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