Re: [linux-pm] [PATCH 02/12] powermac: support G5 CPU hotplug

On Fri, 2007-02-09 at 08:59 +1100, Paul Mackerras wrote:

Johannes Berg writes:

quoted

+#ifdef CONFIG_CPU_HOTPLUG
+	/* if we have fake CPU hotplug just to support suspend to disk
+	 * and can't really turn off a CPU, it may be taking timer interrupts
+	 * even when it is dead. Avoid doing anything in that case so global
+	 * state is not modified for and by a CPU that doesn't really exist. */
+	if (__get_cpu_var(cpu_state) == CPU_DEAD)
+		return;
+#endif

Are you sure this is necessary?  IIRC, the if (!cpu_is_offline(cpu)) a
little bit further down is there to cope with this exact situation.

Yes, but we still do irq_enter() and as it happens with HZ=1000 we
almost always kick the CPU out while it's inbetween irq_enter() and
irq_exit().

You said:

And cpu hotplug already works on G4 SMP powermacs, where we can't
physically remove the cpu either - we just put it into sleep mode with
interrupts disabled, which is what we would be doing on a G5 powermac
also.

and then Benh said:

I've always had doubts about wether SLEEP with EE off was legal and
wouldn't actually still go out of sleep when getting an interrupt... In
that case, at least on CPUs where the DEC acts "level" triggered, we
need to at least reset the DEC to some sane value.

I probably should've mentioned that. I tried going into NAP with EE off
and that seemed to get me a busy loop where the CPU wasn't even napping
at all. I could have done something wrong, of course.

johannes