On Tue, Dec 17, 2013 at 11:51:24PM +0100, Frederic Weisbecker wrote:
> The plan with full system idle detection is to allow the timekeeper
> to sleep when all full dynticks CPUs are sleeping.
>
> Then when a full dynticks CPU wakes up while the whole system is idle,
> it sends an IPI to the timekeeping CPU which then restarts its tick
> and polls on its timekeeping duty on behalf of all other CPUs in the
> system.
>
> But we are using rcu_kick_nohz_cpu() to raise this IPI, which is wrong
> because this function is used to kick full dynticks CPUs when they run
> in the kernel for too long without reporting a quiescent state. And
> this function ignores targets that are not full dynticks, like our
> timekeeper.
>
> To fix this, use the smp_send_reschedule() function directly.
I guess the fact that you needed some change is reassuring. You know
the old saying, "no bugs, no users". ;-)
> Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: John Stultz <john.stu...@linaro.org>
> Cc: Alex Shi <alex....@linaro.org>
> Cc: Kevin Hilman <khil...@linaro.org>
> ---
> kernel/rcu/tree_plugin.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 08004da..84d90c8 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2488,7 +2488,7 @@ void rcu_sysidle_force_exit(void)
> oldstate, RCU_SYSIDLE_NOT);
> if (oldstate == newoldstate &&
> oldstate == RCU_SYSIDLE_FULL_NOTED) {
> - rcu_kick_nohz_cpu(tick_do_timer_cpu);
> + smp_send_reschedule(tick_do_timer_cpu);
Hmmm...
We haven't done any sort of wakeup, and tick_nohz_full_cpu() should
return false for tick_do_timer_cpu, and I don't see that we have
done anything to make got_nohz_idle_kick() return true.
So the idea is that the fact of the interrupt is sufficient, and
the target CPU will figure out that it must turn its scheduling-clock
interrupt when returning from interrupt?
Or is something else going on here?
Thanx, Paul
> return; /* We cleared it, done! */
> }
> oldstate = newoldstate;
> --
> 1.8.3.1
>
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