mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-29 20:30:35 +00:00
![Linus Torvalds](/assets/img/avatar_default.png)
("refactor Kconfig to consolidate KEXEC and CRASH options"). - kernel.h slimming work from Andy Shevchenko ("kernel.h: Split out a couple of macros to args.h"). - gdb feature work from Kuan-Ying Lee ("Add GDB memory helper commands"). - vsprintf inclusion rationalization from Andy Shevchenko ("lib/vsprintf: Rework header inclusions"). - Switch the handling of kdump from a udev scheme to in-kernel handling, by Eric DeVolder ("crash: Kernel handling of CPU and memory hot un/plug"). - Many singleton patches to various parts of the tree -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQTTMBEPP41GrTpTJgfdBJ7gKXxAjgUCZO2GpAAKCRDdBJ7gKXxA juW3AQD1moHzlSN6x9I3tjm5TWWNYFoFL8af7wXDJspp/DWH/AD/TO0XlWWhhbYy QHy7lL0Syha38kKLMXTM+bN6YQHi9AU= =WJQa -----END PGP SIGNATURE----- Merge tag 'mm-nonmm-stable-2023-08-28-22-48' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Pull non-MM updates from Andrew Morton: - An extensive rework of kexec and crash Kconfig from Eric DeVolder ("refactor Kconfig to consolidate KEXEC and CRASH options") - kernel.h slimming work from Andy Shevchenko ("kernel.h: Split out a couple of macros to args.h") - gdb feature work from Kuan-Ying Lee ("Add GDB memory helper commands") - vsprintf inclusion rationalization from Andy Shevchenko ("lib/vsprintf: Rework header inclusions") - Switch the handling of kdump from a udev scheme to in-kernel handling, by Eric DeVolder ("crash: Kernel handling of CPU and memory hot un/plug") - Many singleton patches to various parts of the tree * tag 'mm-nonmm-stable-2023-08-28-22-48' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (81 commits) document while_each_thread(), change first_tid() to use for_each_thread() drivers/char/mem.c: shrink character device's devlist[] array x86/crash: optimize CPU changes crash: change crash_prepare_elf64_headers() to for_each_possible_cpu() crash: hotplug support for kexec_load() x86/crash: add x86 crash hotplug support crash: memory and CPU hotplug sysfs attributes kexec: exclude elfcorehdr from the segment digest crash: add generic infrastructure for crash hotplug support crash: move a few code bits to setup support of crash hotplug kstrtox: consistently use _tolower() kill do_each_thread() nilfs2: fix WARNING in mark_buffer_dirty due to discarded buffer reuse scripts/bloat-o-meter: count weak symbol sizes treewide: drop CONFIG_EMBEDDED lockdep: fix static memory detection even more lib/vsprintf: declare no_hash_pointers in sprintf.h lib/vsprintf: split out sprintf() and friends kernel/fork: stop playing lockless games for exe_file replacement adfs: delete unused "union adfs_dirtail" definition ...
765 lines
30 KiB
ReStructuredText
765 lines
30 KiB
ReStructuredText
=========================
|
|
CPU hotplug in the Kernel
|
|
=========================
|
|
|
|
:Date: September, 2021
|
|
: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>,
|
|
Thomas Gleixner <tglx@linutronix.de>
|
|
|
|
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, but CPU0 is often special
|
|
and excluded from CPU hotplug.
|
|
|
|
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.
|
|
|
|
|
|
The CPU hotplug API
|
|
===================
|
|
|
|
CPU hotplug state machine
|
|
-------------------------
|
|
|
|
CPU hotplug uses a trivial state machine with a linear state space from
|
|
CPUHP_OFFLINE to CPUHP_ONLINE. Each state has a startup and a teardown
|
|
callback.
|
|
|
|
When a CPU is onlined, the startup callbacks are invoked sequentially until
|
|
the state CPUHP_ONLINE is reached. They can also be invoked when the
|
|
callbacks of a state are set up or an instance is added to a multi-instance
|
|
state.
|
|
|
|
When a CPU is offlined the teardown callbacks are invoked in the reverse
|
|
order sequentially until the state CPUHP_OFFLINE is reached. They can also
|
|
be invoked when the callbacks of a state are removed or an instance is
|
|
removed from a multi-instance state.
