(i);
+		if (phys_id < c->phys_proc_id)
+			phys_id = c->phys_proc_id;
+	}
+
+	c->phys_proc_id = phys_id + 1; /* pick up a unused phys_proc_id */
+	c->cpu_core_id = 0; /* always put the logical cpu to core 0 */
+	c->cpu_probe_on = 1;
+}
+
+static void clear_cpu_socket_info(int cpu)
+{
+	struct cpuinfo_x86 *c = &cpu_data(cpu);
+	c->phys_proc_id = 0;
+	c->cpu_core_id = 0;
+	c->cpu_probe_on = 0;
+}
+
+
 ssize_t arch_cpu_probe(const char *buf, size_t count)
 {
 	int nid = 0;

@@ -109,6 +139,7 @@
 	/* register cpu */
 	arch_register_cpu_node(selected, nid);
 	acpi_map_lsapic_emu(selected, nid);
+	fake_cpu_socket_info(selected);
 
 	return count;
 }

@@ -132,10 +163,13 @@
 
 	arch_unregister_cpu(cpu);
 	acpi_unmap_lsapic(cpu);
+	clear_cpu_socket_info(cpu);
+	set_cpu_present(cpu, true);
 
 	return count;
 }
 EXPORT_SYMBOL(arch_cpu_release);
+#endif CONFIG_ARCH_CPU_PROBE_RELEASE
 
 #else /* CONFIG_HOTPLUG_CPU */
 

Thanks & Regards,
Shaohui


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From: Shaohui Zheng <[email protected]>

Add mem_hotplug/add_memory interface to support to memory hotplug emulation. the reserved memory can be added into desired node with this interface.

Add a memory section(128M) to node 3(boots with mem=1024m)

	echo 0x40000000,3 > mem_hotplug/add_memory

And more we make it friendly, it is possible to add memory to do

	echo 3g > mem_hotplug/add_memory
	echo 1024m,3 > mem_hotplug/add_memory

Another format suggested by Dave Hansen:

	echo physical_address=0x40000000 numa_node=3 > mem_hotplug/add_memory

it is more explicit to show meaning of the parameters.

---

Index: linux-hpe4/mm/memory_hotplug.c
===================================================================
@@ -983,4 +983,87 @@
 }
 
 module_init(node_debug_init);
+
+#ifdef CONFIG_ARCH_MEMORY_PROBE
+
+static ssize_t add_memory_store(struct file *file, const char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	u64 phys_addr = 0;
+	int nid = 0;
+	int ret;
+	char *p = NULL, *q = NULL;
+	/* format: physical_address=0x40000000 numa_node=3 */
+	p = strchr(buf, '=');
+	if (p != NULL) {
+		*p = '\0';
+		q = strchr(buf, ' ');
+		if (q == NULL) {
+			if (strcmp(buf, "physical_address") != 0)
+				ret = -EPERM;
+			else
+				phys_addr = memparse(p+1, NULL);
+		} else {
+			*q++ = '\0';
+			p = strchr(q, '=');
+			if (strcmp(buf, "physical_address") == 0)
+				phys_addr = memparse(p+1, NULL);
+			if (strcmp(buf, "numa_node") == 0)
+				nid = simple_strtoul(p+1, NULL, 0);
+			if (strcmp(q, "physical_address") == 0)
+				phys_addr = memparse(p+1, NULL);
+			if (strcmp(q, "numa_node") == 0)
+				nid = simple_strtoul(p+1, NULL, 0);
+		}
+	} else { /* physical_address,numa_node */
+		p = strchr(buf, ',');
+		if (p != NULL && strlen(p+1) > 0) {
+			/* nid specified */
+			*p++ = '\0';
+			nid = simple_strtoul(p, NULL, 0);
+			phys_addr = memparse(buf, NULL);
+		} else {
+			phys_addr = memparse(buf, NULL);
+			nid = memory_add_physaddr_to_nid(phys_addr);
+		}
+	}
+
+	if (nid < 0 || nid > nr_node_ids - 1) {
+		printk(KERN_ERR "Invalid node id %d(0<=nid<%d).\n", nid, nr_node_ids);
+		ret = -EPERM;
+	} else {
+		printk(KERN_INFO "Add a memory section to node: %d.\n", nid);
+		ret = add_memory(nid, phys_addr, PAGES_PER_SECTION << PAGE_SHIFT);
+		if (ret)
+			count = ret;
+	}
+
+	if (ret)
+		count = ret;
+
+	return count;
+}
+
+static const struct file_operations add_memory_file_ops = {
+	.write		= add_memory_store,
+	.llseek		= generic_file_llseek,
+};
+
+static int __init memory_debug_init(void)
+{
+	if (!memhp_debug_root)
+		memhp_debug_root = debugfs_create_dir("mem_hotplug", NULL);
+	if (!memhp_debug_root)
+		return -ENOMEM;
+
+	if (!debugfs_create_file("add_memory", S_IWUSR, memhp_debug_root,
+			NULL, &add_memory_file_ops))
+		return -ENOMEM;
+
+	return 0;
+}
+
+module_init(memory_debug_init);
+
+#endif /* CONFIG_ARCH_MEMORY_PROBE */
 #endif /* CONFIG_DEBUG_FS */

