[RFC][PATCH 1/7] sched: Fix smp_call_function_single_async() usage for ILB

[Peter Zijlstra <peterz@infradead.org>]

The recent commit: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
got smp_call_function_single_async() subtly wrong. Even though it will
return -EBUSY when trying to re-use a csd, that condition is not
atomic and still requires external serialization.

The change in kick_ilb() got this wrong.

While on first reading kick_ilb() has an atomic test-and-set that
appears to serialize the use, the matching 'release' is not in the
right place to actually guarantee this serialization.

Rework the nohz_idle_balance() trigger so that the release is in the
IPI callback and thus guarantees the required serialization for the
CSD.

Fixes: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h      |    4 +
 include/linux/sched/idle.h |   55 ++++++++++++++++++
 kernel/sched/core.c        |  132 +++++++++------------------------------------
 kernel/sched/fair.c        |   18 ++----
 kernel/sched/idle.c        |    3 -
 kernel/sched/sched.h       |    2 
 kernel/smp.c               |    7 ++
 7 files changed, 102 insertions(+), 119 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -637,41 +568,25 @@ void wake_up_nohz_cpu(int cpu)
 		wake_up_idle_cpu(cpu);
 }
 
-static inline bool got_nohz_idle_kick(void)
+static void nohz_csd_func(void *info)
 {
-	int cpu = smp_processor_id();
-
-	if (!(atomic_read(nohz_flags(cpu)) & NOHZ_KICK_MASK))
-		return false;
-
-	if (idle_cpu(cpu) && !need_resched())
-		return true;
+	struct rq *rq = info;
+	int cpu = cpu_of(rq);
+	unsigned int flags;
 
 	/*
-	 * We can't run Idle Load Balance on this CPU for this time so we
-	 * cancel it and clear NOHZ_BALANCE_KICK
+	 * Release the rq::nohz_csd.
 	 */
-	atomic_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
-	return false;
-}
-
-static void nohz_csd_func(void *info)
-{
-	struct rq *rq = info;
+	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
+	WARN_ON(!(flags & NOHZ_KICK_MASK));
 
-	if (got_nohz_idle_kick()) {
-		rq->idle_balance = 1;
+	rq->idle_balance = idle_cpu(cpu);
+	if (rq->idle_balance && !need_resched()) {
+		rq->nohz_idle_balance = flags;
 		raise_softirq_irqoff(SCHED_SOFTIRQ);
 	}
 }
 
-#else /* CONFIG_NO_HZ_COMMON */
-
-static inline bool got_nohz_idle_kick(void)
-{
-	return false;
-}
-
 #endif /* CONFIG_NO_HZ_COMMON */
 
 #ifdef CONFIG_NO_HZ_FULL
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10024,6 +10024,10 @@ static void kick_ilb(unsigned int flags)
 	if (ilb_cpu >= nr_cpu_ids)
 		return;
 
+	/*
+	 * Access to rq::nohz_csd is serialized by NOHZ_KICK_MASK; he who sets
+	 * the first flag owns it; cleared by nohz_csd_func().
+	 */
 	flags = atomic_fetch_or(flags, nohz_flags(ilb_cpu));
 	if (flags & NOHZ_KICK_MASK)
 		return;
@@ -10371,20 +10375,14 @@ static bool _nohz_idle_balance(struct rq
  */
 static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 {
-	int this_cpu = this_rq->cpu;
-	unsigned int flags;
+	unsigned int flags = this_rq->nohz_idle_balance;
 
-	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
+	if (!flags)
 		return false;
 
-	if (idle != CPU_IDLE) {
-		atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
-		return false;
-	}
+	this_rq->nohz_idle_balance = 0;
 
-	/* could be _relaxed() */
-	flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
-	if (!(flags & NOHZ_KICK_MASK))
+	if (idle != CPU_IDLE)
 		return false;
 
 	_nohz_idle_balance(this_rq, flags, idle);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -951,6 +951,7 @@ struct rq {
 
 	struct callback_head	*balance_callback;
 
+	unsigned char		nohz_idle_balance;
 	unsigned char		idle_balance;
 
 	unsigned long		misfit_task_load;