|
|
|
|
If a usage site requires only a callback in one direction of the hotplug
|
|
operations (CPU online or CPU offline) then the other not-required callback
|
|
can be set to NULL when the state is set up.
|
|
|
|
The state space is divided into three sections:
|
|
|
|
* The PREPARE section
|
|
|
|
The PREPARE section covers the state space from CPUHP_OFFLINE to
|
|
CPUHP_BRINGUP_CPU.
|
|
|
|
The startup callbacks in this section are invoked before the CPU is
|
|
started during a CPU online operation. The teardown callbacks are invoked
|
|
after the CPU has become dysfunctional during a CPU offline operation.
|
|
|
|
The callbacks are invoked on a control CPU as they can't obviously run on
|
|
the hotplugged CPU which is either not yet started or has become
|
|
dysfunctional already.
|
|
|
|
The startup callbacks are used to setup resources which are required to
|
|
bring a CPU successfully online. The teardown callbacks are used to free
|
|
resources or to move pending work to an online CPU after the hotplugged
|
|
CPU became dysfunctional.
|
|
|
|
The startup callbacks are allowed to fail. If a callback fails, the CPU
|
|
online operation is aborted and the CPU is brought down to the previous
|
|
state (usually CPUHP_OFFLINE) again.
|
|
|
|
The teardown callbacks in this section are not allowed to fail.
|
|
|
|
* The STARTING section
|
|
|
|
The STARTING section covers the state space between CPUHP_BRINGUP_CPU + 1
|
|
and CPUHP_AP_ONLINE.
|
|
|
|
The startup callbacks in this section are invoked on the hotplugged CPU
|
|
with interrupts disabled during a CPU online operation in the early CPU
|
|
setup code. The teardown callbacks are invoked with interrupts disabled
|
|
on the hotplugged CPU during a CPU offline operation shortly before the
|
|
CPU is completely shut down.
|
|
|
|
The callbacks in this section are not allowed to fail.
|
|
|
|
The callbacks are used for low level hardware initialization/shutdown and
|
|
for core subsystems.
|
|
|
|
* The ONLINE section
|
|
|
|
The ONLINE section covers the state space between CPUHP_AP_ONLINE + 1 and
|
|
CPUHP_ONLINE.
|
|
|
|
The startup callbacks in this section are invoked on the hotplugged CPU
|
|
during a CPU online operation. The teardown callbacks are invoked on the
|
|
hotplugged CPU during a CPU offline operation.
|
|
|
|
The callbacks are invoked in the context of the per CPU hotplug thread,
|
|
which is pinned on the hotplugged CPU. The callbacks are invoked with
|
|
interrupts and preemption enabled.
|
|
|
|
The callbacks are allowed to fail. When a callback fails the hotplug
|
|
operation is aborted and the CPU is brought back to the previous state.
|
|
|
|
CPU online/offline operations
|
|
-----------------------------
|
|
|
|
A successful online operation looks like this::
|
|
|
|
[CPUHP_OFFLINE]
|
|
[CPUHP_OFFLINE + 1]->startup() -> success
|
|
[CPUHP_OFFLINE + 2]->startup() -> success
|
|
[CPUHP_OFFLINE + 3] -> skipped because startup == NULL
|
|
...
|
|
[CPUHP_BRINGUP_CPU]->startup() -> success
|
|
=== End of PREPARE section
|
|
[CPUHP_BRINGUP_CPU + 1]->startup() -> success
|
|
...
|
|
[CPUHP_AP_ONLINE]->startup() -> success
|
|
=== End of STARTUP section
|
|
[CPUHP_AP_ONLINE + 1]->startup() -> success
|
|
...
|
|
[CPUHP_ONLINE - 1]->startup() -> success
|
|
[CPUHP_ONLINE]
|
|
|
|
A successful offline operation looks like this::
|
|
|
|
[CPUHP_ONLINE]
|
|
[CPUHP_ONLINE - 1]->teardown() -> success
|
|
...
|
|
[CPUHP_AP_ONLINE + 1]->teardown() -> success
|
|
=== Start of STARTUP section
|
|
[CPUHP_AP_ONLINE]->teardown() -> success
|
|
...
|
|
[CPUHP_BRINGUP_ONLINE - 1]->teardown()
|
|
...