Index: linux-hpe4/Documentation/memory-hotplug.txt
===================================================================
@@ -19,6 +19,7 @@
   4.1 Hardware(Firmware) Support
   4.2 Notify memory hot-add event by hand
   4.3 Node hotplug emulation
+  4.4 Memory hotplug emulation
 5. Logical Memory hot-add phase
   5.1. State of memory
   5.2. How to online memory

@@ -239,6 +240,30 @@
 Once the new node has been added, it is possible to online the memory by
 toggling the "state" of its memory section(s) as described in section 5.1.
 
+4.4 Memory hotplug emulation
+------------
+With debugfs, it is possible to test memory hotplug with software method, we
+can add memory section to desired node with add_memory interface. It is a much
+more powerful interface than "probe" described in section 4.2.
+
+The add_memory interface is located at "mem_hotplug/add_memory" at the debugfs
+mount point.
+
+Add a memory section(128M) to node 3(boots with mem=1024m)
+
+	echo 0x40000000,3 > mem_hotplug/add_memory
+
+And more we make it friendly, it is possible to add memory to do
+
+	echo 3g > mem_hotplug/add_memory
+	echo 1024m,3 > mem_hotplug/add_memory
+
+Another format suggested by Dave Hansen:
+
+	echo physical_address=0x40000000 numa_node=3 > mem_hotplug/add_memory
+
+Once the new memory section has been added, it is possible to online the memory
+by toggling the "state" described in section 5.1.
 
 ------------------------------
 5. Logical Memory hot-add phase

Thanks & Regards,
Shaohui


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NACK, we don't need such convoluted definitions if debugfs were extended with per-node triggers to add_memory as I suggested in v6 of your proposal:

	/sys/kernel/debug/mem_hotplug/add_node (already exists)
	/sys/kernel/debug/mem_hotplug/node0/add_memory
	/sys/kernel/debug/mem_hotplug/node1/add_memory
	...

You can then write a physical starting address to the add_memory files to hotadd memory to a node other than the one to which it has physical affinity. This is much more extendable if we add additional per-node triggers later.

It would also be helpful if you were to reach consensus on the matters under discussion before posting a new version of your patchset everyday.

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After consider your proposal again, the new add_memory interface under each nodes follow the rule "one file one parameter" better. it make the parser much simpler.

I work out a patch for this proposal, and it works on my side. Any more comments?

Subject: NUMA Hotplug Emulator: Implement add_memory debugfs interface

From: Shaohui Zheng <[email protected]>

Add add_memory interface to support to memory hotplug emulation for each online node under debugfs. The reserved memory can be added into desired node with this interface.

The layout on debufs:

	mem_hotplug/node0/add_memory
	mem_hotplug/node1/add_memory
	mem_hotplug/node2/add_memory
	...

