sched/smt: Update sched_smt_present at runtime

The static key sched_smt_present is only updated at boot time when SMT
siblings have been detected. 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 be updated 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>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 78d8facba456c2fc44c7024ae1a1c8d9db6f0692..0f79bb5ffb5aa3ab184f252b2913ed6c466be575 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5777,6 +5777,18 @@ int sched_cpu_activate(unsigned int cpu)
        struct rq *rq = cpu_rq(cpu);
        struct rq_flags rf;
 
+#ifdef CONFIG_SCHED_SMT
+       /*
+        * The sched_smt_present static key needs to be evaluated on every
+        * hotplug event because at boot time SMT might be disabled when
+        * the number of booted CPUs is limited.
+        *
+        * If then later a sibling gets hotplugged, then the key would stay
+        * off and SMT scheduling would never be functional.
+        */
+       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) {
@@ -5874,22 +5886,6 @@ 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();
@@ -5911,8 +5907,6 @@ void __init sched_init_smp(void)
        init_sched_rt_class();
        init_sched_dl_class();
 
-       sched_init_smt();
-
        sched_smp_initialized = true;
 }
 

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1866e64792a791f8737128c88ae691d7453ff117..cacdbef99b950c6a4dcbdce5aad62c40a1022517 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6238,6 +6238,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
 }
 
 #ifdef CONFIG_SCHED_SMT
+DEFINE_STATIC_KEY_FALSE(sched_smt_present);
 
 static inline void set_idle_cores(int cpu, int val)
 {