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365 lines
16 KiB
365 lines
16 KiB
========================= |
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CPU hotplug in the Kernel |
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========================= |
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:Date: December, 2016 |
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:Author: Sebastian Andrzej Siewior <[email protected]>, |
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Rusty Russell <[email protected]>, |
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Srivatsa Vaddagiri <[email protected]>, |
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Ashok Raj <[email protected]>, |
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Joel Schopp <[email protected]> |
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Introduction |
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============ |
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Modern advances in system architectures have introduced advanced error |
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reporting and correction capabilities in processors. There are couple OEMS that |
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support NUMA hardware which are hot pluggable as well, where physical node |
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insertion and removal require support for CPU hotplug. |
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Such advances require CPUs available to a kernel to be removed either for |
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provisioning reasons, or for RAS purposes to keep an offending CPU off |
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system execution path. Hence the need for CPU hotplug support in the |
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Linux kernel. |
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A more novel use of CPU-hotplug support is its use today in suspend resume |
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support for SMP. Dual-core and HT support makes even a laptop run SMP kernels |
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which didn't support these methods. |
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Command Line Switches |
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===================== |
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``maxcpus=n`` |
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Restrict boot time CPUs to *n*. Say if you have four CPUs, using |
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``maxcpus=2`` will only boot two. You can choose to bring the |
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other CPUs later online. |
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``nr_cpus=n`` |
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Restrict the total amount of CPUs the kernel will support. If the number |
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supplied here is lower than the number of physically available CPUs, then |
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those CPUs can not be brought online later. |
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``additional_cpus=n`` |
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Use this to limit hotpluggable CPUs. This option sets |
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``cpu_possible_mask = cpu_present_mask + additional_cpus`` |
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This option is limited to the IA64 architecture. |
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``possible_cpus=n`` |
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This option sets ``possible_cpus`` bits in ``cpu_possible_mask``. |
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This option is limited to the X86 and S390 architecture. |
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``cpu0_hotplug`` |
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Allow to shutdown CPU0. |
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This option is limited to the X86 architecture. |
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CPU maps |
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======== |
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``cpu_possible_mask`` |
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Bitmap of possible CPUs that can ever be available in the |
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system. This is used to allocate some boot time memory for per_cpu variables |
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that aren't designed to grow/shrink as CPUs are made available or removed. |
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Once set during boot time discovery phase, the map is static, i.e no bits |
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are added or removed anytime. Trimming it accurately for your system needs |
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upfront can save some boot time memory. |
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``cpu_online_mask`` |
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Bitmap of all CPUs currently online. Its set in ``__cpu_up()`` |
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after a CPU is available for kernel scheduling and ready to receive |
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interrupts from devices. Its cleared when a CPU is brought down using |
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``__cpu_disable()``, before which all OS services including interrupts are |
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migrated to another target CPU. |
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``cpu_present_mask`` |
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Bitmap of CPUs currently present in the system. Not all |
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of them may be online. When physical hotplug is processed by the relevant |
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subsystem (e.g ACPI) can change and new bit either be added or removed |
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from the map depending on the event is hot-add/hot-remove. There are currently |
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no locking rules as of now. Typical usage is to init topology during boot, |
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at which time hotplug is disabled. |
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You really don't need to manipulate any of the system CPU maps. They should |
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be read-only for most use. When setting up per-cpu resources almost always use |
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``cpu_possible_mask`` or ``for_each_possible_cpu()`` to iterate. To macro |
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``for_each_cpu()`` can be used to iterate over a custom CPU mask. |
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Never use anything other than ``cpumask_t`` to represent bitmap of CPUs. |
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Using CPU hotplug |
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================= |
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The kernel option *CONFIG_HOTPLUG_CPU* needs to be enabled. It is currently |
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available on multiple architectures including ARM, MIPS, PowerPC and X86. The |
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configuration is done via the sysfs interface: :: |