|
|
=== Start of PREPARE section
|
|
[CPUHP_BRINGUP_CPU]->teardown()
|
|
[CPUHP_OFFLINE + 3]->teardown()
|
|
[CPUHP_OFFLINE + 2] -> skipped because teardown == NULL
|
|
[CPUHP_OFFLINE + 1]->teardown()
|
|
[CPUHP_OFFLINE]
|
|
|
|
A failed online operation looks like this::
|
|
|
|
[CPUHP_OFFLINE]
|
|
[CPUHP_OFFLINE + 1]->startup() -> success
|
|
[CPUHP_OFFLINE + 2]->startup() -> success
|
|
[CPUHP_OFFLINE + 3] -> skipped because startup == NULL
|
|
...
|
|
[CPUHP_BRINGUP_CPU]->startup() -> success
|
|
=== End of PREPARE section
|
|
[CPUHP_BRINGUP_CPU + 1]->startup() -> success
|
|
...
|
|
[CPUHP_AP_ONLINE]->startup() -> success
|
|
=== End of STARTUP section
|
|
[CPUHP_AP_ONLINE + 1]->startup() -> success
|
|
---
|
|
[CPUHP_AP_ONLINE + N]->startup() -> fail
|
|
[CPUHP_AP_ONLINE + (N - 1)]->teardown()
|
|
...
|
|
[CPUHP_AP_ONLINE + 1]->teardown()
|
|
=== Start of STARTUP section
|
|
[CPUHP_AP_ONLINE]->teardown()
|
|
...
|
|
[CPUHP_BRINGUP_ONLINE - 1]->teardown()
|
|
...
|
|
=== Start of PREPARE section
|
|
[CPUHP_BRINGUP_CPU]->teardown()
|
|
[CPUHP_OFFLINE + 3]->teardown()
|
|
[CPUHP_OFFLINE + 2] -> skipped because teardown == NULL
|
|
[CPUHP_OFFLINE + 1]->teardown()
|
|
[CPUHP_OFFLINE]
|
|
|
|
A failed offline operation looks like this::
|
|
|
|
[CPUHP_ONLINE]
|
|
[CPUHP_ONLINE - 1]->teardown() -> success
|
|
...
|
|
[CPUHP_ONLINE - N]->teardown() -> fail
|
|
[CPUHP_ONLINE - (N - 1)]->startup()
|
|
...
|
|
[CPUHP_ONLINE - 1]->startup()
|
|
[CPUHP_ONLINE]
|
|
|
|
Recursive failures cannot be handled sensibly. Look at the following
|
|
example of a recursive fail due to a failed offline operation: ::
|
|
|
|
[CPUHP_ONLINE]
|
|
[CPUHP_ONLINE - 1]->teardown() -> success
|
|
...
|
|
[CPUHP_ONLINE - N]->teardown() -> fail
|
|
[CPUHP_ONLINE - (N - 1)]->startup() -> success
|
|
[CPUHP_ONLINE - (N - 2)]->startup() -> fail
|
|
|
|
The CPU hotplug state machine stops right here and does not try to go back
|
|
down again because that would likely result in an endless loop::
|
|
|
|
[CPUHP_ONLINE - (N - 1)]->teardown() -> success
|
|
[CPUHP_ONLINE - N]->teardown() -> fail
|
|
[CPUHP_ONLINE - (N - 1)]->startup() -> success
|
|
[CPUHP_ONLINE - (N - 2)]->startup() -> fail
|
|
[CPUHP_ONLINE - (N - 1)]->teardown() -> success
|
|
[CPUHP_ONLINE - N]->teardown() -> fail
|
|
|
|
Lather, rinse and repeat. In this case the CPU left in state::
|
|
|
|
[CPUHP_ONLINE - (N - 1)]
|
|
|
|
which at least lets the system make progress and gives the user a chance to
|
|
debug or even resolve the situation.
|
|
|
|
Allocating a state
|
|
------------------
|
|
|
|
There are two ways to allocate a CPU hotplug state:
|
|
|
|
* Static allocation
|
|
|
|
Static allocation has to be used when the subsystem or driver has
|
|
ordering requirements versus other CPU hotplug states. E.g. the PERF core
|
|
startup callback has to be invoked before the PERF driver startup
|
|
callbacks during a CPU online operation. During a CPU offline operation
|
|
the driver teardown callbacks have to be invoked before the core teardown
|
|
callback. The statically allocated states are described by constants in
|
|
the cpuhp_state enum which can be found in include/linux/cpuhotplug.h.
