[PATCH] Updated CPU hotplug documentation
Thanks to Nathan Lynch for the review and comments. Thanks to Joel Schopp for the pointer to add user space scipts. Signed-off-by: Ashok Raj <ashok.raj@intel.com> Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com> Signed-off-by: Joel Schopp <jschopp@austin.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Ashok Raj
Linus Torvalds
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CPU hotplug Support in Linux(tm) Kernel
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Maintainers:
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CPU Hotplug Core:
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Rusty Russell <rusty@rustycorp.com.au>
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Srivatsa Vaddagiri <vatsa@in.ibm.com>
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i386:
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Zwane Mwaikambo <zwane@arm.linux.org.uk>
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ppc64:
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Nathan Lynch <nathanl@austin.ibm.com>
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Joel Schopp <jschopp@austin.ibm.com>
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ia64/x86_64:
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Ashok Raj <ashok.raj@intel.com>
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Authors: Ashok Raj <ashok.raj@intel.com>
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Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
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Joel Schopp <jschopp@austin.ibm.com>
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Introduction
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Modern advances in system architectures have introduced advanced error
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reporting and correction capabilities in processors. CPU architectures permit
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partitioning support, where compute resources of a single CPU could be made
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available to virtual machine environments. There are couple OEMS that
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support NUMA hardware which are hot pluggable as well, where physical
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node insertion and removal require support for CPU hotplug.
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Such advances require CPUs available to a kernel to be removed either for
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provisioning reasons, or for RAS purposes to keep an offending CPU off
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system execution path. Hence the need for CPU hotplug support in the
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Linux kernel.
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A more novel use of CPU-hotplug support is its use today in suspend
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resume support for SMP. Dual-core and HT support makes even
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a laptop run SMP kernels which didn't support these methods. SMP support
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for suspend/resume is a work in progress.
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General Stuff about CPU Hotplug
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--------------------------------
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Command Line Switches
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---------------------
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maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using
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maxcpus=2 will only boot 2. You can choose to bring the
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other cpus later online, read FAQ's for more info.
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additional_cpus=n [x86_64 only] use this to limit hotpluggable cpus.
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This option sets
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cpu_possible_map = cpu_present_map + additional_cpus
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CPU maps and such
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-----------------
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[More on cpumaps and primitive to manipulate, please check
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include/linux/cpumask.h that has more descriptive text.]
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cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
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system. This is used to allocate some boot time memory for per_cpu variables
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that aren't designed to grow/shrink as CPUs are made available or removed.
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Once set during boot time discovery phase, the map is static, i.e no bits
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are added or removed anytime. Trimming it accurately for your system needs
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upfront can save some boot time memory. See below for how we use heuristics
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in x86_64 case to keep this under check.
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cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
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after a cpu is available for kernel scheduling and ready to receive
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interrupts from devices. Its cleared when a cpu is brought down using
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__cpu_disable(), before which all OS services including interrupts are
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migrated to another target CPU.
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cpu_present_map: Bitmap of CPUs currently present in the system. Not all
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of them may be online. When physical hotplug is processed by the relevant
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subsystem (e.g ACPI) can change and new bit either be added or removed
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from the map depending on the event is hot-add/hot-remove. There are currently
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no locking rules as of now. Typical usage is to init topology during boot,
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at which time hotplug is disabled.
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You really dont need to manipulate any of the system cpu maps. They should
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be read-only for most use. When setting up per-cpu resources almost always use
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cpu_possible_map/for_each_cpu() to iterate.
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Never use anything other than cpumask_t to represent bitmap of CPUs.
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#include <linux/cpumask.h>
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for_each_cpu - Iterate over cpu_possible_map
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for_each_online_cpu - Iterate over cpu_online_map
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for_each_present_cpu - Iterate over cpu_present_map
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for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
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#include <linux/cpu.h>
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lock_cpu_hotplug() and unlock_cpu_hotplug():
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The above calls are used to inhibit cpu hotplug operations. While holding the
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cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
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cpus going away, you could also use preempt_disable() and preempt_enable()
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for those sections. Just remember the critical section cannot call any
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function that can sleep or schedule this process away. The preempt_disable()
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will work as long as stop_machine_run() is used to take a cpu down.
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CPU Hotplug - Frequently Asked Questions.
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Q: How to i enable my kernel to support CPU hotplug?
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A: When doing make defconfig, Enable CPU hotplug support
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"Processor type and Features" -> Support for Hotpluggable CPUs
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Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
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You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
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as well.
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Q: What architectures support CPU hotplug?
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A: As of 2.6.14, the following architectures support CPU hotplug.
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i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
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Q: How to test if hotplug is supported on the newly built kernel?
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A: You should now notice an entry in sysfs.
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Check if sysfs is mounted, using the "mount" command. You should notice
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an entry as shown below in the output.
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....
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none on /sys type sysfs (rw)
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....
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if this is not mounted, do the following.
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#mkdir /sysfs
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#mount -t sysfs sys /sys
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now you should see entries for all present cpu, the following is an example
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in a 8-way system.
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#pwd
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#/sys/devices/system/cpu
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#ls -l
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total 0
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drwxr-xr-x 10 root root 0 Sep 19 07:44 .
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drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
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drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
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drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
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Under each directory you would find an "online" file which is the control
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file to logically online/offline a processor.
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Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
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A: The usage of hot-add/remove may not be very consistently used in the code.
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CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel.
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To support physical addition/removal, one would need some BIOS hooks and
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the platform should have something like an attention button in PCI hotplug.
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CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
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Q: How do i logically offline a CPU?
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A: Do the following.
