mirror of https://gitee.com/openkylin/linux.git
Documentation: Update CPU hotplug and move it to core-api
The current CPU hotplug is outdated. During the update to what we currently have I rewrote it partly and moved to sphinx format. Cc: Jonathan Corbet <corbet@lwn.net> Cc: Mauro Carvalho Chehab <mchehab@kernel.org> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com> Cc: Ashok Raj <ashok.raj@intel.com> Cc: Joel Schopp <jschopp@austin.ibm.com> Cc: linux-doc@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Sebastian Andrzej Siewior
Jonathan Corbet
parent
df31175bb4
commit
ff58fa7f55
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=========================
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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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@ -13,6 +13,7 @@ Core utilities
|
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assoc_array
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atomic_ops
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cpu_hotplug
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local_ops
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workqueue
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|
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|
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@ -1,452 +0,0 @@
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CPU hotplug Support in Linux(tm) Kernel
|
||||
|
||||
Maintainers:
|
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CPU Hotplug Core:
|
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Rusty Russell <rusty@rustcorp.com.au>
|
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Srivatsa Vaddagiri <vatsa@in.ibm.com>
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i386:
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Zwane Mwaikambo <zwanem@gmail.com>
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ppc64:
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Nathan Lynch <nathanl@austin.ibm.com>
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||||
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. It's set in __cpu_up()
|
||||
after a CPU is available for kernel scheduling and ready to receive
|
||||
interrupts from devices. It's 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 is offline and 1 when it's 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 send 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 are 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 it's 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 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
|
||||
Loading…
Reference in New Issue