Commit ca38062e authored Oct 03, 2011 by Suresh Siddha Committed by Ingo Molnar Oct 06, 2011

sched: Use resched IPI to kick off the nohz idle balance



Current use of smp call function to kick the nohz idle balance can deadlock
in this scenario.

1. cpu-A did a generic_exec_single() to cpu-B and after queuing its call single
data (csd) to the call single queue, cpu-A took a timer interrupt.  Actual IPI
to cpu-B to process the call single queue is not yet sent.

2. As part of the timer interrupt handler, cpu-A decided to kick cpu-B
for the idle load balancing (sets cpu-B's rq->nohz_balance_kick to 1)
and __smp_call_function_single() with nowait will queue the csd to the
cpu-B's queue. But the generic_exec_single() won't send an IPI to cpu-B
as the call single queue was not empty.

3. cpu-A is busy with lot of interrupts

4. Meanwhile cpu-B is entering and exiting idle and noticed that it has
it's rq->nohz_balance_kick set to '1'. So it will go ahead and do the
idle load balancer and clear its rq->nohz_balance_kick.

5. At this point, csd queued as part of the step-2 above is still locked
and waiting to be serviced on cpu-B.

6. cpu-A is still busy with interrupt load and now it got another timer
interrupt and as part of it decided to kick cpu-B for another idle load
balancing (as it finds cpu-B's rq->nohz_balance_kick cleared in step-4
above) and does __smp_call_function_single() with the same csd that is
still locked.

7. And we get a deadlock waiting for the csd_lock() in the
__smp_call_function_single().

Main issue here is that cpu-B can service the idle load balancer kick
request from cpu-A even with out receiving the IPI and this lead to
doing multiple __smp_call_function_single() on the same csd leading to
deadlock.

To kick a cpu, scheduler already has the reschedule vector reserved. Use
that mechanism (kick_process()) instead of using the generic smp call function
mechanism to kick off the nohz idle load balancing and avoid the deadlock.

   [ This issue is present from 2.6.35+ kernels, but marking it -stable
     only from v3.0+ as the proposed fix depends on the scheduler_ipi()
     that is introduced recently. ]

Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: stable@kernel.org # v3.0+
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111003220934.834943260@sbsiddha-desk.sc.intel.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 9243a169

kernel/sched.c

+19 −2

Original line number	Diff line number	Diff line
		@@ -1404,6 +1404,18 @@ void wake_up_idle_cpu(int cpu)
		smp_send_reschedule(cpu);
		}

		static inline bool got_nohz_idle_kick(void)
		{
		return idle_cpu(smp_processor_id()) && this_rq()->nohz_balance_kick;
		}

		#else /* CONFIG_NO_HZ */

		static inline bool got_nohz_idle_kick(void)
		{
		return false;
		}

		#endif /* CONFIG_NO_HZ */

		static u64 sched_avg_period(void)
		@@ -2717,7 +2729,7 @@ static void sched_ttwu_pending(void)

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

		/*
		@@ -2735,6 +2747,12 @@ void scheduler_ipi(void)
		*/
		irq_enter();
		sched_ttwu_pending();

		/*
		* Check if someone kicked us for doing the nohz idle load balance.
		*/
		if (unlikely(got_nohz_idle_kick() && !need_resched()))
		raise_softirq_irqoff(SCHED_SOFTIRQ);
		irq_exit();
		}

		@@ -8288,7 +8306,6 @@ void __init sched_init(void)
		rq_attach_root(rq, &def_root_domain);
		#ifdef CONFIG_NO_HZ
		rq->nohz_balance_kick = 0;
		init_sched_softirq_csd(&per_cpu(remote_sched_softirq_cb, i));
		#endif
		#endif
		init_rq_hrtick(rq);

kernel/sched_fair.c

+9 −20

Original line number	Diff line number	Diff line
		@@ -4269,22 +4269,6 @@ out_unlock:
		}

		#ifdef CONFIG_NO_HZ

		static DEFINE_PER_CPU(struct call_single_data, remote_sched_softirq_cb);

		static void trigger_sched_softirq(void *data)
		{
		raise_softirq_irqoff(SCHED_SOFTIRQ);
		}

		static inline void init_sched_softirq_csd(struct call_single_data *csd)
		{
		csd->func = trigger_sched_softirq;
		csd->info = NULL;
		csd->flags = 0;
		csd->priv = 0;
		}

		/*
		* idle load balancing details
		* - One of the idle CPUs nominates itself as idle load_balancer, while
		@@ -4450,11 +4434,16 @@ static void nohz_balancer_kick(int cpu)
		}

		if (!cpu_rq(ilb_cpu)->nohz_balance_kick) {
		struct call_single_data *cp;

		cpu_rq(ilb_cpu)->nohz_balance_kick = 1;
		cp = &per_cpu(remote_sched_softirq_cb, cpu);
		__smp_call_function_single(ilb_cpu, cp, 0);

		smp_mb();
		/*
		* Use smp_send_reschedule() instead of resched_cpu().
		* This way we generate a sched IPI on the target cpu which
		* is idle. And the softirq performing nohz idle load balance
		* will be run before returning from the IPI.
		*/
		smp_send_reschedule(ilb_cpu);
		}
		return;
		}

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