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Commit bb118c56 authored by Igor Vuk's avatar Igor Vuk Committed by Jonathan Corbet
Browse files

Documentation: cpu-hotplug: Fix typos 



Fix some minor spelling errors and capitalization issues.

Signed-off-by: default avatarIgor Vuk <parcijala@gmail.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 7b855a12

Documentation/cpu-hotplug.txt

+16 −16
Original line number Original line Diff line number Diff line
@@ -84,9 +84,9 @@ 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

upfront can save some boot time memory. See below for how we use heuristics

in x86_64 case to keep this under check.

in x86_64 case to keep this under check.





cpu_online_mask: Bitmap of all CPUs currently online. Its set in __cpu_up()

cpu_online_mask: Bitmap of all CPUs currently online. It's set in __cpu_up()

after a cpu is available for kernel scheduling and ready to receive

after a CPU is available for kernel scheduling and ready to receive

interrupts from devices. Its cleared when a cpu is brought down using

interrupts from devices. It's cleared when a CPU is brought down using

__cpu_disable(), before which all OS services including interrupts are

__cpu_disable(), before which all OS services including interrupts are

migrated to another target CPU.

migrated to another target CPU.
@@ -181,7 +181,7 @@ 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.





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
@@ -191,15 +191,15 @@ 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 is offline and 1 when it's 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 can't 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.

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

For e.g in IA64 platforms we have ability to send platform interrupts to the

OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI

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

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

current ACPI version doesn't support such re-direction, we disable that CPU
@@ -231,7 +231,7 @@ either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug. 




--Fenghua Yu <fenghua.yu@intel.com>

--Fenghua Yu <fenghua.yu@intel.com>





Q: How do i find out if a particular CPU is not removable?

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

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

absence of the "online" file. This is done if it can be determined ahead of

time that this CPU cannot be removed.

time that this CPU cannot be removed.
@@ -250,7 +250,7 @@ A: The following happen, listed in no particular order :-) 
- All processes are migrated away from this outgoing CPU to new CPUs.

- 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

  The new CPU is chosen from each process' current cpuset, which may be

  a subset of all online CPUs.

  a subset of all online CPUs.

- All interrupts targeted to this CPU is migrated to a new CPU

- All interrupts targeted to this CPU are migrated to a new CPU

- timers/bottom half/task lets are also 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

- Once all services are migrated, kernel calls an arch specific routine

  __cpu_disable() to perform arch specific cleanup.

  __cpu_disable() to perform arch specific cleanup.
@@ -259,10 +259,10 @@ A: The following happen, listed in no particular order :-) 
  CPU is being offlined).

  CPU is being offlined).





  "It is expected that each service cleans up when the CPU_DOWN_PREPARE

  "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

  notifier is called, when CPU_DEAD is called it's expected there is nothing

  running on behalf of this CPU that was offlined"

  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

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.
@@ -311,7 +311,7 @@ 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?

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

   do this:

   do this:





	for_each_online_cpu(i) {

	for_each_online_cpu(i) {
@@ -363,8 +363,8 @@ A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined. 
    callbacks as well as initialize the already online CPUs.

    callbacks as well as initialize the already online CPUs.









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?

A: The following are what is required for CPU hotplug infrastructure to work

A: The following are what is required for CPU hotplug infrastructure to work

   correctly.

   correctly.
@@ -382,8 +382,8 @@ A: The following are what is required for CPU hotplug infrastructure to work 
                          per_cpu state to be set, to ensure the processor

                          per_cpu state to be set, to ensure the processor

                          dead routine is called to be sure positively.

                          dead routine is called to be sure positively.





Q: I need to ensure that a particular cpu is not removed when there is some

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 in progress.

A: There are two ways.  If your code can be run in interrupt context, use

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

   smp_call_function_single(), otherwise use work_on_cpu().  Note that

   work_on_cpu() is slow, and can fail due to out of memory:

   work_on_cpu() is slow, and can fail due to out of memory: