[PATCH 03/13] rcu: Exclude all potential timekeepers from sysidle detection

[Frederic Weisbecker]

The purpose of the full system idle detection is to notify the CPU
handling the timekeeping when the rest of the system is idle so that it
can sleep when nobody needs the jiffies nor GTOD to be maintained.

Now this machinery excludes CPU 0 itself from the range of the idle
detection because if CPU 0 has any non-idle task to execute, it is going
to restart its own tick since it's guaranteed to be outside the full
dynticks range. And as it is the only eligible timekeeper when
CONFIG_NO_HZ_FULL=y anyway, it can handle the timekeeping duty for
and by itself.

Still we also plan to extend the timekeepers affinity and allow every CPU
outside the full dynticks range to handle the timekeeping duty, not just
CPU 0.

So once we reach that step, we can state that all CPUs that are not
full dynticks can be excluded from the full system idle detection,
simply because those CPUs share the same property than CPU 0 today. When
a non full dynticks CPU needs to run some busy task, it restarts its
tick and handles the timekeeping duty for its own needs as is currently
done under CONFIG_NO_HZ_IDLE=y.

To prepare for this support in the sysidle detection, we can use the
tick_timekeeping_cpu() API which checks if a CPU is allowed to handle
timekeeping duty. If so we can conclude that it's not full dynticks and
it can maintain timekeeping by itself and as such it can be excluded
from the sysidle detection.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Alex Shi <alex.shi@xxxxxxxxxx>
Cc: Kevin Hilman <khilman@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6abb03d..08004da 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2539,7 +2539,7 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
 	 * invoke rcu_sysidle_force_exit() directly if it does anything
 	 * more than take a scheduling-clock interrupt.
 	 */
-	if (smp_processor_id() == tick_do_timer_cpu)
+	if (tick_timekeeping_cpu(smp_processor_id()))
 		return;
 
 	/* Update system-idle state: We are clearly no longer fully idle! */
@@ -2563,10 +2563,10 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
 	 * is an offline or the timekeeping CPU, nothing to do.
 	 */
 	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
-	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
+	    cpu_is_offline(rdp->cpu) || tick_timekeeping_cpu(rdp->cpu))
 		return;
 	if (rcu_gp_in_progress(rdp->rsp))
-		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
+		WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id()));
 
 	/* Pick up current idle and NMI-nesting counter and check. */
 	cur = atomic_read(&rdtp->dynticks_idle);
@@ -2729,7 +2729,7 @@ bool rcu_sys_is_idle(void)
 	static struct rcu_sysidle_head rsh;
 	int rss = ACCESS_ONCE(full_sysidle_state);
 
-	if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
+	if (WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id())))
 		return false;
 
 	/* Handle small-system case by doing a full scan of CPUs. */
-- 
1.8.3.1

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