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+                CPU hotplug Support in Linux(tm) Kernel
+
+                Maintainers:
+                CPU Hotplug Core:
+                        Rusty Russell <rusty@rustycorp.com.au>
+                        Srivatsa Vaddagiri <vatsa@in.ibm.com>
+                i386:
+                        Zwane Mwaikambo <zwane@arm.linux.org.uk>
+                ppc64:
+                        Nathan Lynch <nathanl@austin.ibm.com>
+                        Joel Schopp <jschopp@austin.ibm.com>
+                ia64/x86_64:
+                        Ashok Raj <ashok.raj@intel.com>
+
+Authors: Ashok Raj <ashok.raj@intel.com>
+Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
+             Joel Schopp <jschopp@austin.ibm.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 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
+
+Hotplug support for devices is common in Linux today. Its being used today to
+support automatic configuration of network, usb and pci devices. A hotplug
+event can be used to invoke an agent script to perform the configuration task.
+
+You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
+scripts.
+
+        #!/bin/bash
+        # $Id: cpu.agent
+        # Kernel hotplug params include:
+        #ACTION=%s [online or offline]
+        #DEVPATH=%s
+        #
+        cd /etc/hotplug
+        . ./hotplug.functions
+
+        case $ACTION in
+                online)
+                        echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
+                        ;;
+                offline)
+                        echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
+                        ;;
+                *)
+                        debug_mesg CPU $ACTION event not supported
+        exit 1
+        ;;
+        esac