373 lines
16 KiB
ReStructuredText
373 lines
16 KiB
ReStructuredText
=========================
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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 <bigeasy@linutronix.de>,
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Rusty Russell <rusty@rustcorp.com.au>,
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Srivatsa Vaddagiri <vatsa@in.ibm.com>,
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Ashok Raj <ashok.raj@intel.com>,
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Joel Schopp <jschopp@austin.ibm.com>
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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 fourV 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 CPUs the kernel will support. If the number
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supplied here is lower than the number of physically available CPUs than
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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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``cede_offline={"off","on"}``
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Use this option to disable/enable putting offlined processors to an extended
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``H_CEDE`` state on supported pseries platforms. If nothing is specified,
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``cede_offline`` is set to "on".
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This option is limited to the PowerPC 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 <fenghua.yu@intel.com> 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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