|
|
|
|
Insert the state into the enum at the proper place so the ordering
|
|
requirements are fulfilled. The state constant has to be used for state
|
|
setup and removal.
|
|
|
|
Static allocation is also required when the state callbacks are not set
|
|
up at runtime and are part of the initializer of the CPU hotplug state
|
|
array in kernel/cpu.c.
|
|
|
|
* Dynamic allocation
|
|
|
|
When there are no ordering requirements for the state callbacks then
|
|
dynamic allocation is the preferred method. The state number is allocated
|
|
by the setup function and returned to the caller on success.
|
|
|
|
Only the PREPARE and ONLINE sections provide a dynamic allocation
|
|
range. The STARTING section does not as most of the callbacks in that
|
|
section have explicit ordering requirements.
|
|
|
|
Setup of a CPU hotplug state
|
|
----------------------------
|
|
|
|
The core code provides the following functions to setup a state:
|
|
|
|
* cpuhp_setup_state(state, name, startup, teardown)
|
|
* cpuhp_setup_state_nocalls(state, name, startup, teardown)
|
|
* cpuhp_setup_state_cpuslocked(state, name, startup, teardown)
|
|
* cpuhp_setup_state_nocalls_cpuslocked(state, name, startup, teardown)
|
|
|
|
For cases where a driver or a subsystem has multiple instances and the same
|
|
CPU hotplug state callbacks need to be invoked for each instance, the CPU
|
|
hotplug core provides multi-instance support. The advantage over driver
|
|
specific instance lists is that the instance related functions are fully
|
|
serialized against CPU hotplug operations and provide the automatic
|
|
invocations of the state callbacks on add and removal. To set up such a
|
|
multi-instance state the following function is available:
|
|
|
|
* cpuhp_setup_state_multi(state, name, startup, teardown)
|
|
|
|
The @state argument is either a statically allocated state or one of the
|
|
constants for dynamically allocated states - CPUHP_BP_PREPARE_DYN,
|
|
CPUHP_AP_ONLINE_DYN - depending on the state section (PREPARE, ONLINE) for
|
|
which a dynamic state should be allocated.
|
|
|
|
The @name argument is used for sysfs output and for instrumentation. The
|
|
naming convention is "subsys:mode" or "subsys/driver:mode",
|
|
e.g. "perf:mode" or "perf/x86:mode". The common mode names are:
|
|
|
|
======== =======================================================
|
|
prepare For states in the PREPARE section
|
|
|
|
dead For states in the PREPARE section which do not provide
|
|
a startup callback
|
|
|
|
starting For states in the STARTING section
|
|
|
|
dying For states in the STARTING section which do not provide
|
|
a startup callback
|
|
|
|
online For states in the ONLINE section
|
|
|
|
offline For states in the ONLINE section which do not provide
|
|
a startup callback
|
|
======== =======================================================
|
|
|
|
As the @name argument is only used for sysfs and instrumentation other mode
|
|
descriptors can be used as well if they describe the nature of the state
|
|
better than the common ones.
|
|
|
|
Examples for @name arguments: "perf/online", "perf/x86:prepare",
|
|
"RCU/tree:dying", "sched/waitempty"
|
|
|
|
The @startup argument is a function pointer to the callback which should be
|
|
invoked during a CPU online operation. If the usage site does not require a
|
|
startup callback set the pointer to NULL.
|
|
|
|
The @teardown argument is a function pointer to the callback which should
|
|
be invoked during a CPU offline operation. If the usage site does not
|
|
require a teardown callback set the pointer to NULL.
|
|
|
|
The functions differ in the way how the installed callbacks are treated:
|
|
|
|
* cpuhp_setup_state_nocalls(), cpuhp_setup_state_nocalls_cpuslocked()
|
|
and cpuhp_setup_state_multi() only install the callbacks
|
|
|
|
* cpuhp_setup_state() and cpuhp_setup_state_cpuslocked() install the
|
|
callbacks and invoke the @startup callback (if not NULL) for all online
|
|
CPUs which have currently a state greater than the newly installed
|
|
state. Depending on the state section the callback is either invoked on
|
|
the current CPU (PREPARE section) or on each online CPU (ONLINE
|
|
section) in the context of the CPU's hotplug thread.
