Re: [PATCH 1/2] cpuhotplug/nohz: Remove offline cpus from nohz-idle state

* Russell King - ARM Linux [off-list ref] [2013-01-05 10:36:27]:

On Thu, Jan 03, 2013 at 06:58:38PM -0800, Srivatsa Vaddagiri wrote:

quoted

I also think that the
wait_for_completion() based wait in ARM's __cpu_die() can be replaced with a
busy-loop based one, as the wait there in general should be terminated within
few cycles.

Why open-code this stuff when we have infrastructure already in the kernel
for waiting for stuff to happen?  I chose to use the standard infrastructure
because its better tested, and avoids having to think about whether we need
CPU barriers and such like to ensure that updates are seen in a timely
manner.

I was primarily thinking of calling as few generic functions as possible on
a dead cpu. I recall several "am I running on a dead cpu?" checks
(cpu_is_offline(this_cpu) that were put in generic routines during early
versions of cpu hotplug [1] to educate code running on dead cpu, the need for
which went away though with introduction of atomic/stop-machine variant. The
need to add a RCU_NONIDLE() wrapper around ARM's cpu_die() [2] is perhaps a more
recent example of educating code running on dead cpu. As quickly we die as
possible after idle thread of dying cpu gains control, the better!

1. http://lwn.net/Articles/69040/
2. http://lists.infradead.org/pipermail/linux-arm-kernel/2012-July/107971.html

- vatsa
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