sched/rt: Decrease number of calls of push_rt_task() in push_rt_tasks()

The patch aims to decrease the number of calls of push_rt_task() in push_rt_tasks().

It's not necessary to push more than 'num_online_cpus() - 1' tasks. If just pushed task doesn't leave its new CPU during our local call of push_rt_tasks() than we won't push another task to the CPU. If it leave or change priority than it will pull new task by itself.

Signed-off-by Kirill V Tkhai <[email protected]>

CC Steven Rostedt <[email protected]>

CC Ingo Molnar <[email protected]>

CC Peter Zijlstra <[email protected]>

CC linux-rt-users <[email protected]>

---
 kernel/sched/rt.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

kernel/sched/rt.c +3 -1

index 4e8f0f4..edf046d 100644
@@ -1703,8 +1703,10 @@ out:
 
 static void push_rt_tasks(struct rq *rq)
 {
+	int num_cpus = num_online_cpus() - 1;
+
 	/* push_rt_task will return true if it moved an RT */
-	while (push_rt_task(rq))
+	while (push_rt_task(rq) && --num_cpus > 0)
 		;
 }
 

Quoted 4 lines

 The patch aims to decrease the number of calls of push_rt_task() in push_rt_tasks().

Quoted 4 lines

 It's not necessary to push more than 'num_online_cpus() - 1' tasks. If just pushed task doesn't leave its new CPU during our local call of push_rt_tasks() than we won't push another task to the CPU. If it leave or change priority than it will pull new task by itself.

I'm curious. Have you hit situations where this was an issue? Or was this just discovered by code review?

-- Steve

Quoted 23 lines

Signed-off-by Kirill V Tkhai <[email protected]>

CC Steven Rostedt <[email protected]>

CC Ingo Molnar <[email protected]>

CC Peter Zijlstra <[email protected]>

CC linux-rt-users <[email protected]>

 ---
  kernel/sched/rt.c |    4 +++-

  1 file changed, 3 insertions(+), 1 deletion(-)

 diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4e8f0f4..edf046d 100644
 --- a/kernel/sched/rt.c
 +++ b/kernel/sched/rt.c
 @@ -1703,8 +1703,10 @@ out:

  static void push_rt_tasks(struct rq *rq)
  {
 + int num_cpus = num_online_cpus() - 1;
 +
          /* push_rt_task will return true if it moved an RT */
 - while (push_rt_task(rq))
 + while (push_rt_task(rq) && --num_cpus > 0)
                  ;
  }

31.01.2013, 20:08, "Steven Rostedt" <[email protected]>:

Quoted 10 lines · Kirill Tkhai

Quoted 8 lines

The patch aims to decrease the number of calls of push_rt_task() in push_rt_tasks().

It's not necessary to push more than 'num_online_cpus() - 1' tasks. If just pushed task doesn't leave its new CPU during our local call of push_rt_tasks() than we won't push another task to the CPU. If it leave or change priority than it will pull new task by itself.

Quoted 2 lines

I'm curious. Have you hit situations where this was an issue? Or was this just discovered by code review?

No, I did't hit this situation. It's impossible to hook every situation.

Thanks for your explanation.

Kirill