[PATCH] doc: fixing cpu-hotplug documentation
Fixing cpu-hotplug documentation as follows: - moving confusing asterisk on additional_cpus descrition - fixing some typos - unifying indentation for source code and command line example Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com> Cc: Ashok Raj <ashok.raj@intel.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Satoru Takeuchi
Linus Torvalds
parent
fca4edb48b
commit
ca926e80dc
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@ -46,7 +46,7 @@ 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 Use this to limit hotpluggable cpus. This option sets
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additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
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cpu_possible_map = cpu_present_map + additional_cpus
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(*) Option valid only for following architectures
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@ -101,15 +101,15 @@ cpu_possible_map/for_each_possible_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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#include <linux/cpumask.h>
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for_each_possible_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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for_each_possible_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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#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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@ -120,7 +120,7 @@ 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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Q: How to 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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@ -141,39 +141,39 @@ 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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....
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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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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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#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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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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#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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CONFIG_HOTPLUG_CPU 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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@ -181,17 +181,17 @@ 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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#echo 0 > /sys/devices/system/cpu/cpuX/online
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once the logical offline is successful, check
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Once the logical offline is successful, check
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#cat /proc/interrupts
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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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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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#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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@ -234,8 +234,8 @@ 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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#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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@ -279,10 +279,10 @@ 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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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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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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@ -307,38 +307,38 @@ 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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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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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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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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/*
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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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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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