Documentation for CPU hotplug support

Documentation for CPU hotplug support

[Ashok Raj]

Hi Andrew

CPU hotplug support didnt have a readme for general help.

Thanks to Andi for some feedback. 

Consider for -mm so if people have updates they can do when it gets
up the tree.

I added authors as much as i can remember, if roles have changed, please
send updates to this document.

-- 
Cheers,
Ashok Raj
- Open Source Technology Center


First cut Documentation update for cpu hotplug.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
--------------------------------------------------------
 Documentation/cpu-hotplug.txt |  269 ++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 269 insertions(+)

Index: linux-2.6.14-mm1/Documentation/cpu-hotplug.txt
===================================================================
--- /dev/null
+++ linux-2.6.14-mm1/Documentation/cpu-hotplug.txt
@@ -0,0 +1,269 @@
+					CPU hotplug Support in Linux(tm) Kernel
+
+Authors:
+
+General Core Infrastructure:
+
+Rusty Russell: 		rusty@rustycorp.com.au
+Srivatsa Vaddagiri: vatsa@in.ibm.com
+
+i386:
+Zwane:				zwane@arm.linux.org.uk
+
+ppc64:
+Joel Schopp:		jschopp@austin.ibm.com
+Nathan Lynch:		nathanl@austin.ibm.com
+
+
+ia64/x86_64:
+Ashok Raj			ashok.raj@intel.com
+Anil (ACPI support)	anil.s.keshavamurthy@intel.com
+
+Introduction
+
+	Modern advances in system architectures have introduced advanced error
+reporting and correction capabilities in processors. CPU architectures permit
+partitioning support, where compute resources of a single CPU could be made
+available to virtual machine environments. There are couple OEMS that
+support NUMA hardware which are hot pluggable as well, where physical
+node insertion and removal require CPU hotplug support.
+
+Such advances require CPU available to a kernel to be removed either for
+provisioning reasons, or for RAS purposes to keep an offending CPU off
+system execution path. Hence the need for CPU hotplug support in the
+Linux kernel.
+
+A more novel use of CPU-hotplug support is its use in suspend
+resume support for SMP. Dual-core and HT support makes even
+a laptop run SMP kernels which didn't support these methods. SMP support
+for suspend/resume is a work in progress. This is a work in progress in
+ACPI for now.
+
+General Stuff about CPU Hotplug
+--------------------------------
+
+cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
+system. This is used to allocated some boot time memory for per_cpu variables
+that aren't designed to grow/shrink as CPUs are made available or removed.
+Trimming it accurately for your system needs upfront can save some
+boot time memory. See below for how we use heuristics in x86_64 case
+to keep this under check.
+
+cpu_online_map: Bitmap of all CPUs currently online.
+
+cpu_present_map: Bitmap of CPUs currently present in the system. Not all
+of them may be online. One could choose to start with maxcpus=n boot time
+parameter to limit the number of boot time CPUs.
+
+When manipulating with cpu maps, please use one of the appropriate
+ones already defined.
+
+Never use anything other than cpumask_t to represent
+bitmap of CPUs.
+
+#include <linux/cpumask.h>
+
+for_each_cpu			- Iterate over cpu_possible_map
+for_each_online_cpu		- Iterate over cpu_online_map
+for_each_present_cpu	- Iterate over cpu_present_map
+for_each_cpu_mask(x,mask)	- Iterate over some random collection of cpu mask.
+
+#include <linux/cpu.h>
+lock_cpu_hotplug() and unlock_cpu_hotplug():
+Used to stop cpu hotplug from being started.
+
+CPU Hotplug - Frequently Asked Questions.
+
+Q: How to i enable my kernel to support CPU hotplug?
+A: When doing make defconfig, Enable CPU hotplug support
+
+   "Processor type and Features" -> Support for Hotpluggable CPUs
+
+Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
+
+You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
+as well.
+
+Q: What architectures support CPU hotplug?
+A: As of 2.6.14, the following architectures support CPU hotplug.
+
+i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
+
+Q: How to test if hotplug is supported on the newly built kernel?
+A: You should now notice an entry in sysfs.
+
+Check if sysfs is mounted, using the "mount" command. You should notice
+an entry as shown below in the output.
+
+....
+none on /sys type sysfs (rw)
+....
+
+if this is not mounted, do the following.
+
+#mkdir /sysfs
+#mount -t sysfs sys /sys
+
+now you should see entries for all present cpu, the following is an example
+in a 8-way system.
+
+#pwd
+#/sys/devices/system/cpu
+#ls -l
+total 0
+drwxr-xr-x  10 root root 0 Sep 19 07:44 .
+drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
+drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
+
+Under each directory you would find an "online" file which is the control
+file to logically online/offline a processor.
+
+Q: What is logical CPU hotplug and Physical CPU hotplug?
+A: Logical CPU hotplug just removes the presence of the CPU from the
+kernel image. Physical CPU hotplug refers to the electrical isolation
+of the CPU from the system. For support of physical hotplug there needs
+to be a method such as ACPI that interfaces with the system BIOS to
+pass notification of removal to kernel, such as the Attention button in
+PCI based hotplug.
+
+Q: How do i logically offline a CPU?
+A: Do the following.
+
+#echo 0 > /sys/devices/system/cpu/cpuX/online
+
+once the logical offline is successful, check
+
+#cat /proc/interrupts
+
+you should now not see the CPU that you removed.
+
+Q: Why cant i remove CPU0 on some systems?
+A: Some architectures may have some special dependency on a certain CPU.
+
+For e.g in IA64 platforms we have ability to sent platform interrupts to the
+OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
+specifications, we dint have a way to change the target CPU. Hence if the
+current ACPI version doesn't support such re-direction, we disable that CPU
+by making it not-removable.
+
+Q: How do i find out if a particular CPU is not removable?
+A: Depending on the implementation, some architectures may show this by the
+absence of the "online" file. This is done if it can be determined ahead of
+time that this CPU cannot be removed.
+
+In some situations, this can be a run time check, i.e if you try to remove the
+last CPU, this will not be permitted. You can find such failures by
+investigating the return value of the "echo" command.
+
+Q: What happens when a CPU is being logically offlined?
+A: The following happen, listed in no particular order :-)
+
+- A notification is sent to in-kernel registered modules by sending an event
+  CPU_DOWN_PREPARE
+- All process is migrated away from this outgoing CPU to a new CPU
+- All interrupts targeted to this CPU is migrated to a new CPU
+- timers/bottom half/task lets are also migrated to a new CPU
+- Once all services are migrated, kernel calls an arch specific routine
+  __cpu_disable() to perform arch specific cleanup.
+- Once this is successful, an event for successful cleanup is sent by an event
+  CPU_DEAD.
+
+  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
+  notifier is called, when CPU_DEAD is called its expected there is nothing
+  running on behalf of this CPU that was offlined"
+
+Q: If i have some kernel code that needs to be aware of CPU arrival and
+   departure, how to i arrange for proper notification?
+A: This is what you would need in your kernel code to receive notifications.
+
+    #include <linux/cpu.h>
+	static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
+								unsigned long action, void *hcpu)
+	{
+		unsigned int cpu = (unsigned long)hcpu;
+
+		switch (action) {
+		case CPU_ONLINE:
+			foobar_online_action(cpu);
+			break;
+		case CPU_DEAD:
+			foobar_dead_action(cpu);
+			break;
+		}
+		return NOTIFY_OK;
+	}
+
+	static struct notifier_block foobar_cpu_notifer =
+	{
+	   .notifier_call = foobar_cpu_callback,
+	};
+
+
+In your init function,
+
+	register_cpu_notifier(&foobar_cpu_notifier);
+
+You can fail PREPARE notifiers if something doesn't work to prepare resources.
+This will stop the activity and send a following CANCELED event back.
+
+CPU_DEAD should not be failed, its just a goodness indication, but bad
+things will happen if a notifier in path sent a BAD notify code.
+
+Q: I don't see my action being called for all CPUs already up and running?
+A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
+   If you need to perform some action for each cpu already in the system, then
+
+  for_each_online_cpu(i) {
+		foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
+		foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
+  }
+
+Q: If i would like to develop cpu hotplug support for a new architecture,
+   what do i need at a minimum?
+A: The following are what is required for CPU hotplug infrastructure to work
+   correctly.
+
+    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
+	- __cpu_up() 		- Arch interface to bring up a CPU
+	- __cpu_disable()	- Arch interface to shutdown a CPU, no more interrupts
+						  can be handled by the kernel after the routine
+						  returns. Including local APIC timers etc are
+						  shutdown.
+	- __cpu_die()		- This actually supposed to ensure death of the CPU.
+						  Actually look at some example code in other arch
+						  that implement CPU hotplug. The processor is taken
+						  down from the idle() loop for that specific
+						  architecture. __cpu_die() typically waits for some
+						  per_cpu state to be set, to ensure the processor
+						  dead routine is called to be sure positively.
+
+Q: I need to ensure the a particular cpu is not removed when there is some
+   work specific to this cpu is in progress.
+A: Take a look at drivers/cpufreq/cpufreq.c. Typically you would acquire
+   the cpucontrol lock via lock_cpu_hotplug() and unlock_cpu_hotplug().
+
+   When you are in the notification callbacks, you don't need to acquire the
+   lock. use the current_in_hotplug() call to verify is the current
+   thread is performing the hotplug functions. This will ensure you don't
+   end up with deadlocks trying to acquire cpucontrol while its already held
+   by the current caller.
+
+Q: How do we determine how many CPUs are available for hotplug.
+A: There is no clear spec defined way from ACPI that can give us that
+   information today. Based on some input from Natalie of Unisys,
+   that the MADT ACPI tables marks those possible CPUs in a system with
+   disabled status.
+
+   Andi implemented some simple heuristics that count the number of disabled
+   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
+   we assume 1/2 the number of CPUs currently present can be hotplugged.
+
+   Caveat: Today's MADT can only provide 256 entries since the apicid field
+   in MADT is only 8 bits.

