Commit 873b4c65 authored Jun 05, 2013 by Vincent Guittot Committed by Ingo Molnar Jun 19, 2013

sched: Fix clear NOHZ_BALANCE_KICK



I have faced a sequence where the Idle Load Balance was sometime not
triggered for a while on my platform, in the following scenario:

 CPU 0 and CPU 1 are running tasks and CPU 2 is idle

 CPU 1 kicks the Idle Load Balance
 CPU 1 selects CPU 2 as the new Idle Load Balancer
 CPU 2 sets NOHZ_BALANCE_KICK for CPU 2
 CPU 2 sends a reschedule IPI to CPU 2

 While CPU 3 wakes up, CPU 0 or CPU 1 migrates a waking up task A on CPU 2

 CPU 2 finally wakes up, runs task A and discards the Idle Load Balance
       task A quickly goes back to sleep (before a tick occurs on CPU 2)
 CPU 2 goes back to idle with NOHZ_BALANCE_KICK set

Whenever CPU 2 will be selected as the ILB, no reschedule IPI will be sent
because NOHZ_BALANCE_KICK is already set and no Idle Load Balance will be
performed.

We must wait for the sched softirq to be raised on CPU 2 thanks to another
part the kernel to come back to clear NOHZ_BALANCE_KICK.

The proposed solution clears NOHZ_BALANCE_KICK in schedule_ipi if
we can't raise the sched_softirq for the Idle Load Balance.

Change since V1:

- move the clear of NOHZ_BALANCE_KICK in got_nohz_idle_kick if the ILB
  can't run on this CPU (as suggested by Peter)

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1370419991-13870-1-git-send-email-vincent.guittot@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent b0bc225d

kernel/sched/core.c

+17 −4

Original line number	Diff line number	Diff line
		@@ -633,7 +633,19 @@ void wake_up_nohz_cpu(int cpu)
		static inline bool got_nohz_idle_kick(void)
		{
		int cpu = smp_processor_id();
		return idle_cpu(cpu) && test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));

		if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)))
		return false;

		if (idle_cpu(cpu) && !need_resched())
		return true;

		/*
		* We can't run Idle Load Balance on this CPU for this time so we
		* cancel it and clear NOHZ_BALANCE_KICK
		*/
		clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
		return false;
		}

		#else /* CONFIG_NO_HZ_COMMON */
		@@ -1393,8 +1405,9 @@ static void sched_ttwu_pending(void)

		void scheduler_ipi(void)
		{
		if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick()
		&& !tick_nohz_full_cpu(smp_processor_id()))
		if (llist_empty(&this_rq()->wake_list)
		&& !tick_nohz_full_cpu(smp_processor_id())
		&& !got_nohz_idle_kick())
		return;

		/*
		@@ -1417,7 +1430,7 @@ void scheduler_ipi(void)
		/*
		* Check if someone kicked us for doing the nohz idle load balance.
		*/
		if (unlikely(got_nohz_idle_kick() && !need_resched())) {
		if (unlikely(got_nohz_idle_kick())) {
		this_rq()->idle_balance = 1;
		raise_softirq_irqoff(SCHED_SOFTIRQ);
		}

Admin message