387 строки
14 KiB
Plaintext
387 строки
14 KiB
Plaintext
CPU hotplug Support in Linux(tm) Kernel
|
|
|
|
Maintainers:
|
|
CPU Hotplug Core:
|
|
Rusty Russell <rusty@rustycorp.com.au>
|
|
Srivatsa Vaddagiri <vatsa@in.ibm.com>
|
|
i386:
|
|
Zwane Mwaikambo <zwane@arm.linux.org.uk>
|
|
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_map = cpu_present_map + additional_cpus
|
|
|
|
(*) Option valid only for following architectures
|
|
- x86_64, ia64, s390
|
|
|
|
ia64 and x86_64 use 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 doesnt
|
|
mark such hot-pluggable cpus as disabled entries, one could use this
|
|
parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
|
|
|
|
s390 uses the number of cpus it detects at IPL time to also the number of bits
|
|
in cpu_possible_map. If it is desired to add additional cpus at a later time
|
|
the number should be specified using this option or the possible_cpus option.
|
|
|
|
possible_cpus=n [s390 only] use this to set hotpluggable cpus.
|
|
This option sets possible_cpus bits in
|
|
cpu_possible_map. Thus keeping the numbers of bits set
|
|
constant even if the machine gets rebooted.
|
|
This option overrides additional_cpus.
|
|
|
|
CPU maps and such
|
|
-----------------
|
|
[More on cpumaps and primitive to manipulate, please check
|
|
include/linux/cpumask.h that has more descriptive text.]
|
|
|
|
cpu_possible_map: 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_map: 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_map: 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_map/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_map
|
|
for_each_online_cpu - Iterate over cpu_online_map
|
|
for_each_present_cpu - Iterate over cpu_present_map
|
|
for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
|
|
|
|
#include <linux/cpu.h>
|
|
lock_cpu_hotplug() and unlock_cpu_hotplug():
|
|
|
|
The above calls are used to inhibit cpu hotplug operations. While holding the
|
|
cpucontrol mutex, cpu_online_map 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_HOTPLUG, and 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 /sysfs
|
|
#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 if offline and 1 when its online.
|
|
|
|
#To display the current cpu state.
|
|
#cat /sys/devices/system/cpu/cpuX/online
|
|
|
|
Q: Why cant 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 sent 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: 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
|
|
- All process is migrated away from this outgoing CPU to a new CPU
|
|
- All interrupts targeted to this CPU is 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.
|
|
|
|
"It is expected that each service cleans up when the CPU_DOWN_PREPARE
|
|
notifier is called, when CPU_DEAD is called its 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 __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
|
|
unsigned long action, void *hcpu)
|
|
{
|
|
unsigned int cpu = (unsigned long)hcpu;
|
|
|
|
switch (action) {
|
|
case CPU_ONLINE:
|
|
foobar_online_action(cpu);
|
|
break;
|
|
case CPU_DEAD:
|
|
foobar_dead_action(cpu);
|
|
break;
|
|
}
|
|
return NOTIFY_OK;
|
|
}
|
|
|
|
static struct notifier_block __cpuinitdata foobar_cpu_notifer =
|
|
{
|
|
.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
|
|
|
|
for_each_online_cpu(i) {
|
|
foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
|
|
foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
|
|
}
|
|
|
|
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 is in progress.
|
|
A: First switch the current thread context to preferred cpu
|
|
|
|
int my_func_on_cpu(int cpu)
|
|
{
|
|
cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
|
|
int curr_cpu, err = 0;
|
|
|
|
saved_mask = current->cpus_allowed;
|
|
cpu_set(cpu, new_mask);
|
|
err = set_cpus_allowed(current, new_mask);
|
|
|
|
if (err)
|
|
return err;
|
|
|
|
/*
|
|
* If we got scheduled out just after the return from
|
|
* set_cpus_allowed() before running the work, this ensures
|
|
* we stay locked.
|
|
*/
|
|
curr_cpu = get_cpu();
|
|
|
|
if (curr_cpu != cpu) {
|
|
err = -EAGAIN;
|
|
goto ret;
|
|
} else {
|
|
/*
|
|
* Do work : But cant sleep, since get_cpu() disables preempt
|
|
*/
|
|
}
|
|
ret:
|
|
put_cpu();
|
|
set_cpus_allowed(current, saved_mask);
|
|
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: Today's ACPI MADT can only provide 256 entries since the apicid field
|
|
in MADT is only 8 bits.
|
|
|
|
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
|