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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 |