At Thu, 2 Nov 2006 09:19:42 -0800,
Randy Dunlap wrote:
> Good cleanup. Thanks.
> I have a few minor fixes below.
>
>
> > Index: linux-2.6.19-rc4/Documentation/cpu-hotplug.txt
> > ===================================================================
> > --- linux-2.6.19-rc4.orig/Documentation/cpu-hotplug.txt 2006-10-31 12:37:36.000000000 +0900
> > +++ linux-2.6.19-rc4/Documentation/cpu-hotplug.txt 2006-11-02 21:22:15.000000000 +0900
> > @@ -83,8 +87,8 @@ upfront can save some boot time memory.
> > 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
> > +after a CPU is available for kernel scheduling and ready to receive
> > +interrupts from devices. Its cleared when a CPU is brought down using
>
> s/Its/It is/ (or It's)
>
> > __cpu_disable(), before which all OS services including interrupts are
> > migrated to another target CPU.
> >
> > @@ -95,8 +99,8 @@ from the map depending on the event is h
> > 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
> > +You really dont need to manipulate any of the system CPU maps. They should
>
> s/dont/don't/
>
> > +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.
> > @@ -188,21 +194,21 @@ Once the logical offline is successful,
> > #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.
> > +the state as 0 when a CPU is offline and 1 when its online.
>
> s/its/it's/
>
> > @@ -284,7 +290,7 @@ A: Yes, CPU notifiers are called only wh
> > 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,
>
> s/ i / I /
>
> > what do i need at a minimum?
>
Thank you Randy, fixed.
---
Fixing cpu-hotplug document as follows:
- cpucontrol mutex no longer exists and now cpu_bitmask_lock is used instead.
- unifying the notation of CPU to `CPU' in the document
- decolating captions to improve readability
- fixing some minor typos
Signed-off-by: Satoru Takeuchi <[email protected]>
Cc: Ashok Raj <[email protected]>
Index: linux-2.6.19-rc4/Documentation/cpu-hotplug.txt
===================================================================
--- linux-2.6.19-rc4.orig/Documentation/cpu-hotplug.txt 2006-11-02 21:44:42.000000000 +0900
+++ linux-2.6.19-rc4/Documentation/cpu-hotplug.txt 2006-11-06 08:43:34.000000000 +0900
@@ -1,8 +1,9 @@
CPU hotplug Support in Linux(tm) Kernel
+ =======================================
Maintainers:
CPU Hotplug Core:
- Rusty Russell <[email protected]>
+ Rusty Russell <[email protected]>
Srivatsa Vaddagiri <[email protected]>
i386:
Zwane Mwaikambo <[email protected]>
@@ -16,9 +17,11 @@
Authors: Ashok Raj <[email protected]>
Lots of feedback: Nathan Lynch <[email protected]>,
- Joel Schopp <[email protected]>
+ Joel Schopp <[email protected]>
+
Introduction
+============
Modern advances in system architectures have introduced advanced error
reporting and correction capabilities in processors. CPU architectures permit
@@ -32,38 +35,39 @@ provisioning reasons, or for RAS purpose
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
+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=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.
+ 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
- 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
+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.
+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
+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.
+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.
@@ -82,9 +86,9 @@ are added or removed anytime. Trimming
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_online_map: Bitmap of all CPUs currently online. It's set in __cpu_up()
+after a CPU is available for kernel scheduling and ready to receive
+interrupts from devices. It's cleared when a CPU is brought down using
__cpu_disable(), before which all OS services including interrupts are
migrated to another target CPU.
@@ -95,8 +99,8 @@ from the map depending on the event is h
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
+You really don't 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.
@@ -106,24 +110,26 @@ Never use anything other than cpumask_t
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_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()
+The above calls are used to inhibit CPU hotplug operations. While holding the
+cpu_bitmask_lock 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.
+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
+A: When doing make defconfig, enable CPU hotplug support
- "Processor type and Features" -> Support for Hotpluggable CPUs
+ "Processor type and Features" -> Support for hot-pluggable CPUs
Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
@@ -147,10 +153,10 @@ an entry as shown below in the output.
If this is not mounted, do the following.
- #mkdir /sysfs
+ #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 CPUs, the following is an example
in a 8-way system.
#pwd
@@ -171,14 +177,14 @@ in a 8-way system.
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?
+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?
+Q: How do I logically offline a CPU?
A: Do the following.
#echo 0 > /sys/devices/system/cpu/cpuX/online
@@ -188,23 +194,23 @@ Once the logical offline is successful,
#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.
+the state as 0 when a CPU is offline and 1 when it is online.
- #To display the current cpu state.
+ #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
-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
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.
+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?
+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.
@@ -218,20 +224,20 @@ A: The following happen, listed in no pa
- 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 processes are 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
+- timers/bottom halves/tasklets 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
+ notifier is called, when CPU_DEAD is called it's 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?
+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>
@@ -253,7 +259,7 @@ A: This is what you would need in your k
static struct notifier_block __cpuinitdata foobar_cpu_notifer =
{
- .notifier_call = foobar_cpu_callback,
+ .notifier_call = foobar_cpu_callback,
};
You need to call register_cpu_notifier() from your init function.
@@ -272,7 +278,7 @@ For the second case, you should add the
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
+CPU_DEAD should not be failed, it's 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?
@@ -284,8 +290,8 @@ A: Yes, CPU notifiers are called only wh
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: 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.
@@ -303,9 +309,9 @@ A: The following are what is required fo
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
+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)
{
@@ -334,11 +340,11 @@ A: First switch the current thread conte
* Do work : But cant sleep, since get_cpu() disables preempt
*/
}
- ret:
- put_cpu();
- set_cpus_allowed(current, saved_mask);
- return err;
- }
+ ret:
+ put_cpu();
+ set_cpus_allowed(current, saved_mask);
+ return err;
+ }
Q: How do we determine how many CPUs are available for hotplug.
@@ -348,15 +354,17 @@ A: There is no clear spec defined way fr
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
+ 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
+Hotplug support for devices is common in Linux today. It's 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.
@@ -381,6 +389,6 @@ scripts.
;;
*)
debug_mesg CPU $ACTION event not supported
- exit 1
- ;;
+ exit 1
+ ;;
esac
-
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