Re: [PATCH 1/4] 2.6.2-rc2-mm2 CPU Hotplug: cpu_active_map

[Ingo Molnar <mingo@elte.hu>]

* Rusty Russell <rusty@rustcorp.com.au> wrote:

> D: When CPUs are going down, there is a time when cpu_online(cpu) is
> D: false, but they are still scheduling and responding to interrupts
> D: (we are migrating things off the CPU, shutting down per-cpu
> D: threads, etc).  It turns out that RCU cares about these CPUs, so
> D: the decision was made to expose this mask (previously internal to x86,
> D: and only used for IPIs).

these kinds of problems could be avoided by making the CPU-off as much
of an atomic operation as possible. The less atomic it is, the more
kernel code is exposed to the transitional state - and since this is a
rare situation it will always have quality problems. Is there any killer
argument that makes it impossible to down a CPU atomically? Kernel
threads can get their callbacks on other CPUs just fine. Other tasks
should not care. If the migrate-off operation is done 100% atomically
then zero knowledge is needed by unrelated scheduler code about the act
of disabling a CPU.

	Ingo