Re: Documentation for CPU hotplug support

[Joel Schopp]

Ashok Raj wrote:
> Hi Andrew
> 
> CPU hotplug support didnt have a readme for general help.
> 
> Thanks to Andi for some feedback. 
> 
> Consider for -mm so if people have updates they can do when it gets
> up the tree.
> 
> I added authors as much as i can remember, if roles have changed, please
> send updates to this document.
> 

While far from complete (no mention of userspace notification for instance), it 
is a very good start.

Acked-by: Joel Schopp <jschopp@austin.ibm.com>

Re: Documentation for CPU hotplug support

[Nathan Lynch]

Hi Ashok-

Thanks a lot for doing this; it's a good start.  Some feedback and
minor corrections below:


Ashok Raj wrote:
> Index: linux-2.6.14-mm1/Documentation/cpu-hotplug.txt
> ===================================================================
> --- /dev/null
> +++ linux-2.6.14-mm1/Documentation/cpu-hotplug.txt
> @@ -0,0 +1,269 @@
> +					CPU hotplug Support in Linux(tm) Kernel
> +
> +Authors:
> +
> +General Core Infrastructure:
> +
> +Rusty Russell: 		rusty@rustycorp.com.au
> +Srivatsa Vaddagiri: vatsa@in.ibm.com
> +
> +i386:
> +Zwane:				zwane@arm.linux.org.uk
> +
> +ppc64:
> +Joel Schopp:		jschopp@austin.ibm.com
> +Nathan Lynch:		nathanl@austin.ibm.com
> +
> +
> +ia64/x86_64:
> +Ashok Raj			ashok.raj@intel.com
> +Anil (ACPI support)	anil.s.keshavamurthy@intel.com
> +
> +Introduction
> +
> +	Modern advances in system architectures have introduced advanced error
> +reporting and correction capabilities in processors. CPU architectures permit
> +partitioning support, where compute resources of a single CPU could be made
> +available to virtual machine environments. There are couple OEMS that
> +support NUMA hardware which are hot pluggable as well, where physical
> +node insertion and removal require CPU hotplug support.
> +
> +Such advances require CPU available to a kernel to be removed either for
> +provisioning reasons, or for RAS purposes to keep an offending CPU off
> +system execution path. Hence the need for CPU hotplug support in the
> +Linux kernel.
> +
> +A more novel use of CPU-hotplug support is its use in suspend
> +resume support for SMP. Dual-core and HT support makes even
> +a laptop run SMP kernels which didn't support these methods. SMP support
> +for suspend/resume is a work in progress. This is a work in progress in
> +ACPI for now.
> +
> +General Stuff about CPU Hotplug
> +--------------------------------
> +
> +cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
> +system. This is used to allocated some boot time memory for per_cpu variables
> +that aren't designed to grow/shrink as CPUs are made available or removed.
> +Trimming it accurately for your system needs upfront can save some
> +boot time memory. See below for how we use heuristics in x86_64 case
> +to keep this under check.

It think it would be appropriate to stress that cpu_possible_map must
not change after boot.


> +
> +cpu_online_map: Bitmap of all CPUs currently online.

I think a little discussion of what it means for a cpu to be marked
online is in order.  For instance, it is available for running tasks
and servicing interrupts.


> +
> +cpu_present_map: Bitmap of CPUs currently present in the system. Not all
> +of them may be online. One could choose to start with maxcpus=n boot time
> +parameter to limit the number of boot time CPUs.

Shouldn't the maxcpus bit be somewhere else?  maxcpus shouldn't affect
the setting of cpu_present_map.  Maybe it should go with the online
map section.

Also mention that cpu_present_map may change during the system's
lifetime (have we worked out the locking rules for that?)

> +
> +When manipulating with cpu maps, please use one of the appropriate
> +ones already defined.

In general, most code should not manipulate or directly access cpu
maps at all, especially not cpu_online_map or cpu_present_map.
Per-cpu resources should almost always be set up according to
cpu_possible_map/for_each_cpu.


> +
> +Never use anything other than cpumask_t to represent
> +bitmap of CPUs.
> +
> +#include <linux/cpumask.h>
> +
> +for_each_cpu			- Iterate over cpu_possible_map
> +for_each_online_cpu		- Iterate over cpu_online_map
> +for_each_present_cpu	- Iterate over cpu_present_map
> +for_each_cpu_mask(x,mask)	- Iterate over some random collection of cpu mask.
> +
> +#include <linux/cpu.h>
> +lock_cpu_hotplug() and unlock_cpu_hotplug():
> +Used to stop cpu hotplug from being started.

