Re: Documentation for CPU hotplug support

From: Ashok Raj
Date: Wed Nov 23 2005 - 12:41:46 EST

On Thu, Nov 10, 2005 at 11:23:00PM -0800, Nathan Lynch wrote:
> Hi Ashok-
> Thanks a lot for doing this; it's a good start. Some feedback and
> minor corrections below:

Thanks Nathan for the feedback. Beleive it has most of feedback i
got todate. user space side is still missing, i will try to add it
after some time once i have right data.

- spell checked (maybe ;-))
- Added some more code snips
- Some more details liks available_cpus in x86_64.

Andrew: Could you help queue this for next update, so we will have some
starting point.

CPU Hotplug Documentation writeup.

Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Documentation/cpu-hotplug.txt | 315 ++++++++++++++++++++++++++++++++++++++++++
1 files changed, 315 insertions(+)

Index: linux-2.6.15-rc1-mm2/Documentation/cpu-hotplug.txt
--- /dev/null
+++ linux-2.6.15-rc1-mm2/Documentation/cpu-hotplug.txt
@@ -0,0 +1,315 @@
+ CPU hotplug Support in Linux(tm) Kernel
+Modern advances in system architectures have introduced advanced error
+reporting and correction capabilities in processors. CPU architectures permit
+partitioning support, where compute resources of a single CPU could be made
+available to virtual machine environments. There are couple OEMS that
+support NUMA hardware which are hot pluggable as well, where physical
+node insertion and removal require support for CPU hotplug.
+Such advances require CPUs available to a kernel to be removed either for
+provisioning reasons, or for RAS purposes to keep an offending CPU off
+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
+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=2 will only boot 2. You can choose to bring the
+ other cpus later online, read FAQ's for more info.
+additional_cpus=n [x86_64 only] use this to limit hotpluggable cpus.
+ This option sets
+ cpu_possible_map = cpu_present_map + 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
+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
+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
+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_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
+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
+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
+cpu_possible_map/for_each_cpu() to iterate.
+Never use anything other than cpumask_t to represent bitmap of CPUs.
+#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.
+#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()
+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?
+A: When doing make defconfig, Enable CPU hotplug support
+ "Processor type and Features" -> Support for Hotpluggable CPUs
+Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
+You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
+as well.
+Q: What architectures support CPU hotplug?
+A: As of 2.6.14, the following architectures support CPU hotplug.
+i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
+Q: How to test if hotplug is supported on the newly built kernel?
+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)
+if this is not mounted, do the following.
+#mkdir /sysfs
+#mount -t sysfs sys /sys
+now you should see entries for all present cpu, the following is an example
+in a 8-way system.
+#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.
+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
+once the logical offline is successful, check
+#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.
+#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.
+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
+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.
+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.
+In some situations, this can be a run time check, i.e if you try to remove the
+last CPU, this will not be permitted. You can find such failures by
+investigating the return value of the "echo" command.
+Q: What happens when a CPU is being logically offlined?
+A: The following happen, listed in no particular order :-)
+- A notification is sent to in-kernel registered modules by sending an event
+- All process is 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
+- 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
+ "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
+ 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?
+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,
+ unsigned long action, void *hcpu)
+ {
+ unsigned int cpu = (unsigned long)hcpu;
+ switch (action) {
+ case CPU_ONLINE:
+ foobar_online_action(cpu);
+ break;
+ case CPU_DEAD:
+ foobar_dead_action(cpu);
+ break;
+ }
+ return NOTIFY_OK;
+ }
+ static struct notifier_block foobar_cpu_notifer =
+ {
+ .notifier_call = foobar_cpu_callback,
+ };
+In your init function,
+ register_cpu_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.
+CPU_DEAD should not be failed, its 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?
+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) {
+ foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, 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?
+A: The following are what is required for CPU hotplug infrastructure to work
+ correctly.
+ - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
+ - __cpu_up() - Arch interface to bring up a CPU
+ - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
+ can be handled by the kernel after the routine
+ returns. Including local APIC timers etc are
+ shutdown.
+ - __cpu_die() - This actually supposed to ensure death of the CPU.
+ Actually look at some example code in other arch
+ that implement CPU hotplug. The processor is taken
+ down from the idle() loop for that specific
+ architecture. __cpu_die() typically waits for some
+ 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
+ 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.
+A: There is no clear spec defined way from ACPI that can give us that
+ information today. Based on some input from Natalie of Unisys,
+ that the ACPI MADT (Multiple APIC Description Tables) marks those possible
+ 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
+ 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
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