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$ ls -lh /sys/devices/system/cpu |
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total 0 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu0 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu1 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu2 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu3 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu4 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu5 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu6 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu7 |
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drwxr-xr-x 2 root root 0 Dec 21 16:33 hotplug |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 offline |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 online |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 possible |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 present |
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The files *offline*, *online*, *possible*, *present* represent the CPU masks. |
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Each CPU folder contains an *online* file which controls the logical on (1) and |
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off (0) state. To logically shutdown CPU4: :: |
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$ echo 0 > /sys/devices/system/cpu/cpu4/online |
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smpboot: CPU 4 is now offline |
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Once the CPU is shutdown, it will be removed from */proc/interrupts*, |
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*/proc/cpuinfo* and should also not be shown visible by the *top* command. To |
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bring CPU4 back online: :: |
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$ echo 1 > /sys/devices/system/cpu/cpu4/online |
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smpboot: Booting Node 0 Processor 4 APIC 0x1 |
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The CPU is usable again. This should work on all CPUs. CPU0 is often special |
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and excluded from CPU hotplug. On X86 the kernel option |
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*CONFIG_BOOTPARAM_HOTPLUG_CPU0* has to be enabled in order to be able to |
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shutdown CPU0. Alternatively the kernel command option *cpu0_hotplug* can be |
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used. Some known dependencies of CPU0: |
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* Resume from hibernate/suspend. Hibernate/suspend will fail if CPU0 is offline. |
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* PIC interrupts. CPU0 can't be removed if a PIC interrupt is detected. |
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Please let Fenghua Yu <[email protected]> know if you find any dependencies |
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on CPU0. |
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The CPU hotplug coordination |
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============================ |
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The offline case |
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---------------- |
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Once a CPU has been logically shutdown the teardown callbacks of registered |
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hotplug states will be invoked, starting with ``CPUHP_ONLINE`` and terminating |
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at state ``CPUHP_OFFLINE``. This includes: |
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* If tasks are frozen due to a suspend operation then *cpuhp_tasks_frozen* |
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will be set to true. |
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* All processes are migrated away from this outgoing CPU to new CPUs. |
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The new CPU is chosen from each process' current cpuset, which may be |
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a subset of all online CPUs. |
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* All interrupts targeted to this CPU are migrated to a new CPU |
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* timers are also migrated to a new CPU |
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* Once all services are migrated, kernel calls an arch specific routine |
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``__cpu_disable()`` to perform arch specific cleanup. |
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Using the hotplug API |
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--------------------- |
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It is possible to receive notifications once a CPU is offline or onlined. This |
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might be important to certain drivers which need to perform some kind of setup |
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or clean up functions based on the number of available CPUs: :: |
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#include <linux/cpuhotplug.h> |
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ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "X/Y:online", |
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Y_online, Y_prepare_down); |
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*X* is the subsystem and *Y* the particular driver. The *Y_online* callback |
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will be invoked during registration on all online CPUs. If an error |
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occurs during the online callback the *Y_prepare_down* callback will be |
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invoked on all CPUs on which the online callback was previously invoked. |
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After registration completed, the *Y_online* callback will be invoked |
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once a CPU is brought online and *Y_prepare_down* will be invoked when a |
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CPU is shutdown. All resources which were previously allocated in |
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*Y_online* should be released in *Y_prepare_down*. |
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The return value *ret* is negative if an error occurred during the |
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registration process. Otherwise a positive value is returned which |
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contains the allocated hotplug for dynamically allocated states |
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(*CPUHP_AP_ONLINE_DYN*). It will return zero for predefined states. |
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The callback can be remove by invoking ``cpuhp_remove_state()``. In case of a |
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dynamically allocated state (*CPUHP_AP_ONLINE_DYN*) use the returned state. |
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During the removal of a hotplug state the teardown callback will be invoked. |
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Multiple instances |
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~~~~~~~~~~~~~~~~~~ |
