[patch 2/2] nohz: try to avoid IPI when setting tick dependency for task

[Marcelo Tosatti <mtosatti@redhat.com>]

When enabling per-CPU posix timers, an IPI to nohz_full CPUs might be
performed (to re-read the dependencies and possibly not re-enter
nohz_full on a given CPU).

A common case is for applications that run on nohz_full= CPUs
to not use POSIX timers (eg DPDK).

This patch optimizes tick_nohz_dep_set_task to avoid kicking
all nohz_full= CPUs in case the task allowed mask does not
intersect with nohz_full= CPU mask,
when going through tick_nohz_dep_set_task.

This reduces interruptions to nohz_full= CPUs.

---
 kernel/time/tick-sched.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/time/tick-sched.c
===================================================================
--- linux-2.6.orig/kernel/time/tick-sched.c
+++ linux-2.6/kernel/time/tick-sched.c
@@ -383,11 +383,16 @@ void tick_nohz_dep_set_task(struct task_
 			tick_nohz_full_kick();
 			preempt_enable();
 		} else {
+			unsigned long flags;
+
 			/*
 			 * Some future tick_nohz_full_kick_task()
-			 * should optimize this.
+			 * should further optimize this.
 			 */
-			tick_nohz_full_kick_all();
+			raw_spin_lock_irqsave(&tsk->pi_lock, flags);
+			if (cpumask_intersects(&tsk->cpus_mask, tick_nohz_full_mask))
+				tick_nohz_full_kick_all();
+			raw_spin_unlock_irqrestore(&tsk->pi_lock, flags);
 		}
 	}
 }