Used to inhibit cpu hotplug operations.  While the cpucontrol
mutex is held, cpu_online_map will not change.

If you merely need to keep any cpus from going offline, disabling
preemption in your critical section is sufficient.  This is true as
long as cpu_down is implemented with stop_machine_run.

> +
> +CPU Hotplug - Frequently Asked Questions.
> +
> +Q: How to i enable my kernel to support CPU hotplug?
> +A: When doing make defconfig, Enable CPU hotplug support
> +
> +   "Processor type and Features" -> Support for Hotpluggable CPUs
> +
> +Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
> +
> +You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
> +as well.
> +
> +Q: What architectures support CPU hotplug?
> +A: As of 2.6.14, the following architectures support CPU hotplug.
> +
> +i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
> +
> +Q: How to test if hotplug is supported on the newly built kernel?
> +A: You should now notice an entry in sysfs.
> +
> +Check if sysfs is mounted, using the "mount" command. You should notice
> +an entry as shown below in the output.
> +
> +....
> +none on /sys type sysfs (rw)
> +....
> +
> +if this is not mounted, do the following.
> +
> +#mkdir /sysfs
> +#mount -t sysfs sys /sys

Eh, I think the mounting sysfs stuff is superfluous these days, but
that's a nitpick.

> +
> +now you should see entries for all present cpu, the following is an example
> +in a 8-way system.
> +
> +#pwd
> +#/sys/devices/system/cpu
> +#ls -l
> +total 0
> +drwxr-xr-x  10 root root 0 Sep 19 07:44 .
> +drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
> +drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
> +drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
> +
> +Under each directory you would find an "online" file which is the control
> +file to logically online/offline a processor.
> +
> +Q: What is logical CPU hotplug and Physical CPU hotplug?
> +A: Logical CPU hotplug just removes the presence of the CPU from the
> +kernel image.

Well... I think the "hotplug" term is getting overused and is possibly
confusing here.  I'd say it is more precise to distinguish between
online/offline operations and addition/removal.


> ... Physical CPU hotplug refers to the electrical isolation
> +of the CPU from the system. For support of physical hotplug there needs
> +to be a method such as ACPI that interfaces with the system BIOS to
> +pass notification of removal to kernel, such as the Attention button in
> +PCI based hotplug.
> +
> +Q: How do i logically offline a CPU?
> +A: Do the following.
> +
> +#echo 0 > /sys/devices/system/cpu/cpuX/online
> +
> +once the logical offline is successful, check
> +
> +#cat /proc/interrupts
> +
> +you should now not see the CPU that you removed.

Or cat /sys/devices/system/cpu/cpuX/online, it should print 0.

> +
> +Q: Why cant i remove CPU0 on some systems?
> +A: Some architectures may have some special dependency on a certain CPU.
> +
> +For e.g in IA64 platforms we have ability to sent platform interrupts to the
> +OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
> +specifications, we dint have a way to change the target CPU. Hence if the
> +current ACPI version doesn't support such re-direction, we disable that CPU
> +by making it not-removable.
> +
> +Q: How do i find out if a particular CPU is not removable?
> +A: Depending on the implementation, some architectures may show this by the
> +absence of the "online" file. This is done if it can be determined ahead of
> +time that this CPU cannot be removed.
> +
> +In some situations, this can be a run time check, i.e if you try to remove the
> +last CPU, this will not be permitted. You can find such failures by
> +investigating the return value of the "echo" command.
> +
> +Q: What happens when a CPU is being logically offlined?
> +A: The following happen, listed in no particular order :-)
> +- A notification is sent to in-kernel registered modules by sending an event
> +  CPU_DOWN_PREPARE
> +- All process is migrated away from this outgoing CPU to a new CPU
> +- All interrupts targeted to this CPU is migrated to a new CPU
> +- timers/bottom half/task lets are also migrated to a new CPU
> +- Once all services are migrated, kernel calls an arch specific routine
> +  __cpu_disable() to perform arch specific cleanup.
> +- Once this is successful, an event for successful cleanup is sent by an event
> +  CPU_DEAD.
> +
> +  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
> +  notifier is called, when CPU_DEAD is called its expected there is nothing
> +  running on behalf of this CPU that was offlined"
> +
> +Q: If i have some kernel code that needs to be aware of CPU arrival and
> +   departure, how to i arrange for proper notification?
> +A: This is what you would need in your kernel code to receive notifications.
> +
> +    #include <linux/cpu.h>
> +	static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
> +								unsigned long action, void *hcpu)
> +	{
> +		unsigned int cpu = (unsigned long)hcpu;
> +
> +		switch (action) {
> +		case CPU_ONLINE:
> +			foobar_online_action(cpu);
> +			break;
> +		case CPU_DEAD:
> +			foobar_dead_action(cpu);
> +			break;
> +		}
> +		return NOTIFY_OK;
> +	}
> +
> +	static struct notifier_block foobar_cpu_notifer =
> +	{
> +	   .notifier_call = foobar_cpu_callback,
> +	};
> +
> +
> +In your init function,
> +
> +	register_cpu_notifier(&foobar_cpu_notifier);
> +
> +You can fail PREPARE notifiers if something doesn't work to prepare resources.
> +This will stop the activity and send a following CANCELED event back.
> +
> +CPU_DEAD should not be failed, its just a goodness indication, but bad
> +things will happen if a notifier in path sent a BAD notify code.
> +
> +Q: I don't see my action being called for all CPUs already up and running?
> +A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
> +   If you need to perform some action for each cpu already in the system, then
> +
> +  for_each_online_cpu(i) {
> +		foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
> +		foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
> +  }
> +
> +Q: If i would like to develop cpu hotplug support for a new architecture,
> +   what do i need at a minimum?
> +A: The following are what is required for CPU hotplug infrastructure to work
> +   correctly.
> +
> +    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
> +	- __cpu_up() 		- Arch interface to bring up a CPU
> +	- __cpu_disable()	- Arch interface to shutdown a CPU, no more interrupts
> +						  can be handled by the kernel after the routine
> +						  returns. Including local APIC timers etc are
> +						  shutdown.
> +	- __cpu_die()		- This actually supposed to ensure death of the CPU.
> +						  Actually look at some example code in other arch
> +						  that implement CPU hotplug. The processor is taken
> +						  down from the idle() loop for that specific
> +						  architecture. __cpu_die() typically waits for some
> +						  per_cpu state to be set, to ensure the processor
> +						  dead routine is called to be sure positively.