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If a driver has multiple instances and each instance needs to perform the |
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callback independently then it is likely that a ''multi-state'' should be used. |
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First a multi-state state needs to be registered: :: |
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ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, "X/Y:online, |
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Y_online, Y_prepare_down); |
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Y_hp_online = ret; |
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The ``cpuhp_setup_state_multi()`` behaves similar to ``cpuhp_setup_state()`` |
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except it prepares the callbacks for a multi state and does not invoke |
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the callbacks. This is a one time setup. |
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Once a new instance is allocated, you need to register this new instance: :: |
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ret = cpuhp_state_add_instance(Y_hp_online, &d->node); |
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This function will add this instance to your previously allocated |
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*Y_hp_online* state and invoke the previously registered callback |
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(*Y_online*) on all online CPUs. The *node* element is a ``struct |
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hlist_node`` member of your per-instance data structure. |
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On removal of the instance: :: |
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cpuhp_state_remove_instance(Y_hp_online, &d->node) |
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should be invoked which will invoke the teardown callback on all online |
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CPUs. |
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Manual setup |
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~~~~~~~~~~~~ |
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Usually it is handy to invoke setup and teardown callbacks on registration or |
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removal of a state because usually the operation needs to performed once a CPU |
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goes online (offline) and during initial setup (shutdown) of the driver. However |
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each registration and removal function is also available with a ``_nocalls`` |
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suffix which does not invoke the provided callbacks if the invocation of the |
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callbacks is not desired. During the manual setup (or teardown) the functions |
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``get_online_cpus()`` and ``put_online_cpus()`` should be used to inhibit CPU |
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hotplug operations. |
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The ordering of the events |
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-------------------------- |
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The hotplug states are defined in ``include/linux/cpuhotplug.h``: |
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* The states *CPUHP_OFFLINE* … *CPUHP_AP_OFFLINE* are invoked before the |
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CPU is up. |
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* The states *CPUHP_AP_OFFLINE* … *CPUHP_AP_ONLINE* are invoked |
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just the after the CPU has been brought up. The interrupts are off and |
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the scheduler is not yet active on this CPU. Starting with *CPUHP_AP_OFFLINE* |
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the callbacks are invoked on the target CPU. |
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* The states between *CPUHP_AP_ONLINE_DYN* and *CPUHP_AP_ONLINE_DYN_END* are |
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reserved for the dynamic allocation. |
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* The states are invoked in the reverse order on CPU shutdown starting with |
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*CPUHP_ONLINE* and stopping at *CPUHP_OFFLINE*. Here the callbacks are |
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invoked on the CPU that will be shutdown until *CPUHP_AP_OFFLINE*. |
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A dynamically allocated state via *CPUHP_AP_ONLINE_DYN* is often enough. |
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However if an earlier invocation during the bring up or shutdown is required |
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then an explicit state should be acquired. An explicit state might also be |
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required if the hotplug event requires specific ordering in respect to |
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another hotplug event. |
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Testing of hotplug states |
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========================= |
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One way to verify whether a custom state is working as expected or not is to |
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shutdown a CPU and then put it online again. It is also possible to put the CPU |
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to certain state (for instance *CPUHP_AP_ONLINE*) and then go back to |
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*CPUHP_ONLINE*. This would simulate an error one state after *CPUHP_AP_ONLINE* |
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which would lead to rollback to the online state. |
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All registered states are enumerated in ``/sys/devices/system/cpu/hotplug/states``: :: |
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$ tail /sys/devices/system/cpu/hotplug/states |
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138: mm/vmscan:online |
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139: mm/vmstat:online |
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140: lib/percpu_cnt:online |
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141: acpi/cpu-drv:online |
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142: base/cacheinfo:online |
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143: virtio/net:online |
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144: x86/mce:online |
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145: printk:online |
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168: sched:active |
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169: online |
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To rollback CPU4 to ``lib/percpu_cnt:online`` and back online just issue: :: |
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$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
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169 |
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$ echo 140 > /sys/devices/system/cpu/cpu4/hotplug/target |
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$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
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140 |
