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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>
This commit is contained in:
Satoru Takeuchi 2006-10-19 23:29:06 -07:00 committed by Linus Torvalds
parent fca4edb48b
commit ca926e80dc
1 changed files with 70 additions and 70 deletions
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140
Documentation/cpu-hotplug.txt
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@ -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 maxcpus=2 will only boot 2. You can choose to bring the
other cpus later online, read FAQ's for more info. 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 cpu_possible_map = cpu_present_map + additional_cpus
(*) Option valid only for following architectures (*) Option valid only for following architectures
@ -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. 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_possible_cpu - Iterate over cpu_possible_map
for_each_online_cpu - Iterate over cpu_online_map for_each_online_cpu - Iterate over cpu_online_map
for_each_present_cpu - Iterate over cpu_present_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_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
#include <linux/cpu.h> #include <linux/cpu.h>
lock_cpu_hotplug() and unlock_cpu_hotplug(): lock_cpu_hotplug() and unlock_cpu_hotplug():
The above calls are used to inhibit cpu hotplug operations. While holding the 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 cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
@ -120,7 +120,7 @@ will work as long as stop_machine_run() is used to take a cpu down.
CPU Hotplug - Frequently Asked Questions. 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 A: When doing make defconfig, Enable CPU hotplug support
"Processor type and Features" -> Support for Hotpluggable CPUs "Processor type and Features" -> Support for Hotpluggable CPUs
@ -141,39 +141,39 @@ A: You should now notice an entry in sysfs.
Check if sysfs is mounted, using the "mount" command. You should notice Check if sysfs is mounted, using the "mount" command. You should notice
an entry as shown below in the output. 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 #mkdir /sysfs
#mount -t sysfs sys /sys #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. in a 8-way system.
#pwd #pwd
#/sys/devices/system/cpu #/sys/devices/system/cpu
#ls -l #ls -l
total 0 total 0
drwxr-xr-x 10 root root 0 Sep 19 07:44 . 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 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 cpu0
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1 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 cpu2
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3 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 cpu4
drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5 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 07:44 cpu6
drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7 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 Under each directory you would find an "online" file which is the control
file to logically online/offline a processor. file to logically online/offline a processor.
Q: Does hot-add/hot-remove refer to physical add/remove of cpus? 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. 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 To support physical addition/removal, one would need some BIOS hooks and
the platform should have something like an attention button in PCI hotplug. 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. CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
@ -181,17 +181,17 @@ CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
Q: How do i logically offline a CPU? Q: How do i logically offline a CPU?
A: Do the following. 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. the state as 0 when a cpu if offline and 1 when its online.
#To display the current cpu state. #To display the current cpu state.
#cat /sys/devices/system/cpu/cpuX/online #cat /sys/devices/system/cpu/cpuX/online
Q: Why cant i remove CPU0 on some systems? Q: Why cant i remove CPU0 on some systems?
A: Some architectures may have some special dependency on a certain CPU. A: Some architectures may have some special dependency on a certain CPU.
@ -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? departure, how to i arrange for proper notification?
A: This is what you would need in your kernel code to receive notifications. A: This is what you would need in your kernel code to receive notifications.
#include <linux/cpu.h> #include <linux/cpu.h>
static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb, static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
unsigned long action, void *hcpu) unsigned long action, void *hcpu)
{ {
unsigned int cpu = (unsigned long)hcpu; unsigned int cpu = (unsigned long)hcpu;
@ -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. 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 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_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, Q: If i would like to develop cpu hotplug support for a new architecture,
what do i need at a minimum? what do i need at a minimum?
@ -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. work specific to this cpu is in progress.
A: First switch the current thread context to preferred cpu A: First switch the current thread context to preferred cpu
int my_func_on_cpu(int cpu) int my_func_on_cpu(int cpu)
{ {
cpumask_t saved_mask, new_mask = CPU_MASK_NONE; cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
int curr_cpu, err = 0; int curr_cpu, err = 0;
saved_mask = current->cpus_allowed; saved_mask = current->cpus_allowed;
cpu_set(cpu, new_mask); cpu_set(cpu, new_mask);
err = set_cpus_allowed(current, new_mask); err = set_cpus_allowed(current, new_mask);
if (err) if (err)
return err; return err;
/* /*
* If we got scheduled out just after the return from * If we got scheduled out just after the return from
* set_cpus_allowed() before running the work, this ensures * set_cpus_allowed() before running the work, this ensures
* we stay locked. * we stay locked.
*/ */
curr_cpu = get_cpu(); curr_cpu = get_cpu();
if (curr_cpu != cpu) { if (curr_cpu != cpu) {
err = -EAGAIN; err = -EAGAIN;
goto ret; goto ret;
} else { } else {
/* /*
* Do work : But cant sleep, since get_cpu() disables preempt * Do work : But cant sleep, since get_cpu() disables preempt
*/ */
} }
ret: ret:
put_cpu(); put_cpu();
set_cpus_allowed(current, saved_mask); set_cpus_allowed(current, saved_mask);
return err; return err;
} }
Q: How do we determine how many CPUs are available for hotplug. Q: How do we determine how many CPUs are available for hotplug.