[PATCH 09/10] nohz: Switch nohz full timekeeper to dynticks idle on top of sysidle detection

[Frederic Weisbecker <fweisbec@gmail.com>]

In full dynticks, the CPU 0 carries the timekeeping duty on behalf
of all other CPUs in the system. This way full dynticks are left
undisturbed on this regard.

Of course this prevents CPU 0 from entering in dynticks idle mode
because any CPU may need uptodate timekeeping at any time.

Theoretically though, we could put CPU 0 in dynticks idle mode once we
are sure that all other CPUs are dynticks idle as well. Then when a
CPU wakes up and finds the timekeeper idle, it would send an IPI to
wake it up on its duty.

Such a machine state needs to take care of all the races in the way, make
sure that CPU 0 is neither stuck accidentally to sleep for ever, nor
stuck in periodic mode when it could sleep. Also given the amount of
shared data this involves and their access frequency, this must be built
on top of lockless low-overhead state machine.

This is what sysidle provides. The feature is ready for a while, we
were just waiting for the nohz susbsystem to support it. And we just
reached that state.

So lets defer the last call for CPU 0 to enter in dynticks idle to when
we find a full system idle state. And lets wake it up when its duty is
needed.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 kernel/time/tick-sched.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index bcba79d..845aaff 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -547,8 +547,10 @@ static u64 timekeeping_deferment(struct tick_sched *ts, int cpu)
 
 	if (tick_do_timer_cpu == cpu) {
 		time_delta = timekeeping_max_deferment();
-		tick_do_timer_cpu = TICK_DO_TIMER_NONE;
 		ts->do_timer_last = 1;
+		/* In full dynticks mode, CPU 0 always keeps the duty */
+		if (!tick_nohz_full_enabled())
+			tick_do_timer_cpu = TICK_DO_TIMER_NONE;
 	} else if (ts->do_timer_last) {
 		if (tick_do_timer_cpu == TICK_DO_TIMER_NONE)
 			time_delta = timekeeping_max_deferment();
@@ -745,7 +747,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 		 * if there are full dynticks CPUs around
 		 */
 		if (tick_do_timer_cpu == cpu)
-			return false;
+			return rcu_sys_is_idle();
 	}
 
 	return true;
-- 
1.8.3.1