* [PATCH] Documentation: Update CPU hotplug and move it to core-api
@ 2016-12-22 16:19 Sebastian Andrzej Siewior
2016-12-23 8:03 ` Jani Nikula
2017-01-13 17:34 ` Jonathan Corbet
0 siblings, 2 replies; 5+ messages in thread
From: Sebastian Andrzej Siewior @ 2016-12-22 16:19 UTC (permalink / raw)
To: linux-kernel
Cc: tglx, rt, Sebastian Andrzej Siewior, Jonathan Corbet,
Mauro Carvalho Chehab, Rusty Russell, Srivatsa Vaddagiri,
Ashok Raj, Joel Schopp, linux-doc
The current CPU hotplug is outdated.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>
---
Documentation/core-api/cpu_hotplug.rst | 372 +++++++++++++++++++++++++++
Documentation/core-api/index.rst | 1 +
Documentation/cpu-hotplug.txt | 452 ---------------------------------
3 files changed, 373 insertions(+), 452 deletions(-)
create mode 100644 Documentation/core-api/cpu_hotplug.rst
delete mode 100644 Documentation/cpu-hotplug.txt
diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
new file mode 100644
index 000000000000..4a50ab7817f7
--- /dev/null
+++ b/Documentation/core-api/cpu_hotplug.rst
@@ -0,0 +1,372 @@
+=========================
+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 fourV 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 CPUs the kernel will support. If the number
+ supplied here is lower than the number of physically available CPUs than
+ 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.
+
+``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".
+
+ This option is limited to the PowerPC 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
+``get_online_cpus()`` and ``put_online_cpus()`` 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 callbac 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
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 2872ca1a52f1..0d93d8089136 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -13,6 +13,7 @@ Core utilities
assoc_array
atomic_ops
+ cpu_hotplug
local_ops
workqueue
diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
deleted file mode 100644
index d02e8a451872..000000000000
--- a/Documentation/cpu-hotplug.txt
+++ /dev/null
@@ -1,452 +0,0 @@
- CPU hotplug Support in Linux(tm) Kernel
-
- Maintainers:
- CPU Hotplug Core:
- Rusty Russell <rusty@rustcorp.com.au>
- Srivatsa Vaddagiri <vatsa@in.ibm.com>
- i386:
- Zwane Mwaikambo <zwanem@gmail.com>
- ppc64:
- Nathan Lynch <nathanl@austin.ibm.com>
- 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
--
2.11.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] Documentation: Update CPU hotplug and move it to core-api
2016-12-22 16:19 [PATCH] Documentation: Update CPU hotplug and move it to core-api Sebastian Andrzej Siewior
@ 2016-12-23 8:03 ` Jani Nikula
2017-01-04 9:05 ` Sebastian Andrzej Siewior
2017-01-13 17:34 ` Jonathan Corbet
1 sibling, 1 reply; 5+ messages in thread
From: Jani Nikula @ 2016-12-23 8:03 UTC (permalink / raw)
To: Sebastian Andrzej Siewior, linux-kernel
Cc: tglx, rt, Sebastian Andrzej Siewior, Jonathan Corbet,
Mauro Carvalho Chehab, Rusty Russell, Srivatsa Vaddagiri,
Ashok Raj, Joel Schopp, linux-doc
On Thu, 22 Dec 2016, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> 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>
> ---
> Documentation/core-api/cpu_hotplug.rst | 372 +++++++++++++++++++++++++++
> Documentation/core-api/index.rst | 1 +
> Documentation/cpu-hotplug.txt | 452 ---------------------------------
> 3 files changed, 373 insertions(+), 452 deletions(-)
> create mode 100644 Documentation/core-api/cpu_hotplug.rst
> delete mode 100644 Documentation/cpu-hotplug.txt
>
> diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
> new file mode 100644
> index 000000000000..4a50ab7817f7
> --- /dev/null
> +++ b/Documentation/core-api/cpu_hotplug.rst
> @@ -0,0 +1,372 @@
> +=========================
> +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>
Just a drive-by comment, does that do what you want? You might want to
see if adding one :Author: line per author produces better results.
That said, I think git will do a better job of maintaining both the date
and the authorship.
BR,
Jani.
> +
> +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 fourV 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 CPUs the kernel will support. If the number
> + supplied here is lower than the number of physically available CPUs than
> + 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.
> +
> +``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".
> +
> + This option is limited to the PowerPC 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
> +``get_online_cpus()`` and ``put_online_cpus()`` 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 callbac 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
> diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> index 2872ca1a52f1..0d93d8089136 100644
> --- a/Documentation/core-api/index.rst
> +++ b/Documentation/core-api/index.rst
> @@ -13,6 +13,7 @@ Core utilities
>
> assoc_array
> atomic_ops
> + cpu_hotplug
> local_ops
> workqueue
>
> diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
> deleted file mode 100644
> index d02e8a451872..000000000000
> --- a/Documentation/cpu-hotplug.txt
> +++ /dev/null
> @@ -1,452 +0,0 @@
> - CPU hotplug Support in Linux(tm) Kernel
> -
> - Maintainers:
> - CPU Hotplug Core:
> - Rusty Russell <rusty@rustcorp.com.au>
> - Srivatsa Vaddagiri <vatsa@in.ibm.com>
> - i386:
> - Zwane Mwaikambo <zwanem@gmail.com>
> - ppc64:
> - Nathan Lynch <nathanl@austin.ibm.com>
> - 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
--
Jani Nikula, Intel Open Source Technology Center
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] Documentation: Update CPU hotplug and move it to core-api
2016-12-23 8:03 ` Jani Nikula
@ 2017-01-04 9:05 ` Sebastian Andrzej Siewior
2017-01-04 11:19 ` Jani Nikula
0 siblings, 1 reply; 5+ messages in thread
From: Sebastian Andrzej Siewior @ 2017-01-04 9:05 UTC (permalink / raw)
To: Jani Nikula
Cc: linux-kernel, tglx, rt, Jonathan Corbet, Mauro Carvalho Chehab,
Rusty Russell, Srivatsa Vaddagiri, Ashok Raj, Joel Schopp,
linux-doc
On 2016-12-23 10:03:09 [+0200], Jani Nikula wrote:
> > --- /dev/null
> > +++ b/Documentation/core-api/cpu_hotplug.rst
> > @@ -0,0 +1,372 @@
> > +=========================
> > +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>
>
> Just a drive-by comment, does that do what you want? You might want to
> see if adding one :Author: line per author produces better results.