Formatting?  This section doesn't fit in my terminal.


> +
> +Q: I need to ensure the a particular cpu is not removed when there is some
> +   work specific to this cpu is in progress.
> +A: Take a look at drivers/cpufreq/cpufreq.c. Typically you would acquire
> +   the cpucontrol lock via lock_cpu_hotplug() and unlock_cpu_hotplug().
> +
> +   When you are in the notification callbacks, you don't need to acquire the
> +   lock. use the current_in_hotplug() call to verify is the current
> +   thread is performing the hotplug functions. This will ensure you don't
> +   end up with deadlocks trying to acquire cpucontrol while its already held
> +   by the current caller.

Argh, no.  That current_in_cpuhotplug hack has to go.

And actually, the A: to that particular Q: should be "disable
preemption in your critical section".


> +Q: How do we determine how many CPUs are available for hotplug.
> +A: There is no clear spec defined way from ACPI that can give us that
> +   information today. Based on some input from Natalie of Unisys,
> +   that the MADT ACPI tables marks those possible CPUs in a system with
> +   disabled status.
> +
> +   Andi implemented some simple heuristics that count the number of disabled
> +   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
> +   we assume 1/2 the number of CPUs currently present can be hotplugged.
> +
> +   Caveat: Today's MADT can only provide 256 entries since the apicid field
> +   in MADT is only 8 bits.

Re: Documentation for CPU hotplug support

[Paul Jackson]

Nathan wrote:
> It think it would be appropriate to stress that cpu_possible_map must
> not change after boot.

This and some other aspects of cpu_online_map, cpu_possible_map and
cpu_present_map are documented in a long comment in cpumask.h.

You are welcome to steal from or reference that comment.

You are also welcome to fix up any lies you find in that comment ;).

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401

Re: Documentation for CPU hotplug support

[Andrew Morton]

Nathan Lynch <nathanl@austin.ibm.com> wrote:
>
>  Argh, no.  That current_in_cpuhotplug hack has to go.

Yes, Ashok is busily working on removing that ;)

We shouldn't enshrine it in documentation.  Please.

Re: Documentation for CPU hotplug support

[Zwane Mwaikambo]

On Fri, 11 Nov 2005, Andrew Morton wrote:

> Nathan Lynch <nathanl@austin.ibm.com> wrote:
> >
> >  Argh, no.  That current_in_cpuhotplug hack has to go.
> 
> Yes, Ashok is busily working on removing that ;)

Ashok was my patch for the cpufreq driver *that* horrible? Or perhaps we 
just need to move things like the set_cpus_allowed further up in the calls 
and handle everything in one location. Interested?

