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[PATCH] doc: fixing cpu-hotplug documentation
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Fixing cpu-hotplug documentation as follows:

 - moving confusing asterisk on additional_cpus descrition
 - fixing some typos
 - unifying indentation for source code and command line example

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Satoru Takeuchi authored and Linus Torvalds committed Oct 20, 2006
1 parent fca4edb commit ca926e8
Showing 1 changed file with 74 additions and 74 deletions.
148 changes: 74 additions & 74 deletions Documentation/cpu-hotplug.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,7 @@ 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
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
Expand Down Expand Up @@ -101,15 +101,15 @@ 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>
#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.
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():
#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
Expand All @@ -120,7 +120,7 @@ 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
Expand All @@ -141,57 +141,57 @@ 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)
....
....
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?
A: Some architectures may have some special dependency on a certain CPU.
Expand Down Expand Up @@ -234,8 +234,8 @@ 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,
#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;
Expand Down Expand Up @@ -279,10 +279,10 @@ 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) {
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?
Expand All @@ -307,38 +307,38 @@ 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;
}
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.
Expand Down

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