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diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt deleted file mode 100644 index dd68821c22d4..000000000000 --- a/Documentation/cpu-hotplug.txt +++ /dev/null @@ -1,452 +0,0 @@ - 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 <zwanem@gmail.com> - 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(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_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 /sys - #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 -cpu0_hotplug. - -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 -continue. -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 -machines. - -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 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 foobar_cpu_notifier = - { - .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 - do this: - - for_each_online_cpu(i) { - foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); - foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i); - } - - However, if you want to register a hotplug callback, as well as perform - some initialization for CPUs that are already online, then do this: - - Version 1: (Correct) - --------- - - cpu_notifier_register_begin(); - - for_each_online_cpu(i) { - foobar_cpu_callback(&foobar_cpu_notifier, - CPU_UP_PREPARE, i); - foobar_cpu_callback(&foobar_cpu_notifier, - CPU_ONLINE, i); - } - - /* Note the use of the double underscored version of the API */ - __register_cpu_notifier(&foobar_cpu_notifier); - - cpu_notifier_register_done(); - - Note that the following code is *NOT* the right way to achieve this, - because it is prone to an ABBA deadlock between the cpu_add_remove_lock - and the cpu_hotplug.lock. - - Version 2: (Wrong!) - --------- - - get_online_cpus(); - - for_each_online_cpu(i) { - foobar_cpu_callback(&foobar_cpu_notifier, - CPU_UP_PREPARE, i); - foobar_cpu_callback(&foobar_cpu_notifier, - CPU_ONLINE, i); - } - - register_cpu_notifier(&foobar_cpu_notifier); - - put_online_cpus(); - - So always use the first version shown above when you want to register - callbacks as well as initialize the already online CPUs. - - -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: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c - or earlier ACPI version supported, because the apicid field in MADT is only - 8 bits. From ACPI 3.0, this limitation was removed since the apicid field - was extended to 32 bits with x2APIC introduced. - -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 |