Thanks,
	Zwane

Re: Documentation for CPU hotplug support

[Chris Wright]

* Andrew Morton (akpm@osdl.org) wrote:
> Nathan Lynch <nathanl@austin.ibm.com> wrote:
> >
> >  Argh, no.  That current_in_cpuhotplug hack has to go.
> 
> Yes, Ashok is busily working on removing that ;)

That's nice.  As it is, it deadlocks on boot:

cpufreq_stats_init
  lock_cpu_hotplug
  cpufreq_stat_cpu_callback
  ... (trace below)
  cpufreq_governor_performance
  __cpufreq_driver_target
  lock_cpu_hotplug  <-- uh-oh

SysRq : Show State

                                               sibling
  task             PC      pid father child younger older
swapper       D 003D08BC   764     1      0     2 			(L-TLB)
       eff8ed78 ef832030 c184a5f0 003d08bc effc5180 000000d0 df810b00 003d08bc
       00000002 ef832030 c184a5a0 00000000 df810b00 003d08bc ef832030 eff8dab0
       eff8dbd8 c0637140 c0637148 00000282 eff8edb0 c058bad3 eff8dab0 00000001
Call Trace:
 [<c058bad3>] __down+0x76/0xde
 [<c058a4aa>] __down_failed+0xa/0x10
 [<c04609ef>] .text.lock.cpufreq+0xee/0x1ff
 [<c0461049>] cpufreq_governor_performance+0x5d/0x74
 [<c045ff1c>] __cpufreq_governor+0x70/0x106
 [<c04603d8>] __cpufreq_set_policy+0x214/0x2b9
 [<c04605db>] cpufreq_update_policy+0xca/0xf1
 [<c0460fd5>] cpufreq_stat_cpu_callback+0x27/0x3e
 [<c06f657d>] cpufreq_stats_init+0xf0/0x152
 [<c06d59c2>] do_initcalls+0x56/0xba
 [<c06d5a4a>] do_basic_setup+0x24/0x2a
 [<c01003cf>] init+0xb5/0x1fe
 [<c010148d>] kernel_thread_helper+0x5/0xb

Re: Documentation for CPU hotplug support

[Ashok Raj]

On Thu, Nov 10, 2005 at 11:23:00PM -0800, Nathan Lynch wrote:
> 
>    Hi Ashok-
> 
>    Thanks a lot for doing this; it's a good start.  Some feedback and
>    minor corrections below:
> 

Thanks Nathan for the feedback. Beleive it has most of feedback i 
got todate. user space side is still missing, i will try to add it 
after some time once i have right data.

- spell checked (maybe ;-))
- Added some more code snips
- Some more details liks available_cpus in x86_64.

Andrew: Could you help queue this for next update, so we will have some
starting point.

CPU Hotplug Documentation writeup.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
------------------------------------------------------------
 Documentation/cpu-hotplug.txt |  315 ++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 315 insertions(+)