Add a memory section(128M) to node 3(boots with mem=1024m)

	echo 0x40000000 > mem_hotplug/node3/add_memory

And more we make it friendly, it is possible to add memory to do

echo 1024m > mem_hotplug/node3/add_memory

---

Index: linux-hpe4/mm/memory_hotplug.c
===================================================================
@@ -930,6 +930,80 @@
 
 static struct dentry *memhp_debug_root;
 
+#ifdef CONFIG_ARCH_MEMORY_PROBE
+
+static ssize_t add_memory_store(struct file *file, const char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	u64 phys_addr = 0;
+	int nid = file->private_data - NULL;
+	int ret;
+
+	phys_addr = simple_strtoull(buf, NULL, 0);
+	printk(KERN_INFO "Add a memory section to node: %d.\n", nid);
+	phys_addr = memparse(buf, NULL);
+	ret = add_memory(nid, phys_addr, PAGES_PER_SECTION << PAGE_SHIFT);
+
+	if (ret)
+		count = ret;
+
+	return count;
+}
+
+static int add_memory_open(struct inode *inode, struct file *file)
+{
+	file->private_data = inode->i_private;
+	return 0;
+}
+
+static const struct file_operations add_memory_file_ops = {
+	.open		= add_memory_open,
+	.write		= add_memory_store,
+	.llseek		= generic_file_llseek,
+};
+
+/*
+ * Create add_memory debugfs entry under specified node
+ */
+static int debugfs_create_add_memory_entry(int nid)
+{
+	char buf[32];
+	static struct dentry *node_debug_root;
+
+	snprintf(buf, sizeof(buf), "node%d", nid);
+	node_debug_root = debugfs_create_dir(buf, memhp_debug_root);
+
+	/* the nid information was represented by the offset of pointer(NULL+nid) */
+	if (!debugfs_create_file("add_memory", S_IWUSR, node_debug_root,
+			NULL + nid, &add_memory_file_ops))
+		return -ENOMEM;
+
+	return 0;
+}
+
+static int __init memory_debug_init(void)
+{
+	int nid;
+
+	if (!memhp_debug_root)
+		memhp_debug_root = debugfs_create_dir("mem_hotplug", NULL);
+	if (!memhp_debug_root)
+		return -ENOMEM;
+
+	for_each_online_node(nid)
+		 debugfs_create_add_memory_entry(nid);
+
+	return 0;
+}
+
+module_init(memory_debug_init);
+#else
+static debugfs_create_add_memory_entry(int nid)
+{
+	return 0;
+}
+#endif /* CONFIG_ARCH_MEMORY_PROBE */
+
 static ssize_t add_node_store(struct file *file, const char __user *buf,
 				size_t count, loff_t *ppos)
 {

@@ -960,6 +1034,8 @@
 		return -ENOMEM;
 
 	ret = add_memory(nid, start, size);
+
+	debugfs_create_add_memory_entry(nid);
 	return ret ? ret : count;
 }
 

Index: linux-hpe4/Documentation/memory-hotplug.txt
===================================================================
@@ -19,6 +19,7 @@
   4.1 Hardware(Firmware) Support
   4.2 Notify memory hot-add event by hand
   4.3 Node hotplug emulation
+  4.4 Memory hotplug emulation
 5. Logical Memory hot-add phase
   5.1. State of memory
   5.2. How to online memory

@@ -239,6 +240,29 @@
 Once the new node has been added, it is possible to online the memory by
 toggling the "state" of its memory section(s) as described in section 5.1.
 
+4.4 Memory hotplug emulation
+------------
+With debugfs, it is possible to test memory hotplug with software method, we
+can add memory section to desired node with add_memory interface. It is a much
+more powerful interface than "probe" described in section 4.2.
+
+There is an add_memory interface for each online node at the debugfs mount
+point.
+	mem_hotplug/node0/add_memory
+	mem_hotplug/node1/add_memory
+	mem_hotplug/node2/add_memory
+	...
+
+Add a memory section(128M) to node 3(boots with mem=1024m)
+
+	echo 0x40000000 > mem_hotplug/node3/add_memory
+
+And more we make it friendly, it is possible to add memory to do
+
+	echo 1024m > mem_hotplug/node3/add_memory
+
+Once the new memory section has been added, it is possible to online the memory
+by toggling the "state" described in section 5.1.
 
 ------------------------------
 5. Logical Memory hot-add phase

Thanks & Regards,
Shaohui