[tip:sched/urgent] nohz: Affine unpinned timers to housekeepers

[tip-bot for Vatika Harlalka <tipbot@zytor.com>]

Commit-ID:  9642d18eee2cd169b60c6ac0f20bda745b5a3d1e
Gitweb:     http://git.kernel.org/tip/9642d18eee2cd169b60c6ac0f20bda745b5a3d1e
Author:     Vatika Harlalka <vatikaharlalka@gmail.com>
AuthorDate: Tue, 1 Sep 2015 16:50:59 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 2 Sep 2015 10:33:22 +0200

nohz: Affine unpinned timers to housekeepers

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.

Signed-off-by: Vatika Harlalka <vatikaharlalka@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1441119060-2230-2-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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;