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>
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
-(*) Option valid only for following architectures
-- x86_64, ia64, s390
+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".
-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
-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.
+(*) Option valid only for following architectures
+- ia64
-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.
+ia64 uses 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 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_mask.
-possible_cpus=n [s390 only] use this to set hotpluggable cpus.
+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.
- This option overrides 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
+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
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
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
#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
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
projects / linux-flexiantxendom0-3.2.10.git / blobdiff