cpumask: don't recommend set_cpus_allowed hack in Documentation/cpu-hotplug.txt
Rusty Russell [Thu, 17 Dec 2009 17:43:29 +0000 (11:43 -0600)]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Ashok Raj <ashok.raj@intel.com>

Documentation/cpu-hotplug.txt

index 4d4a644..a99d703 100644 (file)
@@ -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