[RFC][PATCH 6/7] sched: Optimize SCHED_SMT

[Peter Zijlstra]

Avoid pointless SCHED_SMT code when running on !SMT hardware.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c      |   19 +++++++++++++++++++
 kernel/sched/fair.c      |    8 +++++++-
 kernel/sched/idle_task.c |    2 --
 kernel/sched/sched.h     |   17 +++++++++++++++++
 4 files changed, 43 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7263,6 +7263,22 @@ int sched_cpu_dying(unsigned int cpu)
 }
 #endif
 
+#ifdef CONFIG_SCHED_SMT
+DEFINE_STATIC_KEY_FALSE(sched_smt_present);
+
+static void sched_init_smt(void)
+{
+	/*
+	 * We've enumerated all CPUs and will assume that if any CPU
+	 * has SMT siblings, CPU0 will too.
+	 */
+	if (cpumask_weight(cpu_smt_mask(0)) > 1)
+		static_branch_enable(&sched_smt_present);
+}
+#else
+static inline void sched_init_smt(void) { }
+#endif
+
 void __init sched_init_smp(void)
 {
 	cpumask_var_t non_isolated_cpus;
@@ -7292,6 +7308,9 @@ void __init sched_init_smp(void)
 
 	init_sched_rt_class();
 	init_sched_dl_class();
+
+	sched_init_smt();
+
 	sched_smp_initialized = true;
 }
 
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5249,7 +5249,7 @@ static inline bool test_idle_cores(int c
  * Since SMT siblings share all cache levels, inspecting this limited remote
  * state should be fairly cheap.
  */
-void update_idle_core(struct rq *rq)
+void __update_idle_core(struct rq *rq)
 {
 	int core = cpu_of(rq);
 	int cpu;
@@ -5281,6 +5281,9 @@ static int select_idle_core(struct task_
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	int core, cpu, wrap;
 
+	if (!static_branch_likely(&sched_smt_present))
+		return -1;
+
 	if (!test_idle_cores(target, false))
 		return -1;
 
@@ -5314,6 +5317,9 @@ static int select_idle_smt(struct task_s
 {
 	int cpu;
 
+	if (!static_branch_likely(&sched_smt_present))
+		return -1;
+
 	for_each_cpu(cpu, cpu_smt_mask(target)) {
 		if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
 			continue;
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -23,8 +23,6 @@ static void check_preempt_curr_idle(stru
 	resched_curr(rq);
 }
 
-extern void update_idle_core(struct rq *rq);
-
 static struct task_struct *
 pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct pin_cookie cookie)
 {
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1811,3 +1811,20 @@ static inline void account_reset_rq(stru
 	rq->prev_steal_time_rq = 0;
 #endif
 }
+
+
+#ifdef CONFIG_SCHED_SMT
+
+extern struct static_key_false sched_smt_present;
+
+extern void __update_idle_core(struct rq *rq);
+
+static inline void update_idle_core(struct rq *rq)
+{
+	if (static_branch_unlikely(&sched_smt_present))
+		__update_idle_core(rq);
+}
+
+#else
+static inline void update_idle_core(struct rq *rq) { }
+#endif