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cpumask: don't recommend set_cpus_allowed hack in Documentation/cpu-h…
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…otplug.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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Rusty Russell committed Dec 17, 2009
1 parent 62ac127 commit 4251417
Showing 1 changed file with 17 additions and 32 deletions.
49 changes: 17 additions & 32 deletions Documentation/cpu-hotplug.txt
Original file line number Diff line number Diff line change
Expand Up @@ -315,41 +315,26 @@ 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
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