On 06/17/2011 10:02 AM, Benjamin Herrenschmidt wrote:

On Tue, 2011-06-07 at 22:00 +0530, Trinabh Gupta wrote:

quoted

diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
index 39a2baa..932392b 100644
--- a/arch/powerpc/kernel/idle.c
+++ b/arch/powerpc/kernel/idle.c

@@ -102,6 +102,24 @@ void cpu_idle(void)
  	}
  }

+static void do_nothing(void *unused)
+{
+}
+
+/*
+ * cpu_idle_wait - Used to ensure that all the CPUs come out of the old
+ * idle loop and start using the new idle loop.
+ * Required while changing idle handler on SMP systems.
+ * Caller must have changed idle handler to the new value before the call.
+ */
+void cpu_idle_wait(void)
+{
+	smp_mb();
+	/* kick all the CPUs so that they exit out of old idle routine */
+	smp_call_function(do_nothing, NULL, 1);
+}
+EXPORT_SYMBOL_GPL(cpu_idle_wait);
+
  int powersave_nap;

  #ifdef CONFIG_SYSCTL

This is gross :-)

Well this is what exists today for x86; so didn't think too
much into this. Maybe there is cleaner way. The requirement
is to completely exit the idle loop and call cpuidle_idle_call()
again. I think sending reschedule may be enough.

With respect to current implementation the arch-independent cpuidle
code needs a cpu_idle_wait() function for any architecture where
CONFIG_SMP is defined. This cpu_idle_wait function is called
whenever we have to pause usage of cpuidle; to switch driver
or governor etc. So maybe there is a cleaner implementation of
cpu_idle_wait instead of smp_call_function(do_nothing...); sending
reschedule may work.

Thanks
-Trinabh

Do you need to absolutely ensure the idle task has changed or just
kicking it with a send reschedule is enough ?

Cheers,
Ben.