|
|
|
|
If a callback fails for CPU N then the teardown callback for CPU
|
|
0 .. N-1 is invoked to rollback the operation. The state setup fails,
|
|
the callbacks for the state are not installed and in case of dynamic
|
|
allocation the allocated state is freed.
|
|
|
|
The state setup and the callback invocations are serialized against CPU
|
|
hotplug operations. If the setup function has to be called from a CPU
|
|
hotplug read locked region, then the _cpuslocked() variants have to be
|
|
used. These functions cannot be used from within CPU hotplug callbacks.
|
|
|
|
The function return values:
|
|
======== ===================================================================
|
|
0 Statically allocated state was successfully set up
|
|
|
|
>0 Dynamically allocated state was successfully set up.
|
|
|
|
The returned number is the state number which was allocated. If
|
|
the state callbacks have to be removed later, e.g. module
|
|
removal, then this number has to be saved by the caller and used
|
|
as @state argument for the state remove function. For
|
|
multi-instance states the dynamically allocated state number is
|
|
also required as @state argument for the instance add/remove
|
|
operations.
|
|
|
|
<0 Operation failed
|
|
======== ===================================================================
|
|
|
|
Removal of a CPU hotplug state
|
|
------------------------------
|
|
|
|
To remove a previously set up state, the following functions are provided:
|
|
|
|
* cpuhp_remove_state(state)
|
|
* cpuhp_remove_state_nocalls(state)
|
|
* cpuhp_remove_state_nocalls_cpuslocked(state)
|
|
* cpuhp_remove_multi_state(state)
|
|
|
|
The @state argument is either a statically allocated state or the state
|
|
number which was allocated in the dynamic range by cpuhp_setup_state*(). If
|
|
the state is in the dynamic range, then the state number is freed and
|
|
available for dynamic allocation again.
|
|
|
|
The functions differ in the way how the installed callbacks are treated:
|
|
|
|
* cpuhp_remove_state_nocalls(), cpuhp_remove_state_nocalls_cpuslocked()
|
|
and cpuhp_remove_multi_state() only remove the callbacks.
|
|
|
|
* cpuhp_remove_state() removes the callbacks and invokes the teardown
|
|
callback (if not NULL) for all online CPUs which have currently a state
|
|
greater than the removed state. Depending on the state section the
|
|
callback is either invoked on the current CPU (PREPARE section) or on
|
|
each online CPU (ONLINE section) in the context of the CPU's hotplug
|
|
thread.
|
|
|
|
In order to complete the removal, the teardown callback should not fail.
|
|
|
|
The state removal and the callback invocations are serialized against CPU
|
|
hotplug operations. If the remove function has to be called from a CPU
|
|
hotplug read locked region, then the _cpuslocked() variants have to be
|
|
used. These functions cannot be used from within CPU hotplug callbacks.
|
|
|
|
If a multi-instance state is removed then the caller has to remove all
|
|
instances first.
|
|
|
|
Multi-Instance state instance management
|
|
----------------------------------------
|
|
|
|
Once the multi-instance state is set up, instances can be added to the
|
|
state:
|
|
|
|
* cpuhp_state_add_instance(state, node)
|
|
* cpuhp_state_add_instance_nocalls(state, node)
|
|
|
|
The @state argument is either a statically allocated state or the state
|
|
number which was allocated in the dynamic range by cpuhp_setup_state_multi().
|
|
|
|
The @node argument is a pointer to an hlist_node which is embedded in the
|
|
instance's data structure. The pointer is handed to the multi-instance
|
|
state callbacks and can be used by the callback to retrieve the instance
|
|
via container_of().
|
|
|
|
The functions differ in the way how the installed callbacks are treated:
|
|
|
|
* cpuhp_state_add_instance_nocalls() and only adds the instance to the
|
|
multi-instance state's node list.
|
|
|
|
* cpuhp_state_add_instance() adds the instance and invokes the startup
|
|
callback (if not NULL) associated with @state for all online CPUs which
|
|
have currently a state greater than @state. The callback is only
|
|
invoked for the to be added instance. Depending on the state section
|
|
the callback is either invoked on the current CPU (PREPARE section) or
|
|
on each online CPU (ONLINE section) in the context of the CPU's hotplug
|
|
thread.
|
|
|
|
If a callback fails for CPU N then the teardown callback for CPU
|
|
0 .. N-1 is invoked to rollback the operation, the function fails and
|
|
the instance is not added to the node list of the multi-instance state.