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It is important to note that the teardown callbac of state 140 have been |
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invoked. And now get back online: :: |
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$ echo 169 > /sys/devices/system/cpu/cpu4/hotplug/target |
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$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
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169 |
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With trace events enabled, the individual steps are visible, too: :: |
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# TASK-PID CPU# TIMESTAMP FUNCTION |
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# | | | | | |
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bash-394 [001] 22.976: cpuhp_enter: cpu: 0004 target: 140 step: 169 (cpuhp_kick_ap_work) |
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cpuhp/4-31 [004] 22.977: cpuhp_enter: cpu: 0004 target: 140 step: 168 (sched_cpu_deactivate) |
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cpuhp/4-31 [004] 22.990: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0 |
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cpuhp/4-31 [004] 22.991: cpuhp_enter: cpu: 0004 target: 140 step: 144 (mce_cpu_pre_down) |
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cpuhp/4-31 [004] 22.992: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0 |
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cpuhp/4-31 [004] 22.993: cpuhp_multi_enter: cpu: 0004 target: 140 step: 143 (virtnet_cpu_down_prep) |
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cpuhp/4-31 [004] 22.994: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0 |
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cpuhp/4-31 [004] 22.995: cpuhp_enter: cpu: 0004 target: 140 step: 142 (cacheinfo_cpu_pre_down) |
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cpuhp/4-31 [004] 22.996: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0 |
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bash-394 [001] 22.997: cpuhp_exit: cpu: 0004 state: 140 step: 169 ret: 0 |
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bash-394 [005] 95.540: cpuhp_enter: cpu: 0004 target: 169 step: 140 (cpuhp_kick_ap_work) |
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cpuhp/4-31 [004] 95.541: cpuhp_enter: cpu: 0004 target: 169 step: 141 (acpi_soft_cpu_online) |
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cpuhp/4-31 [004] 95.542: cpuhp_exit: cpu: 0004 state: 141 step: 141 ret: 0 |
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cpuhp/4-31 [004] 95.543: cpuhp_enter: cpu: 0004 target: 169 step: 142 (cacheinfo_cpu_online) |
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cpuhp/4-31 [004] 95.544: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0 |
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cpuhp/4-31 [004] 95.545: cpuhp_multi_enter: cpu: 0004 target: 169 step: 143 (virtnet_cpu_online) |
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cpuhp/4-31 [004] 95.546: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0 |
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cpuhp/4-31 [004] 95.547: cpuhp_enter: cpu: 0004 target: 169 step: 144 (mce_cpu_online) |
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cpuhp/4-31 [004] 95.548: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0 |
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cpuhp/4-31 [004] 95.549: cpuhp_enter: cpu: 0004 target: 169 step: 145 (console_cpu_notify) |
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cpuhp/4-31 [004] 95.550: cpuhp_exit: cpu: 0004 state: 145 step: 145 ret: 0 |
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cpuhp/4-31 [004] 95.551: cpuhp_enter: cpu: 0004 target: 169 step: 168 (sched_cpu_activate) |
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cpuhp/4-31 [004] 95.552: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0 |
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bash-394 [005] 95.553: cpuhp_exit: cpu: 0004 state: 169 step: 140 ret: 0 |
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As it an be seen, CPU4 went down until timestamp 22.996 and then back up until |
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95.552. All invoked callbacks including their return codes are visible in the |
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trace. |
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Architecture's requirements |
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=========================== |
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The following functions and configurations are required: |
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``CONFIG_HOTPLUG_CPU`` |
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This entry needs to be enabled in Kconfig |
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``__cpu_up()`` |
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Arch interface to bring up a CPU |
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``__cpu_disable()`` |
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Arch interface to shutdown a CPU, no more interrupts can be handled by the |
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kernel after the routine returns. This includes the shutdown of the timer. |
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``__cpu_die()`` |
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This actually supposed to ensure death of the CPU. Actually look at some |
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example code in other arch that implement CPU hotplug. The processor is taken |
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down from the ``idle()`` loop for that specific architecture. ``__cpu_die()`` |
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typically waits for some per_cpu state to be set, to ensure the processor dead |
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routine is called to be sure positively. |
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User Space Notification |
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======================= |
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After CPU successfully onlined or offline udev events are sent. A udev rule like: :: |
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SUBSYSTEM=="cpu", DRIVERS=="processor", DEVPATH=="/devices/system/cpu/*", RUN+="the_hotplug_receiver.sh" |
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will receive all events. A script like: :: |
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#!/bin/sh |
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if [ "${ACTION}" = "offline" ] |
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then |
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echo "CPU ${DEVPATH##*/} offline" |
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elif [ "${ACTION}" = "online" ] |
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then |
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echo "CPU ${DEVPATH##*/} online" |
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fi |
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can process the event further. |
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Kernel Inline Documentations Reference |
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====================================== |
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.. kernel-doc:: include/linux/cpuhotplug.h
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