On Tue, Nov 06, 2012 at 12:24:20PM -0800, Steve Muckle wrote:

On 11/05/12 01:15, Russell King - ARM Linux wrote:

quoted

On Mon, Nov 05, 2012 at 09:23:38AM +0530, Taniya Das wrote:

quoted

I was looking at the arch/arm/process.c:cpu_die, I was wondering why we
do not exit from NOHZ for a hotplugged/offline cpu?

Offlined CPUs are not supposed to just 'return' from cpu_die(), and
in actual fact they can't.

Although an offline CPU will not come back into the idle thread by
returning from cpu_die(), I believe the question about exiting nohz mode
before cpu_die() still stands.

Well, that depends on the design of all that.  I don't know that, but
what I do know is we're doing exactly the same as x86 which I presume
is correct.

quoted

From arch/arm/process.c:cpu_idle() it appears a CPU could enter

pm_idle() for a while (in nohz mode), then execute the inner loop again
and call cpu_die(). Is there the potential for cpu idle time accounting
to get messed up here by not exiting nohz mode prior to the cpu dying,
so that missed ticks from nohz are properly accounted for?

Why does the idle time matter if the CPU has been taken offline?

In any case, I think this is a question for the CPU idle people; it's not
a feature that I've ever knowingly used or have any interest in using at
the moment.