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diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt new file mode 100644 index 000000000000..08c5d04f3086 --- /dev/null +++ b/Documentation/cpu-hotplug.txt @@ -0,0 +1,357 @@ + 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 |