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cpumask: don't recommend set_cpus_allowed hack in Documentation/cpu-hotplug.txt
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Cc: Gautham R Shenoy <ego@in.ibm.com> Cc: Ashok Raj <ashok.raj@intel.com>
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@ -315,41 +315,26 @@ A: The following are what is required for CPU hotplug infrastructure to work
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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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A: There are two ways. If your code can be run in interrupt context, use
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smp_call_function_single(), otherwise use work_on_cpu(). Note that
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work_on_cpu() is slow, and can fail due to out of memory:
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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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int err;
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get_online_cpus();
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if (!cpu_online(cpu))
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err = -EINVAL;
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else
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#if NEEDS_BLOCKING
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err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
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#else
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smp_call_function_single(cpu, __my_func_on_cpu, &err,
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true);
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#endif
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put_online_cpus();
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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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