| 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> |
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.
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.
This option sets possible_cpus bits in cpu_possible_mask.
This option is limited to the X86 and S390 architecture.
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.
Allow to shutdown CPU0.
This option is limited to the X86 architecture.
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.
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:
Please let Fenghua Yu <fenghua.yu@intel.com> know if you find any dependencies on CPU0.
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:
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.
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.
should be invoked which will invoke the teardown callback on all online CPUs.
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 hotplug states are defined in include/linux/cpuhotplug.h:
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.
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.
The following functions and configurations are required:
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.
Setup hotplug state callbacks with calling the callbacks
Parameters
Description
Installs the callback functions and invokes the startup callback on the present cpus which have already reached the state.
Setup hotplug state callbacks without calling the callbacks
Parameters
Description
Same as cpuhp_setup_state except that no calls are executed are invoked during installation of this callback. NOP if SMP=n or HOTPLUG_CPU=n.
Add callbacks for multi state
Parameters
Description
Sets the internal multi_instance flag and prepares a state to work as a multi instance callback. No callbacks are invoked at this point. The callbacks are invoked once an instance for this state are registered via cpuhp_state_add_instance or cpuhp_state_add_instance_nocalls.
Add an instance for a state and invoke startup callback.
Parameters
Description
Installs the instance for the state and invokes the startup callback on the present cpus which have already reached the state. The state must have been earlier marked as multi-instance by cpuhp_setup_state_multi.
Add an instance for a state without invoking the startup callback.
Parameters
Description
Installs the instance for the state The state must have been earlier marked as multi-instance by cpuhp_setup_state_multi.
Remove hotplug state callbacks and invoke the teardown
Parameters
Description
Removes the callback functions and invokes the teardown callback on the present cpus which have already reached the state.
Remove hotplug state callbacks without invoking teardown
Parameters
Remove hotplug multi state callback
Parameters
Description
Removes the callback functions from a multi state. This is the reverse of cpuhp_setup_state_multi(). All instances should have been removed before invoking this function.
Remove hotplug instance from state and invoke the teardown callback
Parameters
Description
Removes the instance and invokes the teardown callback on the present cpus which have already reached the state.
Remove hotplug instance from state without invoking the reatdown callback
Parameters
Description
Removes the instance without invoking the teardown callback.