[PATCH 1/2] nohz: Affine unpinned timers to housekeepers

[Frederic Weisbecker <fweisbec@gmail.com>]

From: Vatika Harlalka <vatikaharlalka@gmail.com>

The problem addressed in this patch is about affining unpinned timers.
Adaptive or Full Dynticks CPUs are currently disturbed by unnecessary
jitter due to firing of such timers on them.

This patch will affine timers to online CPUs which are not full dynticks
in NOHZ_FULL configured systems. It should not introduce overhead in
nohz full off case due to static keys.

Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Signed-off by: Vatika Harlalka <vatikaharlalka@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 include/linux/tick.h | 9 +++++++++
 kernel/sched/core.c  | 7 +++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 48d901f..e312219 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -147,11 +147,20 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
 		cpumask_or(mask, mask, tick_nohz_full_mask);
 }
 
+static inline int housekeeping_any_cpu(void)
+{
+	return cpumask_any_and(housekeeping_mask, cpu_online_mask);
+}
+
 extern void tick_nohz_full_kick(void);
 extern void tick_nohz_full_kick_cpu(int cpu);
 extern void tick_nohz_full_kick_all(void);
 extern void __tick_nohz_task_switch(void);
 #else
+static inline int housekeeping_any_cpu(void)
+{
+	return smp_processor_id();
+}
 static inline bool tick_nohz_full_enabled(void) { return false; }
 static inline bool tick_nohz_full_cpu(int cpu) { return false; }
 static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8b864ec..0902e4d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -623,18 +623,21 @@ int get_nohz_timer_target(void)
 	int i, cpu = smp_processor_id();
 	struct sched_domain *sd;
 
-	if (!idle_cpu(cpu))
+	if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu))
 		return cpu;
 
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
 		for_each_cpu(i, sched_domain_span(sd)) {
-			if (!idle_cpu(i)) {
+			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
 				cpu = i;
 				goto unlock;
 			}
 		}
 	}
+
+	if (!is_housekeeping_cpu(cpu))
+		cpu = housekeeping_any_cpu();
 unlock:
 	rcu_read_unlock();
 	return cpu;
-- 
2.1.4