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Diffstat (limited to 'Documentation/cpu-hotplug.txt')
| -rw-r--r-- | Documentation/cpu-hotplug.txt | 166 |
1 files changed, 114 insertions, 52 deletions
diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt index 9d620c153b0..a0b005d2bd9 100644 --- a/Documentation/cpu-hotplug.txt +++ b/Documentation/cpu-hotplug.txt @@ -2,10 +2,10 @@ Maintainers: CPU Hotplug Core: - Rusty Russell <rusty@rustycorp.com.au> + Rusty Russell <rusty@rustcorp.com.au> Srivatsa Vaddagiri <vatsa@in.ibm.com> i386: - Zwane Mwaikambo <zwane@arm.linux.org.uk> + Zwane Mwaikambo <zwanem@gmail.com> ppc64: Nathan Lynch <nathanl@austin.ibm.com> Joel Schopp <jschopp@austin.ibm.com> @@ -47,7 +47,13 @@ maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using other cpus later online, read FAQ's for more info. additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets - cpu_possible_map = cpu_present_map + additional_cpus + cpu_possible_mask = cpu_present_mask + additional_cpus + +cede_offline={"off","on"} Use this option to disable/enable putting offlined + processors to an extended H_CEDE state on + supported pseries platforms. + If nothing is specified, + cede_offline is set to "on". (*) Option valid only for following architectures - ia64 @@ -58,11 +64,11 @@ 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 doesn't 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. +cpu_possible_mask. possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus. This option sets possible_cpus bits in - cpu_possible_map. Thus keeping the numbers of bits set + cpu_possible_mask. Thus keeping the numbers of bits set constant even if the machine gets rebooted. CPU maps and such @@ -70,7 +76,7 @@ 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 +cpu_possible_mask: 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 @@ -78,13 +84,13 @@ 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() +cpu_online_mask: 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 +cpu_present_mask: 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 @@ -93,22 +99,22 @@ 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_possible_cpu() to iterate. +cpu_possible_mask/for_each_possible_cpu() to iterate. Never use anything other than cpumask_t to represent bitmap of CPUs. #include <linux/cpumask.h> - 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_possible_cpu - Iterate over cpu_possible_mask + for_each_online_cpu - Iterate over cpu_online_mask + for_each_present_cpu - Iterate over cpu_present_mask for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask. #include <linux/cpu.h> get_online_cpus() and put_online_cpus(): The above calls are used to inhibit cpu hotplug operations. While the -cpu_hotplug.refcount is non zero, the cpu_online_map will not change. +cpu_hotplug.refcount is non zero, the cpu_online_mask 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 @@ -122,7 +128,7 @@ 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. +Make sure that you have CONFIG_SMP turned on as well. You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support as well. @@ -190,7 +196,7 @@ 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? +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 @@ -201,6 +207,30 @@ by making it not-removable. In such cases you will also notice that the online file is missing under cpu0. +Q: Is CPU0 removable on X86? +A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is +removable by default. Otherwise, CPU0 is also removable by kernel option +cpu0_hotplug. + +But some features depend on CPU0. Two known dependencies are: + +1. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if +CPU0 is offline and you need to online CPU0 before hibernate/suspend can +continue. +2. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt +is detected. + +It's said poweroff/reboot may depend on CPU0 on some machines although I haven't +seen any poweroff/reboot failure so far after CPU0 is offline on a few tested +machines. + +Please let me know if you know or see any other dependencies of CPU0. + +If the dependencies are under your control, you can turn on CPU0 hotplug feature +either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug. + +--Fenghua Yu <fenghua.yu@intel.com> + 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 @@ -237,8 +267,8 @@ Q: If i have some kernel code that needs to be aware of CPU arrival and 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) + static int foobar_cpu_callback(struct notifier_block *nfb, + unsigned long action, void *hcpu) { unsigned int cpu = (unsigned long)hcpu; @@ -255,7 +285,7 @@ A: This is what you would need in your kernel code to receive notifications. return NOTIFY_OK; } - static struct notifier_block __cpuinitdata foobar_cpu_notifer = + static struct notifier_block foobar_cpu_notifier = { .notifier_call = foobar_cpu_callback, }; @@ -282,12 +312,57 @@ 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 + do this: for_each_online_cpu(i) { foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i); } + However, if you want to register a hotplug callback, as well as perform + some initialization for CPUs that are already online, then do this: + + Version 1: (Correct) + --------- + + cpu_notifier_register_begin(); + + for_each_online_cpu(i) { + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_UP_PREPARE, i); + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_ONLINE, i); + } + + /* Note the use of the double underscored version of the API */ + __register_cpu_notifier(&foobar_cpu_notifier); + + cpu_notifier_register_done(); + + Note that the following code is *NOT* the right way to achieve this, + because it is prone to an ABBA deadlock between the cpu_add_remove_lock + and the cpu_hotplug.lock. + + Version 2: (Wrong!) + --------- + + get_online_cpus(); + + for_each_online_cpu(i) { + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_UP_PREPARE, i); + foobar_cpu_callback(&foobar_cpu_notifier, + CPU_ONLINE, i); + } + + register_cpu_notifier(&foobar_cpu_notifier); + + put_online_cpus(); + + So always use the first version shown above when you want to register + callbacks as well as initialize the already online CPUs. + + 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 @@ -309,41 +384,26 @@ A: The following are what is required for CPU hotplug infrastructure to work 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 +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 + work_on_cpu() is slow, and can fail due to out of memory: 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; - } - + int err; + get_online_cpus(); + if (!cpu_online(cpu)) + err = -EINVAL; + else +#if NEEDS_BLOCKING + err = work_on_cpu(cpu, __my_func_on_cpu, NULL); +#else + smp_call_function_single(cpu, __my_func_on_cpu, &err, + true); +#endif + put_online_cpus(); + 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 @@ -355,8 +415,10 @@ A: There is no clear spec defined way from ACPI that can give us that 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. + Caveat: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c + or earlier ACPI version supported, because the apicid field in MADT is only + 8 bits. From ACPI 3.0, this limitation was removed since the apicid field + was extended to 32 bits with x2APIC introduced. User Space Notification |