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+ CPU hotplug Support in Linux(tm) Kernel
+ Maintainers:
+ CPU Hotplug Core:
+ Rusty Russell <rusty@rustcorp.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>
+ s390:
+ Heiko Carstens <heiko.carstens@de.ibm.com>
+Authors: Ashok Raj <ashok.raj@intel.com>
+Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
+ Joel Schopp <jschopp@austin.ibm.com>
+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 (*) Use this to limit hotpluggable cpus. This option sets
+ cpu_possible_mask = cpu_present_mask + additional_cpus
+cede_offline={"off","on"} Use this option to disable/enable putting offlined
+ processors to an extended H_CEDE state on
+ supported pseries platforms.
+ If nothing is specified,
+ cede_offline is set to "on".
+(*) Option valid only for following architectures
+- ia64
+ia64 uses the number of disabled local apics in ACPI tables MADT to
+determine the number of potentially hot-pluggable cpus. The implementation
+should only rely on this to count the # of cpus, but *MUST* not rely
+on the apicid values in those tables for disabled apics. In the event
+BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
+use this parameter "additional_cpus=x" to represent those cpus in the
+possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
+ This option sets possible_cpus bits in
+ cpu_possible_mask. Thus keeping the numbers of bits set
+ constant even if the machine gets rebooted.
+CPU maps and such
+[More on cpumaps and primitive to manipulate, please check
+include/linux/cpumask.h that has more descriptive text.]
+cpu_possible_mask: 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_mask: 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_mask: 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_mask/for_each_possible_cpu() to iterate.
+Never use anything other than cpumask_t to represent bitmap of CPUs.
+ #include <linux/cpumask.h>
+ for_each_possible_cpu - Iterate over cpu_possible_mask
+ for_each_online_cpu - Iterate over cpu_online_mask
+ for_each_present_cpu - Iterate over cpu_present_mask
+ for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
+ #include <linux/cpu.h>
+ get_online_cpus() and put_online_cpus():
+The above calls are used to inhibit cpu hotplug operations. While the
+cpu_hotplug.refcount is non zero, the cpu_online_mask 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 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_HOTPLUG_CPU 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 can't 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: Is CPU0 removable on X86?
+A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
+removable by default. Otherwise, CPU0 is also removable by kernel option
+But some features depend on CPU0. Two known dependencies are:
+1. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
+CPU0 is offline and you need to online CPU0 before hibernate/suspend can
+2. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
+is detected.
+It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
+seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
+Please let me know if you know or see any other dependencies of CPU0.
+If the dependencies are under your control, you can turn on CPU0 hotplug feature
+either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
+--Fenghua Yu <fenghua.yu@intel.com>
+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 or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
+ CPU is being offlined while tasks are frozen due to a suspend operation in
+ progress
+- All processes are migrated away from this outgoing CPU to new CPUs.
+ The new CPU is chosen from each process' current cpuset, which may be
+ a subset of all online CPUs.
+- 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 (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
+ CPU is being offlined).
+ "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 __cpuinitdata foobar_cpu_notifer =
+ {
+ .notifier_call = foobar_cpu_callback,
+ };
+You need to call register_cpu_notifier() from your init function.
+Init functions could be of two types:
+1. early init (init function called when only the boot processor is online).
+2. late init (init function called _after_ all the CPUs are online).
+For the first case, you should add the following to your init function
+ register_cpu_notifier(&foobar_cpu_notifier);
+For the second case, you should add the following to your init function
+ register_hotcpu_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: There are two ways. If your code can be run in interrupt context, use
+ smp_call_function_single(), otherwise use work_on_cpu(). Note that
+ work_on_cpu() is slow, and can fail due to out of memory:
+ int my_func_on_cpu(int cpu)
+ {
+ int err;
+ get_online_cpus();
+ if (!cpu_online(cpu))
+ err = -EINVAL;
+ else
+ err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
+ smp_call_function_single(cpu, __my_func_on_cpu, &err,
+ true);
+ put_online_cpus();
+ 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
+ #!/bin/bash
+ # $Id: cpu.agent
+ # Kernel hotplug params include:
+ #ACTION=%s [online or offline]
+ #
+ 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