Index: linux-2.6.15-rc1-mm2/Documentation/cpu-hotplug.txt
===================================================================
--- /dev/null
+++ linux-2.6.15-rc1-mm2/Documentation/cpu-hotplug.txt
@@ -0,0 +1,315 @@
+		CPU hotplug Support in Linux(tm) Kernel
+
+
+Introduction
+
+Modern advances in system architectures have introduced advanced error
+reporting and correction capabilities in processors. CPU architectures permit
+partitioning support, where compute resources of a single CPU could be made
+available to virtual machine environments. There are couple OEMS that
+support NUMA hardware which are hot pluggable as well, where physical
+node insertion and removal require support for CPU hotplug.
+
+Such advances require CPUs available to a kernel to be removed either for
+provisioning reasons, or for RAS purposes to keep an offending CPU off
+system execution path. Hence the need for CPU hotplug support in the
+Linux kernel.
+
+A more novel use of CPU-hotplug support is its use today in suspend
+resume support for SMP. Dual-core and HT support makes even
+a laptop run SMP kernels which didn't support these methods. SMP support
+for suspend/resume is a work in progress.
+
+General Stuff about CPU Hotplug
+--------------------------------
+
+Command Line Switches
+---------------------
+maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
+             maxcpus=2 will only boot 2. You can choose to bring the
+             other cpus later online, read FAQ's for more info.
+
+additional_cpus=n	[x86_64 only] use this to limit hotpluggable cpus.
+                        This option sets
+			cpu_possible_map = cpu_present_map + additional_cpus
+
+CPU maps and such
+-----------------
+[More on cpumaps and primitive to manipulate, please check
+include/linux/cpumask.h that has more descriptive text.]
+
+cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
+system. This is used to allocate some boot time memory for per_cpu variables
+that aren't designed to grow/shrink as CPUs are made available or removed.
+Once set during boot time discovery phase, the map is static, i.e no bits
+are added or removed anytime.  Trimming it accurately for your system needs
+upfront can save some boot time memory. See below for how we use heuristics
+in x86_64 case to keep this under check.
+
+cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
+after a cpu is available for kernel scheduling and ready to receive
+interrupts from devices. Its cleared when a cpu is brought down using
+__cpu_disable(), before which all OS services including interrupts are
+migrated to another target CPU.
+
+cpu_present_map: Bitmap of CPUs currently present in the system. Not all
+of them may be online. When physical hotplug is processed by the relevant
+subsystem (e.g ACPI) can change and new bit either be added or removed
+from the map depending on the event is hot-add/hot-remove. There are currently
+no locking rules as of now. Typical usage is to init topology during boot,
+at which time hotplug is disabled.
+
+You really dont need to manipulate any of the system cpu maps. They should
+be read-only for most use. When setting up per-cpu resources almost always use
+cpu_possible_map/for_each_cpu() to iterate.
+
+Never use anything other than cpumask_t to represent bitmap of CPUs.
+
+#include <linux/cpumask.h>
+
+for_each_cpu              - Iterate over cpu_possible_map
+for_each_online_cpu       - Iterate over cpu_online_map
+for_each_present_cpu      - Iterate over cpu_present_map
+for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
+
+#include <linux/cpu.h>
+lock_cpu_hotplug() and unlock_cpu_hotplug():
+
+The above calls are used to inhibit cpu hotplug operations. While holding the
+cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
+cpus going away, you could also use preempt_disable() and preempt_enable()
+for those sections. Just remember the critical section cannot call any
+function that can sleep or schedule this process away. The preempt_disable()
+will work as long as stop_machine_run() is used to take a cpu down.
+
+CPU Hotplug - Frequently Asked Questions.
+
+Q: How to i enable my kernel to support CPU hotplug?
+A: When doing make defconfig, Enable CPU hotplug support
+
+   "Processor type and Features" -> Support for Hotpluggable CPUs
+
+Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
+
+You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
+as well.
+
+Q: What architectures support CPU hotplug?
+A: As of 2.6.14, the following architectures support CPU hotplug.
+
+i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
+
+Q: How to test if hotplug is supported on the newly built kernel?
+A: You should now notice an entry in sysfs.
+
+Check if sysfs is mounted, using the "mount" command. You should notice
+an entry as shown below in the output.
+
+....
+none on /sys type sysfs (rw)
+....
+
+if this is not mounted, do the following.
+
+#mkdir /sysfs
+#mount -t sysfs sys /sys
+
+now you should see entries for all present cpu, the following is an example
+in a 8-way system.
+
+#pwd
+#/sys/devices/system/cpu
+#ls -l
+total 0
+drwxr-xr-x  10 root root 0 Sep 19 07:44 .
+drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
+drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
+drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
+
+Under each directory you would find an "online" file which is the control
+file to logically online/offline a processor.
+
+Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
+A: The usage of hot-add/remove may not be very consistently used in the code.
+CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel.
+To support physical addition/removal, one would need some BIOS hooks and
+the platform should have something like an attention button in PCI hotplug.
+CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
+
+Q: How do i logically offline a CPU?
+A: Do the following.
+
+#echo 0 > /sys/devices/system/cpu/cpuX/online
+
+once the logical offline is successful, check
+
+#cat /proc/interrupts
+
+you should now not see the CPU that you removed. Also online file will report
