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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>
-
-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 (*) 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
-cpu_possible_mask.
-
-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: 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:
- case CPU_ONLINE_FROZEN:
- foobar_online_action(cpu);
- break;
- case CPU_DEAD:
- case CPU_DEAD_FROZEN:
- 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
-#if NEEDS_BLOCKING
- err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
-#else
- smp_call_function_single(cpu, __my_func_on_cpu, &err,
- true);
-#endif
- 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
-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