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

[Marcelo Tosatti]

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);
                }
        }
 }