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#echo 0 > /sys/devices/system/cpu/cpuX/online
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once the logical offline is successful, check
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#cat /proc/interrupts
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you should now not see the CPU that you removed. Also online file will report
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the state as 0 when a cpu if offline and 1 when its online.
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#To display the current cpu state.
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#cat /sys/devices/system/cpu/cpuX/online
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Q: Why cant i remove CPU0 on some systems?
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A: Some architectures may have some special dependency on a certain CPU.
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For e.g in IA64 platforms we have ability to sent platform interrupts to the
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OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
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specifications, we didn't have a way to change the target CPU. Hence if the
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current ACPI version doesn't support such re-direction, we disable that CPU
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by making it not-removable.
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In such cases you will also notice that the online file is missing under cpu0.
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Q: How do i find out if a particular CPU is not removable?
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A: Depending on the implementation, some architectures may show this by the
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absence of the "online" file. This is done if it can be determined ahead of
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time that this CPU cannot be removed.
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In some situations, this can be a run time check, i.e if you try to remove the
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last CPU, this will not be permitted. You can find such failures by
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investigating the return value of the "echo" command.
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Q: What happens when a CPU is being logically offlined?
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A: The following happen, listed in no particular order :-)
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- A notification is sent to in-kernel registered modules by sending an event
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CPU_DOWN_PREPARE
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- All process is migrated away from this outgoing CPU to a new CPU
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- All interrupts targeted to this CPU is migrated to a new CPU
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- timers/bottom half/task lets are also migrated to a new CPU
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- Once all services are migrated, kernel calls an arch specific routine
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__cpu_disable() to perform arch specific cleanup.
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- Once this is successful, an event for successful cleanup is sent by an event
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CPU_DEAD.
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"It is expected that each service cleans up when the CPU_DOWN_PREPARE
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notifier is called, when CPU_DEAD is called its expected there is nothing
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running on behalf of this CPU that was offlined"
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Q: If i have some kernel code that needs to be aware of CPU arrival and
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departure, how to i arrange for proper notification?
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A: This is what you would need in your kernel code to receive notifications.
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#include <linux/cpu.h>
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static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
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unsigned long action, void *hcpu)
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{
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unsigned int cpu = (unsigned long)hcpu;
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switch (action) {
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case CPU_ONLINE:
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foobar_online_action(cpu);
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break;
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case CPU_DEAD:
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foobar_dead_action(cpu);
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break;
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}
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return NOTIFY_OK;
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}
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static struct notifier_block foobar_cpu_notifer =
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{
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.notifier_call = foobar_cpu_callback,
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};
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In your init function,
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register_cpu_notifier(&foobar_cpu_notifier);
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You can fail PREPARE notifiers if something doesn't work to prepare resources.
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This will stop the activity and send a following CANCELED event back.
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CPU_DEAD should not be failed, its just a goodness indication, but bad
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things will happen if a notifier in path sent a BAD notify code.
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Q: I don't see my action being called for all CPUs already up and running?
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A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
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If you need to perform some action for each cpu already in the system, then
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for_each_online_cpu(i) {
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foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
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foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
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}
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Q: If i would like to develop cpu hotplug support for a new architecture,
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what do i need at a minimum?
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A: The following are what is required for CPU hotplug infrastructure to work
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correctly.
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- Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
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- __cpu_up() - Arch interface to bring up a CPU
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- __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
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can be handled by the kernel after the routine
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returns. Including local APIC timers etc are
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shutdown.
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- __cpu_die() - This actually supposed to ensure death of the CPU.
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Actually look at some example code in other arch
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that implement CPU hotplug. The processor is taken
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down from the idle() loop for that specific
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architecture. __cpu_die() typically waits for some
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per_cpu state to be set, to ensure the processor
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dead routine is called to be sure positively.
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Q: I need to ensure that a particular cpu is not removed when there is some
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work specific to this cpu is in progress.
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A: First switch the current thread context to preferred cpu
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int my_func_on_cpu(int cpu)
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{
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cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
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int curr_cpu, err = 0;
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saved_mask = current->cpus_allowed;
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cpu_set(cpu, new_mask);
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err = set_cpus_allowed(current, new_mask);
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if (err)
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return err;
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/*
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* If we got scheduled out just after the return from
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* set_cpus_allowed() before running the work, this ensures
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* we stay locked.
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*/
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curr_cpu = get_cpu();
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if (curr_cpu != cpu) {
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err = -EAGAIN;
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goto ret;
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} else {
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/*
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* Do work : But cant sleep, since get_cpu() disables preempt
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*/
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}
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ret:
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put_cpu();
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set_cpus_allowed(current, saved_mask);
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return err;
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}
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Q: How do we determine how many CPUs are available for hotplug.
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A: There is no clear spec defined way from ACPI that can give us that
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information today. Based on some input from Natalie of Unisys,
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that the ACPI MADT (Multiple APIC Description Tables) marks those possible
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CPUs in a system with disabled status.
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Andi implemented some simple heuristics that count the number of disabled
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CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
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we assume 1/2 the number of CPUs currently present can be hotplugged.
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Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
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in MADT is only 8 bits.
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User Space Notification
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Hotplug support for devices is common in Linux today. Its being used today to
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support automatic configuration of network, usb and pci devices. A hotplug
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event can be used to invoke an agent script to perform the configuration task.
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You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
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scripts.
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#!/bin/bash
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# $Id: cpu.agent
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# Kernel hotplug params include:
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#ACTION=%s [online or offline]
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#DEVPATH=%s
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#
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cd /etc/hotplug
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. ./hotplug.functions
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case $ACTION in
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online)
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echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
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;;
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offline)
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echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
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;;
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*)
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debug_mesg CPU $ACTION event not supported
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exit 1
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;;
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esac
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