I see an Author: line for each author, I like better the way it is.
> That said, I think git will do a better job of maintaining both the date
> and the authorship.
If you see the generated html output in your browser you don't have git
around to check the date or the Author. The date should help you to keep
track of the latest version. As for the Author, git would only show me
as the author but I did not write everything myself. Some parts are
still from old documentation.
> BR,
> Jani.
Sebastian
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] Documentation: Update CPU hotplug and move it to core-api
2017-01-04 9:05 ` Sebastian Andrzej Siewior
@ 2017-01-04 11:19 ` Jani Nikula
0 siblings, 0 replies; 5+ messages in thread
From: Jani Nikula @ 2017-01-04 11:19 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: linux-kernel, tglx, rt, Jonathan Corbet, Mauro Carvalho Chehab,
Rusty Russell, Srivatsa Vaddagiri, Ashok Raj, Joel Schopp,
linux-doc
On Wed, 04 Jan 2017, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> On 2016-12-23 10:03:09 [+0200], Jani Nikula wrote:
>> > --- /dev/null
>> > +++ b/Documentation/core-api/cpu_hotplug.rst
>> > @@ -0,0 +1,372 @@
>> > +=========================
>> > +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>
>>
>> Just a drive-by comment, does that do what you want? You might want to
>> see if adding one :Author: line per author produces better results.
>
> I see an Author: line for each author, I like better the way it is.
>
>> That said, I think git will do a better job of maintaining both the date
>> and the authorship.
>
> If you see the generated html output in your browser you don't have git
> around to check the date or the Author. The date should help you to keep
> track of the latest version. As for the Author, git would only show me
> as the author but I did not write everything myself. Some parts are
> still from old documentation.
Just to be clear, I don't really care about this file specifically one
way or the other. Do as you wish, and please don't let my comments block
you.
---
In general, I don't think maintaining either the date or the authors
manually is a good idea. This applies for both documentation and source
code.
The obvious downside is that updating them will be overlooked and
forgotten. Arguably not having them at all is better than having
incorrect or stale information (when the accurate information can be
retrieved from git).
Perhaps we could add more automatically updated metadata to the
generated documentation. We could even add the date from git if we
really wanted to. But do we? The generated documentation already
includes the kernel version, is that not enough?
Even when people do remember to update the fields, they're faced with a
dilemma: Do their changes warrant an update in date or adding themselves
as author? Are the changes significant enough? This is completely
subjective. What if there have been bigger changes by others without
updates to authors? Should you remove authors when their contributions
have been long since been removed or rewritten, and forgotten, and don't
have any relevance to the current text?
I think listing authors in files, whether they're text or source code,
in collaborative projects, is counter-productive. The best possible end
result, not the promotion of individuals, should be the shared goal. No
contributor should feel that their contribution promotes the people in
authors rather than improves the end result. No contributor should shy
away from updating a file because it seems to be "owned" by someone. Of
course, credit to whom credit is due, but in an objective and fair
manner.
I argue that git log and blame are objective, and the history is all
there for anyone that cares to look. It's not without flaws, as you
point out, but I think it's superior to maintaining authors
manually. (Copyright notices may need to be updated in the files, but
they could be in reStructuredText comments, not unlike in source code.)
BR,
Jani.
--
Jani Nikula, Intel Open Source Technology Center
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] Documentation: Update CPU hotplug and move it to core-api
2016-12-22 16:19 [PATCH] Documentation: Update CPU hotplug and move it to core-api Sebastian Andrzej Siewior
2016-12-23 8:03 ` Jani Nikula
@ 2017-01-13 17:34 ` Jonathan Corbet
1 sibling, 0 replies; 5+ messages in thread
From: Jonathan Corbet @ 2017-01-13 17:34 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: linux-kernel, tglx, rt, Mauro Carvalho Chehab, Rusty Russell,
Srivatsa Vaddagiri, Ashok Raj, Joel Schopp, linux-doc
On Thu, 22 Dec 2016 17:19:34 +0100
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> The current CPU hotplug is outdated. During the update to what we
> currently have I rewrote it partly and moved to sphinx format.
OK, I've applied this (finally) to the docs tree, even though I do kind
of agree with Jani about keeping the author information in Git.
Thanks,
jon
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2017-01-13 17:34 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2016-12-22 16:19 [PATCH] Documentation: Update CPU hotplug and move it to core-api Sebastian Andrzej Siewior
2016-12-23 8:03 ` Jani Nikula
2017-01-04 9:05 ` Sebastian Andrzej Siewior
2017-01-04 11:19 ` Jani Nikula
2017-01-13 17:34 ` Jonathan Corbet
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