+the state as 0 when a cpu if offline and 1 when its online.
+
+#To display the current cpu state.
+#cat /sys/devices/system/cpu/cpuX/online
+
+Q: Why cant i remove CPU0 on some systems?
+A: Some architectures may have some special dependency on a certain CPU.
+
+For e.g in IA64 platforms we have ability to sent platform interrupts to the
+OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
+specifications, we didn't have a way to change the target CPU. Hence if the
+current ACPI version doesn't support such re-direction, we disable that CPU
+by making it not-removable.
+
+In such cases you will also notice that the online file is missing under cpu0.
+
+Q: How do i find out if a particular CPU is not removable?
+A: Depending on the implementation, some architectures may show this by the
+absence of the "online" file. This is done if it can be determined ahead of
+time that this CPU cannot be removed.
+
+In some situations, this can be a run time check, i.e if you try to remove the
+last CPU, this will not be permitted. You can find such failures by
+investigating the return value of the "echo" command.
+
+Q: What happens when a CPU is being logically offlined?
+A: The following happen, listed in no particular order :-)
+
+- A notification is sent to in-kernel registered modules by sending an event
+  CPU_DOWN_PREPARE
+- All process is migrated away from this outgoing CPU to a new CPU
+- All interrupts targeted to this CPU is migrated to a new CPU
+- timers/bottom half/task lets are also migrated to a new CPU
+- Once all services are migrated, kernel calls an arch specific routine
+  __cpu_disable() to perform arch specific cleanup.
+- Once this is successful, an event for successful cleanup is sent by an event
+  CPU_DEAD.
+
+  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
+  notifier is called, when CPU_DEAD is called its expected there is nothing
+  running on behalf of this CPU that was offlined"
+
+Q: If i have some kernel code that needs to be aware of CPU arrival and
+   departure, how to i arrange for proper notification?
+A: This is what you would need in your kernel code to receive notifications.
+
+    #include <linux/cpu.h>
+	static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
+								unsigned long action, void *hcpu)
+	{
+		unsigned int cpu = (unsigned long)hcpu;
+
+		switch (action) {
+		case CPU_ONLINE:
+			foobar_online_action(cpu);
+			break;
+		case CPU_DEAD:
+			foobar_dead_action(cpu);
+			break;
+		}
+		return NOTIFY_OK;
+	}
+
+	static struct notifier_block foobar_cpu_notifer =
+	{
+	   .notifier_call = foobar_cpu_callback,
+	};
+
+
+In your init function,
+
+	register_cpu_notifier(&foobar_cpu_notifier);
+
+You can fail PREPARE notifiers if something doesn't work to prepare resources.
+This will stop the activity and send a following CANCELED event back.
+
+CPU_DEAD should not be failed, its just a goodness indication, but bad
+things will happen if a notifier in path sent a BAD notify code.
+
+Q: I don't see my action being called for all CPUs already up and running?
+A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
+   If you need to perform some action for each cpu already in the system, then
+
+  for_each_online_cpu(i) {
+		foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
+		foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
+  }
+
+Q: If i would like to develop cpu hotplug support for a new architecture,
+   what do i need at a minimum?
+A: The following are what is required for CPU hotplug infrastructure to work
+   correctly.
+
+    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
+    - __cpu_up()        - Arch interface to bring up a CPU
+    - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
+                          can be handled by the kernel after the routine
+                          returns. Including local APIC timers etc are
+                          shutdown.
+     - __cpu_die()      - This actually supposed to ensure death of the CPU.
+                          Actually look at some example code in other arch
+                          that implement CPU hotplug. The processor is taken
+                          down from the idle() loop for that specific
+                          architecture. __cpu_die() typically waits for some
+                          per_cpu state to be set, to ensure the processor
+                          dead routine is called to be sure positively.
+
+Q: I need to ensure that a particular cpu is not removed when there is some
+   work specific to this cpu is in progress.
+A: First switch the current thread context to preferred cpu
+
+   int my_func_on_cpu(int cpu)
+   {
+       cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
+       int curr_cpu, err = 0;
+
+       saved_mask = current->cpus_allowed;
+       cpu_set(cpu, new_mask);
+       err = set_cpus_allowed(current, new_mask);
+
+       if (err)
+           return err;
+
+       /*
+        * If we got scheduled out just after the return from
+        * set_cpus_allowed() before running the work, this ensures
+        * we stay locked.
+        */
+       curr_cpu = get_cpu();
+
+       if (curr_cpu != cpu) {
+	   err = -EAGAIN;
+           goto ret;
+       } else {
+       	   /*
+	    * Do work : But cant sleep, since get_cpu() disables preempt
+	    */
+       }
+    ret:
+    	put_cpu();
+	set_cpus_allowed(current, saved_mask);
+	return err;
+    }
+
+
+Q: How do we determine how many CPUs are available for hotplug.
+A: There is no clear spec defined way from ACPI that can give us that
+   information today. Based on some input from Natalie of Unisys,
+   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
+   CPUs in a system with disabled status.
+
+   Andi implemented some simple heuristics that count the number of disabled
+   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
+   we assume 1/2 the number of CPUs currently present can be hotplugged.
+
+   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
+   in MADT is only 8 bits.
+
+User Space Notification
+
+TBD:

