Maintainers:
CPU Hotplug Core:
- Rusty Russell <rusty@rustycorp.com.au>
+ Rusty Russell <rusty@rustcorp.com.au>
Srivatsa Vaddagiri <vatsa@in.ibm.com>
i386:
Zwane Mwaikambo <zwane@arm.linux.org.uk>
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
+additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
+ cpu_possible_mask = cpu_present_mask + additional_cpus
-(*) Option valid only for following architectures
-- x86_64, ia64, s390
+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".
-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.
+(*) Option valid only for following architectures
+- ia64
-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.
+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 only] use this to set hotpluggable cpus.
+possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
This option sets possible_cpus bits in
- cpu_possible_map. Thus keeping the numbers of bits set
+ cpu_possible_mask. 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
+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
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()
+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_map: Bitmap of CPUs currently present in the system. Not all
+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
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_cpu() to iterate.
+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>
+ #include <linux/cpumask.h>
-for_each_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.
+ 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_mask(x,mask) - Iterate over some random collection of cpu mask.
-#include <linux/cpu.h>
-lock_cpu_hotplug() and unlock_cpu_hotplug():
+ #include <linux/cpu.h>
+ get_online_cpus() and put_online_cpus():
-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
+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 i enable my kernel to support CPU hotplug?
+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
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)
-....
+ ....
+ none on /sys type sysfs (rw)
+ ....
-if this is not mounted, do the following.
+If this is not mounted, do the following.
-#mkdir /sysfs
-#mount -t sysfs sys /sys
+ #mkdir /sysfs
+ #mount -t sysfs sys /sys
-now you should see entries for all present cpu, the following is an example
+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
+ #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_CPU_HOTPLUG enables logical online/offline capability in the kernel.
+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
+ #echo 0 > /sys/devices/system/cpu/cpuX/online
-once the logical offline is successful, check
+Once the logical offline is successful, check
-#cat /proc/interrupts
+ #cat /proc/interrupts
-you should now not see the CPU that you removed. Also online file will report
+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
+ #To display the current cpu state.
+ #cat /sys/devices/system/cpu/cpuX/online
-Q: Why cant i remove CPU0 on some systems?
+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 sent platform interrupts to the
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
+ 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 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.
+ 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 its expected there is nothing
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,
+ #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:
+ 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_notifer =
+ 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).
-In your init function,
+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.
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) {
+ for_each_online_cpu(i) {
foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
- foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, 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?
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;
- }
+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
projects / linux-flexiantxendom0-3.2.10.git / blobdiff