[RFC PATCH 1/5] sched: Convert nohz_flags to atomic_t

[Peter Zijlstra]

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c  |    6 +++---
 kernel/sched/fair.c  |   23 +++++++++++++++--------
 kernel/sched/sched.h |   11 ++++++-----
 3 files changed, 24 insertions(+), 16 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -608,7 +608,7 @@ static inline bool got_nohz_idle_kick(vo
 {
 	int cpu = smp_processor_id();
 
-	if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)))
+	if (!(atomic_read(nohz_flags(cpu)) & NOHZ_BALANCE_KICK))
 		return false;
 
 	if (idle_cpu(cpu) && !need_resched())
@@ -618,7 +618,7 @@ static inline bool got_nohz_idle_kick(vo
 	 * 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));
+	atomic_andnot(NOHZ_BALANCE_KICK, nohz_flags(cpu));
 	return false;
 }
 
@@ -6002,7 +6002,7 @@ void __init sched_init(void)
 		rq_attach_root(rq, &def_root_domain);
 #ifdef CONFIG_NO_HZ_COMMON
 		rq->last_load_update_tick = jiffies;
-		rq->nohz_flags = 0;
+		atomic_set(&rq->nohz_flags, 0);
 #endif
 #ifdef CONFIG_NO_HZ_FULL
 		rq->last_sched_tick = 0;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8991,6 +8991,7 @@ static inline int find_new_ilb(void)
  */
 static void nohz_balancer_kick(void)
 {
+	unsigned int flags;
 	int ilb_cpu;
 
 	nohz.next_balance++;
@@ -9000,7 +9001,8 @@ static void nohz_balancer_kick(void)
 	if (ilb_cpu >= nr_cpu_ids)
 		return;
 
-	if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu)))
+	flags = atomic_fetch_or(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu));
+	if (flags & NOHZ_BALANCE_KICK)
 		return;
 	/*
 	 * Use smp_send_reschedule() instead of resched_cpu().
@@ -9014,7 +9016,9 @@ static void nohz_balancer_kick(void)
 
 void nohz_balance_exit_idle(unsigned int cpu)
 {
-	if (unlikely(test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))) {
+	unsigned int flags = atomic_read(nohz_flags(cpu));
+
+	if (unlikely(flags & NOHZ_TICK_STOPPED)) {
 		/*
 		 * Completely isolated CPUs don't ever set, so we must test.
 		 */
@@ -9022,7 +9026,8 @@ void nohz_balance_exit_idle(unsigned int
 			cpumask_clear_cpu(cpu, nohz.idle_cpus_mask);
 			atomic_dec(&nohz.nr_cpus);
 		}
-		clear_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu));
+
+		atomic_andnot(NOHZ_TICK_STOPPED, nohz_flags(cpu));
 	}
 }
 
@@ -9076,7 +9081,7 @@ void nohz_balance_enter_idle(int cpu)
 	if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
 		return;
 
-	if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))
+	if (atomic_read(nohz_flags(cpu)) & NOHZ_TICK_STOPPED)
 		return;
 
 	/*
@@ -9087,7 +9092,7 @@ void nohz_balance_enter_idle(int cpu)
 
 	cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
 	atomic_inc(&nohz.nr_cpus);
-	set_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu));
+	atomic_or(NOHZ_TICK_STOPPED, nohz_flags(cpu));
 }
 #endif
 
@@ -9225,8 +9230,10 @@ static void nohz_idle_balance(struct rq
 	unsigned long next_balance = jiffies + 60*HZ;
 	int update_next_balance = 0;
 
-	if (idle != CPU_IDLE ||
-	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
+	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_BALANCE_KICK))
+		return;
+
+	if (idle != CPU_IDLE)
 		goto end;
 
 	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
@@ -9272,7 +9279,7 @@ static void nohz_idle_balance(struct rq
 	if (likely(update_next_balance))
 		nohz.next_balance = next_balance;
 end:
-	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+	atomic_andnot(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
 }
 
 /*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -723,7 +723,7 @@ struct rq {
 #ifdef CONFIG_SMP
 	unsigned long last_load_update_tick;
 #endif /* CONFIG_SMP */
-	unsigned long nohz_flags;
+	atomic_t nohz_flags;
 #endif /* CONFIG_NO_HZ_COMMON */
 #ifdef CONFIG_NO_HZ_FULL
 	unsigned long last_sched_tick;
@@ -2003,10 +2003,11 @@ extern void cfs_bandwidth_usage_inc(void
 extern void cfs_bandwidth_usage_dec(void);
 
 #ifdef CONFIG_NO_HZ_COMMON
-enum rq_nohz_flag_bits {
-	NOHZ_TICK_STOPPED,
-	NOHZ_BALANCE_KICK,
-};
+#define NOHZ_TICK_STOPPED_BIT	0
+#define NOHZ_BALANCE_KICK_BIT	1
+
+#define NOHZ_TICK_STOPPED	BIT(NOHZ_TICK_STOPPED_BIT)
+#define NOHZ_BALANCE_KICK	BIT(NOHZ_BALANCE_KICK_BIT)
 
 #define nohz_flags(cpu)	(&cpu_rq(cpu)->nohz_flags)