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>
diff --git a/kernel/sched.c b/kernel/sched.c
index 4cdc91c..9e49af0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1404,6 +1404,18 @@
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 @@
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 @@
*/
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 @@
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);
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index fef0bfd..6c5fa10 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -4269,22 +4269,6 @@
}
#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 @@
}
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;
}