RE: Documentation for CPU hotplug support

[Raj, Ashok]

>
>Ashok was my patch for the cpufreq driver *that* horrible? Or perhaps
we
>just need to move things like the set_cpus_allowed further up in the
calls
>and handle everything in one location. Interested?
>

I have been on to multiple things recently, I think I saw your post, but
didn't look at it closer.

Yes, moving things higher up would definitely help, especially the
set_cpus_allowed(). 

We should also do the same for the case where we have the list of
dependent cpus in the mask before calling the lower level functions. 

Zwane, if you want to take a shot at it, that would be awesome.. I might
not be able to get to this immediately.

RE: Documentation for CPU hotplug support

[Zwane Mwaikambo]

On Sat, 12 Nov 2005, Raj, Ashok wrote:

> >
> >Ashok was my patch for the cpufreq driver *that* horrible? Or perhaps
> we
> >just need to move things like the set_cpus_allowed further up in the
> calls
> >and handle everything in one location. Interested?
> >
> 
> I have been on to multiple things recently, I think I saw your post, but
> didn't look at it closer.
> 
> Yes, moving things higher up would definitely help, especially the
> set_cpus_allowed(). 
> 
> We should also do the same for the case where we have the list of
> dependent cpus in the mask before calling the lower level functions. 
> 
> Zwane, if you want to take a shot at it, that would be awesome.. I might
> not be able to get to this immediately.

Urgh making it generic turned out to be fairly complicated due to the 
number of sleeping calls done from the lowlevel drivers and handling the 
disabling and enabling of preemption just got too ugly.