[patch V2 01/12] sched/smt: Update sched_smt_present at runtime

From: Thomas Gleixner <tglx@linutronix.de>
To: speck@linutronix.de
Subject: [patch V2 01/12] sched/smt: Update sched_smt_present at runtime
Date: Wed, 06 Jun 2018 21:27:15 +0200	[thread overview]
Message-ID: <20180606192807.014995293@linutronix.de> (raw)
In-Reply-To: 20180606192714.754943543@linutronix.de

Subject: [patch V2 01/12] sched/smt: Update sched_smt_present at runtime
From: Peter Zijlstra <peterz@infradead.org>

The static key sched_smt_present is only updated at boot time when SMT
siblings have been deteced. Booting with maxcpus=1 and bringing the
siblings online after boot rebuilds the scheduling domains correctly but
does not update the static key, so the SMT code is not enabled.

Let the key update in the scheduler CPU hotplug code to fix this.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/sched/core.c |   32 ++++++++++++++------------------
 kernel/sched/fair.c |    1 +
 2 files changed, 15 insertions(+), 18 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5771,6 +5771,20 @@ int sched_cpu_activate(unsigned int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	struct rq_flags rf;
 
+#ifdef CONFIG_SCHED_SMT
+	/*
+	 * We can't rely on the sched domains themselves to reliably inform us
+	 * of SMT, but if we ever see a CPU with siblings pass by, enable the
+	 * SMT code.
+	 *
+	 * The failure case for domains is when we boot with SMT disabled, then
+	 * create partitions that split all cores and then hotplug the
+	 * siblings. In that case we'll never build an SMT domain.
+	 */
+	if (cpumask_weight(cpu_smt_mask(cpu)) > 1)
+		static_branch_enable_cpuslocked(&sched_smt_present);
+#endif
+
 	set_cpu_active(cpu, true);
 
 	if (sched_smp_initialized) {
@@ -5868,22 +5882,6 @@ 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)
 {
 	sched_init_numa();
@@ -5905,8 +5903,6 @@ 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
@@ -6238,6 +6238,7 @@ static inline int find_idlest_cpu(struct
 }
 
 #ifdef CONFIG_SCHED_SMT
+DEFINE_STATIC_KEY_FALSE(sched_smt_present);
 
 static inline void set_idle_cores(int cpu, int val)
 {