|
|
|
|
To remove an instance from the state's node list these functions are
|
|
available:
|
|
|
|
* cpuhp_state_remove_instance(state, node)
|
|
* cpuhp_state_remove_instance_nocalls(state, node)
|
|
|
|
The arguments are the same as for the cpuhp_state_add_instance*()
|
|
variants above.
|
|
|
|
The functions differ in the way how the installed callbacks are treated:
|
|
|
|
* cpuhp_state_remove_instance_nocalls() only removes the instance from the
|
|
state's node list.
|
|
|
|
* cpuhp_state_remove_instance() removes the instance and invokes the
|
|
teardown callback (if not NULL) associated with @state for all online
|
|
CPUs which have currently a state greater than @state. The callback is
|
|
only invoked for the to be removed instance. Depending on the state
|
|
section the callback is either invoked on the current CPU (PREPARE
|
|
section) or on each online CPU (ONLINE section) in the context of the
|
|
CPU's hotplug thread.
|
|
|
|
In order to complete the removal, the teardown callback should not fail.
|
|
|
|
The node list add/remove operations and the callback invocations are
|
|
serialized against CPU hotplug operations. These functions cannot be used
|
|
from within CPU hotplug callbacks and CPU hotplug read locked regions.
|
|
|
|
Examples
|
|
--------
|
|
|
|
Setup and teardown a statically allocated state in the STARTING section for
|
|
notifications on online and offline operations::
|
|
|
|
ret = cpuhp_setup_state(CPUHP_SUBSYS_STARTING, "subsys:starting", subsys_cpu_starting, subsys_cpu_dying);
|
|
if (ret < 0)
|
|
return ret;
|
|
....
|
|
cpuhp_remove_state(CPUHP_SUBSYS_STARTING);
|
|
|
|
Setup and teardown a dynamically allocated state in the ONLINE section
|
|
for notifications on offline operations::
|
|
|
|
state = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "subsys:offline", NULL, subsys_cpu_offline);
|
|
if (state < 0)
|
|
return state;
|
|
....
|
|
cpuhp_remove_state(state);
|
|
|
|
Setup and teardown a dynamically allocated state in the ONLINE section
|
|
for notifications on online operations without invoking the callbacks::
|
|
|
|
state = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "subsys:online", subsys_cpu_online, NULL);
|
|
if (state < 0)
|
|
return state;
|
|
....
|
|
cpuhp_remove_state_nocalls(state);
|
|
|
|
Setup, use and teardown a dynamically allocated multi-instance state in the
|
|
ONLINE section for notifications on online and offline operation::
|
|
|
|
state = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, "subsys:online", subsys_cpu_online, subsys_cpu_offline);
|
|
if (state < 0)
|
|
return state;
|
|
....
|
|
ret = cpuhp_state_add_instance(state, &inst1->node);
|
|
if (ret)
|
|
return ret;
|
|
....
|
|
ret = cpuhp_state_add_instance(state, &inst2->node);
|
|
if (ret)
|
|
return ret;
|
|
....
|
|
cpuhp_remove_instance(state, &inst1->node);
|
|
....
|
|
cpuhp_remove_instance(state, &inst2->node);
|
|
....
|
|
remove_multi_state(state);
|
|
|
|
|
|
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.
|
|
|
|
When changes to the CPUs in the system occur, the sysfs file
|
|
/sys/devices/system/cpu/crash_hotplug contains '1' if the kernel
|
|
updates the kdump capture kernel list of CPUs itself (via elfcorehdr),
|
|
or '0' if userspace must update the kdump capture kernel list of CPUs.
|
|
|
|
The availability depends on the CONFIG_HOTPLUG_CPU kernel configuration
|
|
option.
|
|
|
|
To skip userspace processing of CPU hot un/plug events for kdump
|
|
(i.e. the unload-then-reload to obtain a current list of CPUs), this sysfs
|
|
file can be used in a udev rule as follows:
|
|
|
|
SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
|
|
|
|
For a CPU hot un/plug event, if the architecture supports kernel updates
|
|
of the elfcorehdr (which contains the list of CPUs), then the rule skips
|
|
the unload-then-reload of the kdump capture kernel.
|
|
|
|
Kernel Inline Documentations Reference
|
|
======================================
|
|
|
|
.. kernel-doc:: include/linux/cpuhotplug.h
|